GEOLOGICAL SURVEY OF ETHIOPIA GEOTHERMAL RESOURCE EXPLORATION AND EVALUATION DIRECTORATE

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1 GEOLOGICAL SURVEY OF ETHIOPIA GEOTHERMAL RESOURCE EXPLORATION AND EVALUATION DIRECTORATE Review and Reinterpretation of Geophysical Data of Tendaho Geothermal Field By Berhanu Bekele August 2012 Addis Ababa 0

2 Abstract Tendaho geothermal field is one of the first priorities for the development of power generation on the basis of its proximity to electric power grid, level of study compared to other geothermal prospects, and its huge potential since it is found in the afar triple junction. The present review and reinterpretation of geophysical data were performed with the objectives of improving the understanding of the subsurface of Tendaho geothermal fields, delineate geothermal reservoirs, and locate exploration drilling sites. After correcting for the magnetic and gravity field variations that are unrelated to the earth s crust, several filters were applied to the reduced data, so that anomalies of interest can be enhanced and displayed in a more interpretable manner. Three high residual gravity ridges separated by three gravity ridges separated gravity lows are reflection of subsurface rift configurations imprinted on Afar stratoid basalts. Computation of horizontal gravity gradients enabled to map the contact between masses of different densities. With such edge enhancement fault/contacts, calderas, craters, domes and vents are outlined. The magnetic method defined the axial region of the Tendaho rift which coincides with the central high gravity ridge. Estimates of vertical and horizontal positions have been made using Euler 3D deconvolution for fault, and sill/dyke models. Such models enabled to construct faults blocks constituting two half grabens separated by hinged high blocks with central depression. Geologically mapped thermal manifestations and craters bear positional correlations with the geophysical interpreted faults, ring fractures, calderas, craters, and vents. Both gravity and magnetic data show that the Tendaho graben is severely affected by fault systems, trending NW, NE, EW, and NS Stacking of the direct current (DC) and magnetotelluric geo electric sections, digitizing resistivity structures and integrating the various geo data helped to extract useful subsurface information. Depths to resistive basement, estimated using the DC and MT methods, imaged the corrugated nature of the subsurface structure. The central uplifted 1

3 resistivity structures and discontinuities are well correlated with density structures and magnetic fault blocks. Temperature gradients were recalculated and used with estimated Curie depth in order to predict Curie temperature of C which indicates that the magnetic mineral is magnetite. The MT method revealed a low resistivity diapir, interpreted as a magmatic melt. From temperature resistivity relation, the interpreted magmatic melt has an estimated temperature reaching C. Temperature gradient map outlined areas of high heat flow, where geothermal reservoirs occur. The axial rift region coincides with the high heat flow area and the magmatic melt occurs just below it at an average depth of 6.17 km. As a first approximation, contour closure of C/km may have delineated the geothermal reservoir. Based on the obtained results, it is highly recommended to drill a deep well at Airobera, where faults and ring fractures intersect and coincide with steaming grounds. Other highly recommended drilling site is at southeast position of the mud pools. But this has to be done after conducting high resolution MT survey at the recommended sites. Other recommended geothermal works are Heat flow survey to estimate the resource and micro seismicity to locate fractures and monitor drilling activities. 2

4 Contents Abstract Introduction Location and Accessibility of Tendaho Geothermal Field Surface Geoscientific Investigations Regional Geological and Tectonic Setting Regional Tectonic Setting Regional Geological Setting Local Geological Setting Hydrothermal Activity Objective Tendaho Geophysical Data Geophysical Surveys Geophysical Data Reduction and Processing Data Reduction Data Processing in space and Fourier Domains Gravity Anomaly Description and Interpretation Simple Bouguer Gravity Anomaly Residual Gravity anomaly of Tendabho Horizontal Gravity Gradient Magnetic Anomaly Description and Interpretation Total Field Magnetic Anomaly Spectral Analysis of Magnetic Data Residual Magnetic Anomaly of Tendaho Causative Source Distribution of Magnetic anomalies

5 4.4.1 Magnetic Fault Model Magnetic Dyke/sill Model DC Resistivity Structure Stacked Geoelectric Sections Correlation of Resistivity Layers with Borehole Geological Logs Magnetotelluric Resistivity Structure Temperature gradients Summary and Discussions of Geophysical Results conclusion and Recommendation Conclusion Recommendation List of Tables Table 1: Results of Spectral analysis of magnetic data.39 Table 2: summary of processing parameters while applying Euler Deconvolution..43 Table 3: processing parametrs of windowing on uncertainities and offsets 44 Table 4: Temperature gradient.73 Table 5: Depth estimates as obtained from different methods.81 Table 6: Summary of results for temperature & depth ranges, and temperature gradient

6 List of Figures Figure 1.1: Location Map of Tendaho geothermal field with respect to the Ethiopian Rift system 9 Figure 1.2: Geological Map of the explored part of Tendaho Graben (NW Tendaho) and Manda Hararo Zone.16 Figure 1.3: Schematic Geological well logs of TD1, TD2, and TD3, Lines connecting Afar Stratoid Basalt Series from well to well..18 Figure 2.1: Geophysical Survey layout Plotted against Topographic map of Tendaho Geothermal Prospect 24 Figure 3.1: Simple Bouguer gravity Map of Tendaho Geothermal Prospect. 29 Figure 3.2: Residual gravity map of Tendaho 32 Figure 3.3: Horizontal gravity gradient Map..33 Figure 4.1: Total Field Magnetic anomaly Map of Tendaho 37 Figure 4.2: Power Spectrum of Total Magnetic field of Tendaho 39 Figure 4.3: Redual Manetic Anomaly Map of Tendaho.41 Figure 4.5.1a: Solution Plot of Subsurface Fault with varying depth shown with different colours.45 Figure 4.5.1b: 3D Solution Plot of Subsurface Fault with varying depth shown with different colours 47 Figure 4.5.2a: Solution Plot of Subsurface Dyke/sill with varying depth shown with different colour..50 Figure 4.2.3a: Section map showing plots of multiple solution peaks and interpolated points constituting the curves.52 Figure 4.2.3a: Section map showing plots of multiple solution peaks and interpolated points constituting the curves.53 5

7 Figure 5.1: Stacked Geo electric Section of Tendaho rift.56 Figure 5.2: Map of Interpreted resistivity structures superimposed 58 on resistivity contour of the bottom substratum..60 Figure 5.2a: Geoelectric section along profile 16 found south of the well area b: Geoelectric section along profile 15 found north of the well area.. 64 Figure 5.3: Reinterpreted Geoelectric and Pseudo resistivity sections along Profile Figure 6.1: Stacked MT Resistivity sections assembled from individual section Figure 6.2: One Dimensional MT Resistivity section. 69 Figure 6.3: Three Dimensional View of Deep Resistivity structure assembled from depth slice of resistivity map and geo electric section of Profile line Figure 7.1: Map showing temperature gradients 75 Figure 8.1: Map showing Superposition of Horizontal gradient Fault locations and craters..79 Figure 8.2: Graph showing relationship between Resistivity & Temperature from laboratory experiment..83 Figure 8.3: Interpretative schematic cross sections portraying possible geometries 85 Appendix 1a: Regional Gravity Field Map of Tendaho.93 Appendix 1b: Regional Total Field Map of Tendaho.94 Appendix 3a: Profiles of Elevation Fault and Dyke/Sill.95 Appendix 2a: Three Dimensional Solution plot of Dyke/Sill 96 6

8 1 Introduction At present, in Ethiopia, Tendaho is relatively the most explored geothermal field next to the Alutu field that can be progressed to the development stage for inclusion in the schedule of power projects to be executed at any time soon. Already a 30 MW electric power from hydro is exported to Djibouti. The transmission line of 230kV to Djibouti with a substation at Semera, the capital city of Afar region is 10km away from drilled area which is a primary target for geothermal power development in Tendaho. Between 1993 and 1998 six exploratory wells were drilled, of these three are shallow and the rest are deep. The drilled wells intersected a shallow resource having O C temperature in the depth range of m in Dubti farm area. It could represent an outflow from a deep resource, probably situated in the southeast. If Dubti geothermal resource is fully studied involving geochemical monitoring, well testing, and reservoir engineering program in a continuous manner, at least, for about 6 months, the shallow resource can be well defined and its potential known. This may lead to the development of generating local electric power supply at small scale, which can replace the existing 3.7MW diesel generators in Semera and Dubti. As a first step, a well head turbine for electricity generation can be installed until a power plant is in place. Exploration drilling from the two deep wells (TD1 & TD2) also showed the existence of temperatures reaching C at deeper depth within the Afar Stratoid basalt Series, which is covered by a thick layer of sedimentary rocks. At the drilled positions the Afar Stratoid Series may have low permeability and as a result no significant geothermal fluid flows into the wells at the level penetrated by the deep wells. Geochemical information has interpreted the existence of a deep parental geothermal fluid at a temperature of about 300 O C or more. A high temperature reservoir is thought to underlie the area immediately to the southeast of the drilled area. Geophysical survey data interpreted in light of information provided by the drilling supports the above interpretation. 7

9 Increasingly, the greatest success in developing a geothermal resource comes through the appropriate consideration of all types of available information in a synergistic interpretation. At the initial stage, upstream geo scientific exploration data leads to a better informed choice of well sites and effective application of exploration capital. The greater availability of reliable information on the geothermal resources of the country, therefore, would encourage public and/ or private power developers to invest in electricity generation from geothermal resource and supply power to a geographically expanding market, following a fast growing Ethiopian economy. Review of previous geo scientific reports and reinterpretation of Tendaho geophysical data, which is the subject of this report, could be helpful to comprehend the Tendaho geothermal system and identify knowledge gaps in order to formulate project ideas for definitive feasibility study. 1.1 Location and Accessibility of Tendaho Geothermal Field The Tendaho graben is located within the central NE part of Ethiopia in Afar regional state, about 600 kms from the capital city, Addis Ababa (Figure 1). On the average the graben is a 50km wide depression of late Quaternary age. Tendaho is a prime target for geothermal energy development owing to its position with respect to Afar Triple Junction, where the Red Sea, Gulf of Aden and Main Ethiopian rift join. Altitudes on this part of Tendaho graben floor vary from 800m to 219m. As has been indicated above, Tendaho is a region of multiple geothermal prospect areas, only a few of which have been investigated beyond the reconnaissance stage. Dubti, Ayrobera, Logiya, and Alalobeda geothermal fields are situated in the northwestern part of Tendaho graben. Of these fields, the drilled area is found only at Dubti. River Awash enters the graben through a ravine (Figure 2) in the rift shoulder, the southwestern graben bounding up lifted block, and flows towards southeast into Lake Abe. 8

10 . Figure 1.1: Location Map of Tendaho geothermal field with respect to the Ethiopian Rift system 9

11 The region s populations are mostly pastoralist engaged mainly in camel and goat herding. Owing to the fertile alluvial soils and water supply of Awash River, the most important economic activity for the past 4 to 5 decades has been cultivation of cotton at Dubti, Det Bahri, Tangaye Koma and Asayita plantations. Dubti, the wereda seat of government (population 15,342) and Det Bahri started as satellite towns to the farms that bear their names. An asphaltic road from Addis Ababa bifurcates at Semera to continue to Djibouti and Assab. The main transport route for Ethiopia s foreign trade at present is only through Djibouti. Because of this temporary facility of storage area has been built at Semera to minimize cost of portal service. The town of Logiya is a product of the road traffic which provides service to the truck traffic on that highway. Semera, which started out as a road maintenance camp, also served as the campsite for the geothermal exploration project since the 1990s. Semera has become the capital of Afar region since 2004, and it is growing up at a fast rate. The population of Dubti wereda in July 2004 was estimated at 83,987(60% male) of which 25% or 21,095 people lived in the above four towns. Energy consumption of the rural population is mainly based on biomass fuel use: wood from acacia trees, crop residue and camel and goat droppings. However all four towns have electricity supply, used primarily in household lighting and the commercial sector. Dubti and Logiya are now connected to the EEPCo diesel generator in Semera. Dubti farm generates its own electricity for use at its ginnery and workshop, and for air conditioning and lighting. Diesel fuel is used to generate power for irrigation and drainage pumping. 1.2 Surface Geoscientific Investigations The first exploration work, following the reconnaissance survey of , was carried out under a technical cooperation program with the Italian Ministry of Foreign Affairs (MAE). MAE paid for the technical services of Aquater spa of the ENI group. Aquater provided a team of geologists, geochemists and geophysicists who worked with counterparts from GSE under a project managed by GSE. Exploration equipment was brought from Italy for the duration of the project. 10

12 Detailed geoscientific studies (Geology, Geochemistry, Geophysics) followed by the drilling of shallow temperature gradient holes were carried out in two phases during the late 1970s to the early 1980s. The work aimed at gaining an understanding of the probable heating and circulation system and the citing of exploration wells aimed at discovering an economically exploitable geothermal reservoir. After a long interval following the completion of these surveys, exploration drilling was carried out during with financial and technical assistance from MAE. GSE carried out further drilling during using its own budget. To define and for more understanding of the geothermal system, fill in surveys, reservoir engineering & geochemical monitoring, and magnetotelluric (MT) survey were carried out after the cease of drilling activity. Here below, results of Geological studies by different groups and geothermal exploration activities are outlined and summarized. 1.3 Regional Geological and Tectonic Setting Regional Tectonic Setting The majority of the Ethiopian flood basalts and associated felsic rocks were erupted between 31 and 29 Ma (Hofmann C., et al., 1997), covering 1000 km diameter region. The plateau up lift together with Cenozoic volcanism led to the inference that a hot mantle beneath Afar was responsible for voluminous magmatism and initiation of the NE directed extension in the southern Red Sea and Gulf of Aden (Menzies M. et al., 1990 & 1992; and Wolfeden E., et al., 2003). The petrologic character of the basalts shows sources in primary magma generation in the DUPAL anomaly region of the mantle. Proposed models for the origin of rifting, volcanism and plateau uplift in East Africa as summarized by Nyblade A. A., et al., 2004, are: stationary plume head; Runny plume head (Ebinger C., & sleep, 1998); and broad thermal upwelling from the lower mantle (Lithyow Belton & Sheer, 1998) with a support from petrological character of the basalts showing sources in primary magma generation in the Dupal anomaly region of the mantle (Kieffer B., et. al., 2004). Whichever model accounts for the observed first order features (rifting, 11

13 volcanism & uplift), Rogers N., 2004 applied trace element and Os isotope analysis of a detailed Trap Series basalt section and that Afar plume was at a depth between 120 and 150 km. The flood basalt stage was followed by the eruptions of trachytic and rhyolitic volcanism in close proximity to the zones where the rift margin and transverse structures were to develop later. Rifting thus developed across a flood basalt plateau, as a further development of crustal processes taking place under the influence of the Afar plume (Demssie G., 2011). Initial rifting in the southern Red Sea occurred soon after or concurrent with flood basalt magmatism when the African and Arabian plate split apart (Menzies M., et al., 1990 & 1992). Border faulting of Afar initial rifting began between 25 and 23 Ma (ELC, 1987). This is equivalent to stage 1 of EGLE (the Ethiopian Afar Geoscientific Lithospheric Experiment) that the rift formation in the southern Red sea occurred between the time span of 28 to 19 Ma. Stage 2 of rifting in both the southern Red Sea (19 12 Ma) and the northern MER (Main Ethiopian Rift) between 2 Ma and the present are marked by a rift ward migration of strains to narrow zone of aligned eruptive centers and normal fault swarms (Ebinger C., 2004). Stage 3, oceanization of the southern Red Sea with extrusion of thick piles of basaltic lavas known as the Stratoid Series probably occurred between 3.5 to 4 Ma (ELC, 1987), and the triple junction reorganized itself in the Pliocene to Recent, while the magmatic segments of the southern Red Sea were abandoned and the northern MER propagates northward (Wolfeden E., et al., 2004; and Ebinger C., et al., 2004). The Red Sea as an oceanic rift seems to be more and more obliterated south of 15 0 North (Ross & Schlee 1973; Gas Mallick & Cox 1973) and it is even inactive in the areas from Hanish dubbie up to Baab el Mandab (Barberi F., & Varet J. 1975). Diffuse extension may give way to separation along a narrow spreading center, or new plate boundaries may form at the expense of other boundaries that became inactive (Garfunkel Z., Beyth M., 2006). 12

14 Thus in response to the diminishing of opening (T. B. Christiansen, H. U. Schaefer and M. Schonfeld, 1975), the Red Sea spreading center south of N latitude bifurcates into two; the SSW branch forming an active volcanic range along a narrow spreading center propagates southward on land at the northern tip of the Afar triangle, while the southeastern branch preserves the southern continuation of the Red sea (Chu D.,& Gorden R. G., 1998; Abbate E., Passerini P., & Zan L. 1995; Christiansen T. B., Schaefer H. U., and Schonfeld M., 1975; and Beyene A., and Abdelsalam M. G., 2005). Erta Ale volcanic range is active, belonging to the SSW branch of the axial structure of northern Afar rift which indicates the shifted extension of the Red Sea spreading axis (Barberi F., Cheminee J. C., & Varet J. 1973). It branches into a southeastern segment, Tat Ali, which ends in the Dadar graben and a southwestern segment, Alyata. Manda Hararo is the southern continuation of Alyata (the western segment) which propagates into central Afar. Southward of Manda Hararo are the Tendaho Goba Ad propagators that terminate at the western side of Ali Sabieh block (Abbate E., Passerini P., & Zan L. 1995; Beyene A., Abdelsalam M. G., 2005; and Lahitte p., et al., 2001 & 2003). In between Manda Hararo and Alyata is Dabbahu magmatic segment, where a major rifting episode occurred in 2005, associated with a volcanic eruption along 60 km length fed from a shallow magma chamber occurring between depths of 2 & 9 km. Such dyking that opened by up to 8 meters was accompanied by seismic activity (Wright et al., 2006; Ayele et al., 2007). The northern part of the Main Ethiopian Rift (MER) in southern Afar, starting from the area of south Fantale forms a funnel shape, widening towards the north. The MER propagated northeast in the last 11 Ma with a predominantly N E Miocene border faulting. After ~ 3 Ma the border faulting was superseded by N15 0 E striking faults (Wolfenden et al., 2004). The NNE SSW trending Wonji Fault Belt (Mohr, 1967 &1972), is a rift in a rift structure referred to as where faulting and dyking constitute its extension. It is characterized by right stepping of magmatic segments or en echelon zones of magmatism and faulting with localization of strain (Ebinger and Casey, 2001). 13

15 As the MER comes nearer to the Tendaho propagator, it branches into two; the western branch is the Karrayu rift and the eastern being the main branch of the Ethiopian rift. Tendaho graben interrupts the Karrayu rift sharply, having cross cutting relationships between them Regional Geological Setting Regionally, the pre Stratoid succession consists of Adolie basalt, Mabla rhyolites and the overlying late Miocene Dalha basalt. On the basis of the relative stratigraphical positions and nature of volcanic products, Barberi F., et al, (1975) recognized these three volcanic formations. They can be observed in eastern margin of Afar, Ali Sabieh Aischa and south east of Danakil Alps (Barberi F., et al, (1975); Black et al., (1972b); and Civetta et al., (1974a)). Dahla basalt outcrops near the western Afar margin (Mille region) and also in southern Afar (Adda do graben). Adolie basalt is the oldest of these rock formations, associated with the initial and 2 nd stages of rifting in Afar, for its formation spans between 22.3 to 14.5 Ma in accordance with ages determined by Chessex et al, (1974a), and Barberi F., et al, (1975). Mabla rhyolite series is characterized by the deposition of a large volume of acid volcanics with subordinate basaltic and intermediate lavas (Mabla rhyolites) dated between 15.3 and 3.3 million years. In the Ali Sabieh region, according to Chessex et al. (1974a), have age around 14 m.y., but ages based on Barberi F., et al, (1975) determinations are ranging from 14.2 to 9.7 Ma. It occurred during late second stage of rifting. Its outcrops are largely observed in the area north of the gulf of Tadjura and consists of thick flows, domes and ignimbrites of slightly peralkaline (comenditic ) rhyolites with some intercalated basaltic flows, pumice and cinerite deposits (Barberi F., et al, (1975)). Dahla basalt: As cited by Barberi et al., (1975), five age determinations have been obtained for the Dalha series s.s. in T.F.A.I. outcrops ranging from 8 to 6.5 m.y. Ribita rhoylite gave a 3.5 m.y. age and an age of 4 m.y. was obtained for the uppermost flow of 14

16 basalt covered by the Stratoid Series has been dated by Civetta et al. (1974a, 1975) in the Beylul region. Results are the same as those we obtained in T.F.A.I. ( m.y.). This unit consists of a series of basaltic flows with rare intercalations of ignimbrites and detritic deposits, and reaches a thickness of 800 meters. Figure 1.2 is a regional geological map of central Afar with emphasis on Manda Harraro and the northwest part of Tendaho graben. Barberi et al. (1972) have shown that the central Afar is floored by a dominantly basaltic Stratoid formation without any important break. Since about 4 million years ago, the inner part of the rift floor became affected by tensional tectonics and accompanying intense volcanic activity, producing mostly the fissural Afar Stratoid Series basalts and subordinate rhyolites. The Afar rift is believed to have reached its present geological setting during the Pleistocene period, with the emergence of the axial zones of crustal separation. Intense tensional tectonics affects the entire depression, thus forming a complex mosaic of horsts and grabens which are still active and form localized sedimentary basins. Tendaho graben was formed on this geological landscape (Demssie G., et al., 2011). 15

17 Figure 1.2: Geological Map of the explored part of Tendaho Graben (NW Tendaho) and Manda Hararo Zone (After Getahun Demessie, 2011). 16

18 1.3.3 Local Geological Setting Here in this section, a very brief account of surface and subsurface geology of northwest Tendaho rift is presented from summaries given in various project proposals for Tendaho geothermal field. Few among them are proposals (see reference for the list) for ARGEo (African Rift Geothermal Development Facility), MOFED (Ministry of Finance and Economic Development), and AFD (French Development Agency). The portion of Tendaho graben, that covers the project area, consists of a NW SE elongated broad plain, about 50 km in width having an area of about 4000 Km 2 (Figure 2.1). The Afar Stratoid series, a Pliocene to early Pleistocene (<4 million years) formation, consisting mostly of basalts of fissural origin, with minor rhyolitic bodies in its upper part, make up the graben bounding fault blocks. The series have been encountered by deep drilling, forming the basement rocks on which the sediments have been laid. The sedimentary sequences that fill the graben floor are fine to medium grained sandstone, siltstone and clay. Intercalation of basaltic lava sheets with the sediment was observed in the geological log. The youngest rock units that form the upper volcanic complexes, both fissural and central by origin make up the positive topographic features inside the graben. The volcanic complexes are mainly composed of basalt & subordinate rhyoloite, and Pleistocene volcanites of Manda Hararo. With a view to establish the stratigraphic succession in the drilled area (Figure 1.3), and to study the different types of alteration mineral assemblages in the rocks: Geological logs were prepared for each well based on cutting samples that were collected at 5 m intervals; and, Core samples were recovered below a depth of 500 m, especially at major changes of formation, with a minimum of 200 m interval between core samples. The samples were petrographically examined in order to identify the different types of hydrothermal minerals present in the collected drill cutting and core samples. Furthermore, detailed laboratory analyses including absolute age dating of hydrothermal phases, X ray Fluorescence (XRF), X ray Diffractometry (XRD), Electron Microprobe, and, 17

19 Figure 1.3: Schematic Geological well logs of TD1, TD2, and TD3, Lines connecting Afar Stratoid Basalt Series from well to well 18

20 for samples from TD1 to TD4, Fluid Inclusion studies were performed in various laboratories in Italy: The results obtained from the study of geological logs showed that: o permeable intervals are found in the upper part of the sequence and essentially in the basaltic intercalations within the sedimentary sequence; o sedimentary horizons with better permeability may exist in the more arenaceous levels; o The Afar Stratoid series has poor primary permeability and it is presumed that only secondary permeability induced by recent tectonic activity may exist. The heat source for the hydrothermal activity within the graben appears to be related to the recent and widespread fissural basaltic volcanism specifically to magma that may be injected into fractures in this tectonically active zone. Petrographic study of cuttings and core samples showed: o evidence of an early stage of calcite, zeolite (wairakite or laumontite) and quartz crystallization, while calcite underwent different stages of dissolution/precipitation, possibly due to abrupt changes in ph and CO 2 partial pressures; and o The occurrence of epidote, garnet, prehnite, pyroxene, and amphibole crystallization occurred after wairakite or laumontite. Chemical analysis of core samples (altered basalts) showed increased Ca, Fe, Mg, Al content (owing to the dissolution of plagioclase and femics and the precipitation of wairakite, laumontite, epidote, garnet, calcite and clay minerals) and decreased Na, K, Si and Ti (owing mostly to the dissolution of glass matrix). The mineralogical features of well TD1 and fluid inclusion data indicate a recent heating of the geothermal fluid. Wells TD2, TD4, TD5 and TD6 on the other hand, are under more or less stable conditions. It is likely that all wells have been drilled in the proximity of the up flow zone of the system. The mineralogy and fluid inclusion data from TD3 indicated the opposite to be true. TD3 was drilled far from any up flow zone, probably in an area characterized by self sealing, cooling, very low permeability and very shallow hot water flow. 19

21 1.2.2 Hydrothermal Activity The inventory of hydrothermal features was carried out in Tendaho area on two occasions: during 1969 by a ground based survey by GSE and during the 1971 an airborne thermal infrared survey carried out in conjunction with ground truth work that was successfully used for locating areas of surface thermal anomaly in unknown areas of poor access (UNDP, 1973). According to summaries by GSE in various project proposals, this work has provided the following information on the distribution and characteristics of hydrothermal manifestations: A spouting hot spring and hot water pools at boiling temperature at Alalobeda along the western graben margin, Magenta range, about 20 km SW of Dubti; Fumaroles on Magenta range, the south western graben bounding fault block adjacent to Alalobeda; zones of steaming and warm ground on Airobera plain about 15kms north of Dubti; Mud volcanoes and steaming/ altered ground on the north eastern margin of Dubti plantation; Thermal springs (50 O C) on the shore of Begaadoloma crater lake, located approximately 30 km NW of the Dubti; solfatara of various degrees of activity associated with a number of rhyolitic edifices located in the southeastern part of the graben thermal springs occurring on the shores of L. Abhe and the higher standing terrain to the southwest; Very high temperature fumaroles occurring in the annular collapse structure of Dama'ale volcano near L. Abhe. The above inventory of hydrothermal manifestations formed the basis for the determination of Tendaho graben as a target for exploration. It should be noted that Tendaho is a generic name for Dubti, Alalobeda, Airobera and Begaadoloma, etc. 20

22 1.4 Objective A lot has been changed in software and computer technology since the time of exploration works for geothermal resource in Tendaho have been conducted. The advance in digital processing offers the opportunities to perform advanced processing, to integrate various data and extract new information that would have been impossible decades ago. Such opportunities and the need for more power generation initiated the performance of review and reinterpretation of previous geophysical data. Broad objective The overall objective is to contribute to the realization of geothermal power generation through the provision of subsurface information and identify knowledge gap to propose future studies. In short, the main scope of this work is then to improve the understanding of the subsurface of Tendaho geothermal fields Specific Objectives Prior to the progress of the Tendaho geothermal field into the development stage, specific objectives have to be set and achieved in order to realize power development. As part of this endeavour, the objectives to be met are: Delineate the geothermal reservoir through outlining and defining the subsurface geological structures Locate target sites for exploration drilling for the discovery of an exploitable geothermal resource which the previous multidisciplinary exploration information has indicated to exist and; Propose future geoscientific studies for more understanding of the source though filling any knowlrdge gap. 21

23 2 Tendaho Geophysical Data Through the years, large amount of geophysical data have been generated by the Geological Survey of Ethiopia, research institutions and agencies. This section gives a brief account of the various geophysical surveys from which the data were obtained. Reprocessing of the geophysical data and review of the available information were performed after retrieving them from the web based geothermal database. The sub section devoted to data processing elaborates the applied processing procedures. 2.1 Geophysical Surveys The Geological Survey of Ethiopia (GSE) and Aquater in 1979/80 conducted detailed gravity, magnetic and resistivity surveys in and around Tendaho rift. A micro seismic survey was also performed from May, 1989 up to January 1990 using data recorded by two sets of 3 seismometers. Later in 1996 & 2004, GSE alone ran in fill surveys along accessible tracks at Dubti plantation and its vicinity. Portions of the Tendaho rift covered by all these surveys are approximately 50 km by 41 km. Observation points for the gravity & magnetic surveys were 2379 and 3443 respectively. The surveys were conducted along profiles and rarely on random points. Station intervals varied between 250 m and 500m. Vertical Electrical Sounding (VES) data were taken on 177 points at an interval of three km along profiles separated by 5 km. Figure 2 depicts the resistivity survey layout and station locations occupied for gravity and magnetic observations. During 2007, a joint GSE BGR resistivity survey consisting of transient electromagnetic (TEM) and magnetotelluric (MT) was carried out along widely separated profiles with sparse measurement station density. The MT survey attained great depth of penetration but was unable to delineate what may be interpreted to be a geothermal reservoir or any other related feature, since it lacks resolution both in the horizontal and vertical directions, owing to sparse measurement stations. To attain higher data density and resolution of the 22

24 subsurface resistivity structure, GSE continues detailed MT surveying at the time of writing this review work. Other relevant geophysical data can be obtained from aeromagnetic survey by Girdler (1969), gravity mapping by Geophysical Observatory (1970), and Starting from the 1970s, considerable international research interest in plate tectonic and continental rifting processes, initiated especially by the Italian and French national research institutions, provided a large volume of fundamental information on the evolution and current state of the Afar. The works of research institutions from the then German Federal Republic have provided regional seismic, gravimetric and magnetotelluric data which have led to the better understanding of crust mantle interactions and temperature distribution at depth. These have been fundamental to the understanding of the gross geothermal structure of the shallow subsurface of the Afar. Such data may give a regional overview of Tendaho with respect to Afar depression. Though we focus on the detailed surveys, such fundamental information will be used for the review and reinterpretation of the geophysical data of Tendaho geothermal field. 23

25 Tendaho Figure 2.1: Geophysical Survey layout Plotted against Topographic map of Tendaho Geothermal Prospect 24

26 2.2 Geophysical Data Reduction and Processing Oasis Montage version 4.3 of Geosoft software package was used for reduction and processing of potential field data collected in parts of Tendaho rift Data Reduction Diurnal magnetic variations and non dipolar broad variations of the magnetic field arising from the earth s core are of external and internal origins, respectively, which are unrelated to the earth s crust. It is thus essential to remove these variations in order to obtain observations due to only the earth s crust. Correction for diurnal variations was performed during the data acquisition stage by monitoring the variation on a daily basis using a base station. The non dipole core magnetic field variations were corrected for 1979/80 and 2005 magnetic data in the office using 1980 and 2005 IGRF models respectively. Then the two data sets were merged for further processing and interpretation. In gravity data reduction, the factors that affect the gravity observations are earth s tide, instrument drift, latitude, altitude, topographic terrain, the mass between sea level and observation points and density. These effects are predictable and the 1979/80 gravity data have been corrected for all of them, since density variation is of interest for gravity exploration. This was done first by applying tidal, & drift corrections then latitudinal correction was made employing the 1967 formula. Finally free air and Bouguer reductions were employed using a density of In case of the 2005 gravity data, only the former five have been removed. Thus when combining the two data sets, only simple Bouguer values were used for processing and interpretation. Gridding The irregularly distributed gravity and magnetic data were interpolated on equally spaced grids using the method of minimum curvature. To prevent overshooting/undershooting in areas of sparse data, a tension factors from 0.25 to 0.5 was applied. This method was chosen owing to its suitability for a sparse data distribution and honors the individual random values to generate a smooth surface. Regular grids with 124 by 102 points and mesh size of 750x750 meters for gravity data processing were selected employing UTM 25

27 coordinate projection with a datum of Adindan to produce the required basic grids. In case of magnetic data interpolation, the mesh size is 500x500 meters using the same projection Data Processing in space and Fourier Domains First the measured data were inspected for spikes and a notch filter was applied to despike them. A variety of filtering techniques were applied to the grid data sets in order to obtain new interpretative information by enhancing particular trends or wavelength of the observed anomalies. These techniques included a smoothing filter (hanning), high, low & band pass filters, horizontal derivatives in X & Y directions and vertical derivative filter in Z direction. Except the smoothing process, all the filters were applied to the data in a wave number domain. FFT (Fast Fourier Transformation) algorithm was employed in transforming the spatial data into wave number domain or spatial frequency. Radially averaged power spectrum computed in the wave number domain has been used to determine the spatial frequencies for filtering. The filtering results were then transformed into space domain and contoured to produce the required maps. The power spectrum was also used to estimate depths. Regional Residual Separation A regional component in the Bouguer gravity or in total magnetic fields distorts or obscures the effects or signals of the near surface geology. A band pass filter with wave number cut offs between cycles/km was used to remove the masking effect of high amplitude, long wavelength anomaly and the resulting residual gravity can be judged to reflect the near surface geology. In the case of magnetic data the residual field is obtained by applying a band pass filter with cut off spatial frequencies between cycles/km, or 10 3 km wavelength. Even after removing the long wavelength part of the anomaly, there remain the overlapping gravity or magnetic field effects due to the closeness of geological bodies in which the details are obscured. Thus derivative filters 26

28 were applied to the gravity/magnetic fields in order to improve the resolution or enhance the edge effects in the application of Euler deconvolution. Analytical signal computation was performed in order to match the magnetic highs and the causative magnetic bodies. This helps to minimize interpretation difficulties due to effects of low magnetic latitude and remnant field. Euler 3D deconvolution was applied to understand shape, depth, orientations, distribution, size and intensity (frequency) of subsurface features. Derivative filters and upward continuation were employed while applying Euler deconvolution and producing analytical signal map. Resistivity data processing and Curve fitting According to Aqauter (1980), the calculated apparent resistivity for each VES was plotted against half electrode separation (AB/2) on a double logarithmic paper. Approximate interpretation method i.e. the auxiliary point method was employed for the determination of the resistivity and thickness which were used as initial model parameters for a more computer assisted iterative interpretation to determine the layer parameters, resistivity and thickness of the layers. Details of resistivity data processing & interpretations are given by Aquater (1980 & 1995). The processing and interpretation results were summarized by individual geoelectric section. Summary of MT data processing and interpretation is given in the section dedicated for this purpose, and more details are found in the report by Kalberkamp U (2010). In this review, stacking of the various resistivity sections was conducted in order to obtain the overall subsurface picture of Tendho graben. 27

29 3 Gravity Anomaly Description and Interpretation 3.1 Simple Bouguer Gravity Anomaly The Tendaho gravity field over the surveyed area ranges from 19 to 57 mgals with a mean value of 31 mgals. Figure 2 is a graphical display of the Tendaho simple Bouguer gravity map along with a histogram view of the gravity data. The observed gravity anomalies can be classified in to three categories, namely, high, intermediate and low anomalous zones. The high gravity anomalous zones are bounded by a contour line of 30 mgals. Within the zones there are very high anomalies of short wavelength with 25 mgals contour closure. A conspicuous, very high gravity anomaly is found between Gum Atmali, Airobera and Begaadoloma Crater Lake. The anomaly center occurs over a topographically depressed area with its margins on high grounds. It coincides with parts of the upper extrusive complex consisting mainly of basalt and subordinate rhyolite. The extrusive complex may not be the cause of the anomaly, through its association cannot be ruled out. Dense intrusion, probably within the zone of the rift axis, could be the cause of this anomaly. Other less significant, very high gravity anomalies occur over and around the rift margins at Tendaho, Allalobeda and Serdo. The gravity low zones are bounded by 35 mgals contour line. As in the high zone, very low gravity anomalies are hosted within the low gravity zones. They are outlined by a contour line of 40 mgals. The one found Southeast of Dubti town could be attributed to low density intrusion of magnetic origin. A broad, intermediate gravity zone is observed in the central part of the surveyed area. The zone is characterized by a smooth pattern of iso anomalies, which decrease from 30 to 35 mgals. It is open on its ends & extends from East to West. Another zone sandwiched between two high anomalies is found East of Ayrobera. The origin of the intermediate zone could be ascribed to gradational contacts between two anomalous masses of differing densities. 28

30 The Bouguer gravity field of Tendaho is dominated by regional field component and similar except the short wavelength anomalies contained in the Bouguer gravity field. The regional gravity field map is presented in appendix 1a for interested readers. Figure 3.1: Simple Bouguer gravity Map of Tendaho Geothermal Prospect 29

31 3.2 Residual Gravity anomaly of Tendabho Short wavelength anomalies are assumed to reflect the geophysical property of the geological bodies occurring at the surface or at a shallow depth. The gravity field obtained by applying a high pass filter with a spatial frequency greater than 0.06 cycles per meter or wavelength less than 16 km is a residual gravity to characterize the near surface feature. The Tendaho residual gravity field over the mapped area varies between 13 and 10 mgals with an average value of zero. Figure 3 is a graphical presentation of the Tendaho residual gravity map with its associated histogram view. Three high gravity ridges are outlined by a zero contour line. They run parallel to each other in a NNW direction, but just south of Logya Dubti axis the central and the western gravity ridges curved toward the east, forming curvilinear gravity ridges. Each of them hosts short wavelength, high gravity anomalies which are circular or elongated in the direction of the ridge. The Western high gravity ridge is caused by Afar stratoid basalts that are exposed or occur at the western Tendaho rift margin. Step faults of the basalt and crisscross faults may give rise to several high anomalies with short wavelength. The central high gravity ridge is interpreted as a buried axial range in which the recent basalt and associated minor rhyolite occur. The short wavelength high anomalies within the ridge could be volcanic centers which are responsible for intrusive or extrusive volcanism. The Southern tail of the eastern high gravity ridge is not mapped owing to lack of gravity data. Sandwiched between these ridges are two elongated gravity lows. They are interpreted as graben structures that are filled with sediments. The gravity ridges are also covered by sediment covers except in the areas where outcrops of stratiod basalt and recent extrusive volcanic complex occur. 30

32 Figure 3.2: Residual gravity map of Tendaho At the initial stage, the Tendaho rift could be only a graben structure formed due to fault extension of the upper elastic crust. The development of the axial ridge at later stage may indicate the initiations of magmatic extension which may have dominated the extension 31

33 mechanism to form a central host or a central uplift due to fault block adjustment. The observed gravity ridges may reflect either of such features. 3.3 Horizontal Gravity Gradient The horizontal gravity gradient field superimposed with plots of the locations of high gradient magnitudes are depicted on a map given by figure 2. The high magnitude plots obtained from the edge enhancement by the horizontal derivative outline lateral mass inhomogeneities and these are expressions of structural and lithological boundaries. The plots are designated by a strike & dip symbol ( or ). At the apex of the ridge gradient is zero and the central region possesses low gradients. High gradients are then at the contacts between high and low density masses. The central gravity ridge observed in the residual gravity map (fig 3.2) is bounded by high gravity gradient as observed in figure 3.3. Linear or curvilinear shaped low gradients are bounded by high gradients, or vice versa. But the plots occur only over the high gradients and are aligned along the longest direction. In case of a low density mass, dip directions marked by the plots face each other from the opposite sides of its boundaries or dip toward the low gradient (gravity depression). In case of high density mass, dip directions diverge away from the low gradient. Such patterns are readily observed on the map in figure 3.3. In this way, buried graben and horst structures are easily identified on the map. There are circular, nearly circular or elliptical plots occurring within the gravity depressions or gravity ridge. Some of them are not fully circular or elliptical. Daorre crater falls at the edge of a feature outlined by circular plots with converging dip directions, similar in shape to observed topographic expression of the crater. Thus, plots of converging dip directions towards the center of circular or elliptical features are interpreted to be buried craters, calderas or collapsing structures; however, plots with diverging dip 32

34 directions having circular or near circular shapes are attributed to buried doming structures or volcanic cones. Figure 3.3: Horizontal gravity gradient Map There also exist several east west trending linear features; the main ones occur at the geothermal well location passing between TD 1 and other wells (TD 2, TD 4, TD 5, and TD 6), east of Logya crossing the road to Assaita, east of Tendaho town reaching the road to Det Bahri, and south of the gradient well GBH 2. Their origin is unknown, but they are discrete and intermittently extend in space. From bore hole geological logs it can be observed that the Afar stratoid series is about 280 meters deeper in well TD 1 than in well TD 2. This may attest that the east west linear feature that passes between the two 33

35 geothermal wells is a fault responsible for the vertical displacement of the Afar stratoid series. In general, it can be said, however, that topographic up and downs or undulation along the gravity ridge or gravity depressions may occur owing to different trending faults, and or due to doming or crater expressed by gradient lineaments and closed or open curvilinear features, respectively. 34

36 4 Magnetic Anomaly Description and Interpretation 4.1 Total Field Magnetic Anomaly The Tendaho total magnetic field over the surveyed area ranges from to gammas with a mean value of gammas. Figure. is a wet colour shaded map of Tendaho total magnetic field. It is illuminated from northwest with declination angle of Azimuth and inclination of Thus it should be noted that features are displaced to southeast from their actual positions. In interpreting the magnetic field it is assumed that Induced magnetization is dominant and remnant magnetization is neglected. Unlike gravity, magnetic field is dipolar having inseparable positive & negative anomalies as caused by a shallow magnetic body with limited depth extent. Thus the magnetic anomalies are observed to be characterized by high and low anomalies of complex nature. Since Tendaho is found at the low magnetic latitude or magnetic equator, the lows are due to high magnetic susceptibility of the causative magnetic bodies, with positive lobs on either side of the low if the causative bodies are shallow and of limited depth extent. Magnetic bodies with large depth extent may have very small positive lobs, or no lobs. More complication arises if the causative magnetic bodies are close together since their corresponding fields superpose either to give exaggerated highs or resulted in the cancellation of positive & negative magnetic fields. Such theory allows description and interpretation of magnetic anomalies The Tendaho magnetic field is dominated by short wavelength anomalies. Two zones of low anomalies, with an alignment of Northwest are identified. The central low zone consisting of several very low short wavelength anomalies straddles from Bagaadoloma in the northwest to the southeast end of the surveyed area. Their attendant positive lobs are very high spreading out possibly over non magnetic bodies. All of the geothermal exploration wells and most temperature gradient shallow bore holes fall within this zone. 35

37 The second low zone is found at southwest in Det Bahri area. It is displaced to southeast at the location between Allao Beda & Tendaho before reaching logya. There are northeast trending magnetic features that can be readily observed in figure.. They cross the low zones and are also crossed by the zones. Such crisscross are of geothermal interest specially the point of intersections may create space for the accumulation of geothermal fluids. Minor east west trending features exist which also need to be considered. The total magnetic field of Tendaho is dominated by regional field component and they similar except the short wavelength anomalies contained in the total magnetic field. The regional magnetic field map is presented in appendix 1b for interested readers. 36

38 Figure 4.1: Total Field Magnetic anomaly Map of Tendaho 37

39 4.2 Spectral Analysis of Magnetic Data The radially averaged power spectrum of the total magnetic field data, shown in figure, takes the form of three segments with changes in slopes at about and wave numbers. Depths to magnetic layers are estimated by applying slope method (grant & west, 1975) given by Slope = 2Пh, where h is depth. Figure 4.2: Power Spectrum of Total Magnetic field of Tendaho Depth to the bottom of magnetic basement or curie depth is also estimated using the formula (Botler, 1978) given by f max = ln(d/h)/(d h) 38

40 Where D is depth to the bottom of magnetic basement and f max is peak frequency, where maximum amplitude occurs. The base of the magnetic basement derived from peak frequency is interpreted as the position of Curie point isotherm (Bhattacharyya and Leu, 1975; Byerly et al, 1977; Connard et al, 1983; Blakely, 1996). Estimated Curie temperature and corresponding magnetic minerals will be discussed in section 7. Employing spectral slope method, depth to the top of magnetic basement is 4.16 km. For this part of Tendaho rift, the peak frequency is 0.03 cycles per km and by applying the above relation, depth to the base of magnetic basement is 5.25 km, implying its thickness to be 1.09 km. The results obtained from spectral analysis, given in table 2 below, are ensemble averages and in the absence of detailed information this could be accurate enough for a first approximation. Table 1: Results of Spectral analysis of magnetic data No slope Depth (km) Magnetic source rock Remark Afar stratoid basalt series It is also an average thickness of sediment fills intercalated with basaltic rocks Magnetic basement Dahla basalt could be the magnetic basement rock Depth to the bottom of magnetic basement As pointed out previously, the power spectrum of magnetic data is used in separating the residual and regional fields. However, since there is no clear cut breaks of the power spectrum curve (shown above in fig ), there is some degree of subjectivism in determining the spatial frequencies to be used for filtering. To minimize such subjectivism different spatial frequencies were tried out and the filtering results that reflect known geology were selected. 39

41 4.3 Residual Magnetic Anomaly of Tendaho The Tendaho residual magnetic field over the surveyed area ranges from to gammas with a mean value of 0.18 gammas. Figure 4.3 is a colour shaded map of Tendaho residual magnetic field. It is illuminated from northeast with declination angle of 45 0 Azimuth and inclination of Because of such illumination there could be a shift of the anomalies toward the southwest from their actual positions. The Tendaho residual magnetic field is characterized by circular short wavelength anomalies. The anomalies are caused by magnetic bodies of limited depth extent defined by negative anomalies and their attendant positive fields. More aligned features composed of short wavelength anomalies are observed in the residual field than in the total magnetic field, since short wavelength anomalies were enhanced at the expense of long wavelength magnetic field by the applied band pass filter. Such alignments define linear magnetic features of the surveyed area. In all cases positive anomalies are observed with attendant negative field due to the dipolar nature of magnetism, indicating that the causative magnetic bodies have shallow depth extents. Three alignment directions of these anomalies can be easily distinguished in the residual magnetic map (figure ). These are northeast, northwest and east west trending magnetic lineaments, composed of alignments of small anomalies. The crisscrosses are more conspicuous than has been observed in the total magnetic map. The northeast (NE) trending magnetic linear features are mostly found in the western side of the residual magnetic map (figure 4.3), and their crisscrosses with northwest are observed. Their existences in the northeastern part persist, though crossed or offset by the NW features. Two northwest (NW) trending linear magnetic features are conspicuous in the central part of the map area, probably defining the axial rift region. They are interrupted by northeast trending features at and around well GBH2 in the south and Ayrobera in the north. 40

42 Figure 4.3: Redual Manetic Anomaly Map of Tendaho 41

43 Geothermal fluids may circulate in the space provided by these intersections to form the thermal manifestations observed at Airobera. There is another NW trending features south east of Kurub which is crossed by an east west trending anomaly. The EW striking features occur at Gum Atmali, Logya, around well area of TD1, and at Kurub northeast. Unlike the other trending features they are very small with limited strike extent and the NW linear features are mostly connected by them. Anomalies observed at and around Serdo are likely not to be real since magnetic data were collected by a single east west profile.. The area, where an east west trending feature meets the NW trend at the well location of TD1 (and TD2, TD4, TD5, TD6) coincide with occurrences of the geothermal manifestations. In general, the observed features in the residual anomalies are similar to those observed in the total magnetic field. But, the magnetic anomalies and lineaments are well defined and the intersection areas can be easily spotted. 4.4 Causative Source Distribution of Magnetic anomalies Estimates of depth and location of source positions were obtained using Euler 3D Deconvolution method. This was done first by using or computing derivatives of residual magnetic data and then inverting the gradient (in x, y, & z) data to obtain depth and positions of the anomaly sources, after specifying a square window size, structural index (SI) and depth tolerance (in percent). According to Reid et al (1990) anomalies derived from a real data are likely to be caused by sources with various structural indices, which are measures of the rate of change with distance from the causative sources. Thus a range of indices were tried out and their plotted results were examined to locate reliable solution clustering, where real anomalies exist. Reid et al (1990) pointed out that a minimum and maximum depth returns are about the same as and twice the window size, respectively. It is, therefore, necessary to choose an optimum window size so that deep and shallow sources are fairly represented. Table 3 presents the processing parameters used while applying Euler Deconvolution. 42

44 Table 2: summary of processing parameters while applying Euler Deconvolution Magnetic model Structural Index (SI) Window Size (Grid points) Grid Point Interval (km) Depth tolerance (dz in %) Max. Depth Tolerance (m) Fault Dyke/Sill Vertical Pipe / horizontal cylinder Sphere Inversion of Euler s homogeneity equation over a certain window of data gives solutions at every grid points even in areas that are free of anomalies. These are then refined by applying located euler deconvolution to find solutions based on peaks in analytical signa, or windowing on solutions of depth, off sets in x and y positions, and uncertainities(in %) of depth (dz) & horizontal position (dxy). Even though located Euler deconvolution produces fewer reliable solution, windowing on solutions, in case of Tendaho, reflect the geology better than the former. Table 4 contains windowing parametrs to refine the solutions obtained by the preceeding parametrs given in table 3. Since solution depth is less than the maximum depth tolerance no windowing on depth was performed. Having done all these features with correct index based on tight clustering and data density were chosen as reliable results.. 43

45 Table 3: processing parametrs of windowing on uncertainities and offsets Magnetic model Structural Index (SI) Depth uncertainty (dz in %) Horizontal uncertainty (dxy in %) X offset (in meters) y offset (in meters) Fault ±2000 ±2000 Dyke/Sill ±2250 ±2250 Vertical Pipe / horizontal cylinder ±2500 ±3000 Sphere ±2500 ±3000 The plot results from SI=2 & 1 are not presented here since mapping faults and dyke/sills are our focus to be dealt in the next sections Magnetic Fault Model For source position estimates of a fault model in 3D Euler deconvolution method, a structral index of 0.5 was employed to invert Euler s homogeneity equation over a window size of 15 with a maximum accepted distance of 3000 meters. Fault solutions, obtained in this way, are plotted in 2 and 3 dimensional maps and these are given in figures 4.4.1a & 4.4.1b, respectively. Horizontal positions, in figure 4.5.1a, are represented by circles and depths are indicated by zone colours and proportionality of sizes. Depths to sources below ground surface vary from 450m to 2575m with mean depth of 970 m. As can be observed in figure 4.4.1a, most of the faults occur at depth less than 600meters below ground surface. 44

46 Figure 4.4.1a: Solution Plot of Subsurface Fault with varying depth shown with different colours 45

47 Linear features, on the map, possess different orientations, trending NW, NE, EW and NS, the former being two the dominant ones. Other observed features are circular, elliptical, semicircular or curved fault solution plots. Features, be it linear or curved as revealed by gravity gradients, due to bodies of different densities, though vary in details, have also similar trends, patterns and positional associations with the solution plots of magnetic fault model. Dubti fault inferred (Aquater, 1996) from the alignment of mud cones coincides with a northwest trending fault solution plot. This may confirm its real presence. Parallel to Dubti fault there is another NW striking feature occurring east of well TD1. It extends up to south of Gum Atmali or well TD3, but seems to be terminated by a NE trending feature. At that locality, an east west striking features discontinue the NW and NS features. Parallel to TD1 TD3 axis, there are also features with the same trend that extend for about 15.2 km. The area south of the mud pools and the TD wells is featureless. After an interruption by a circular feature in the area between Airobera and Gum Atmali, the NW striking features continue starting from bagaadoloma crater region and their southern ends interact with a NE linear feature. The occurrence of the NW striking features dominantly in the axial region of Tendaho rift may indicate that these features are extensional faults or fractures, where emphasis should be made for well citing purpose. Geologically observed (UNDP, 1973; Aquater, 1980 &1996) faults striking NW and NE, at the western rift margin west of allalobeda, intersect each other. Their depth wise continuations can be easily observed from the coincidence of the solution plot at the same location. 46

48 Figure 4.4.1b: 3D Solution Plot of Subsurface Fault with varying depth shown with different colours 47

49 The NE striking features are observed west of Dubti fault, between Logya and Gum Atmali, north east of Airobera, in the region of Daorre crater, and north of well GBH2. These could be interpreted as subsurface continuations of the NE trending MER (Main Ethiopian rift). Features with east west trend are dominant in the area between Logya & Dubti, in the central south region of the map area, and south of well GBH 2. These are all coincident with gravity gradient features, presented in figure 3.3. The features west of Serdo have nearly the same trend. The NS trending features seem insignificant and are not dealt here. The various curved features expressed by circular, semicircular, or elliptical plot patterns in most cases have positional correlations with the pattern of gravity gradient plots, the difference being that of size. However, in areas where gravity gradients reveal more features, fault solution plots may show less features and vise versa. The circular feature occurring in the area between Airobera and Gum Atmali may outline buried crater rim, where ring fractures, or circular faults exist. The linear faults are either discontinued or become tangent to the ring feature. Tract of warm ground at Airobera is a peculiar geothermal development, owing to its association with the buried ring fractures which interacts with the linear faults. The east west trending warm ground overlies the tangential east west linear fault. The NW trending warm ground extends across the northern part ring fracture. There are several circular and elliptical features around well GBH2 and semicircular ones at Allalobeda and north of Kurub. The hot springs and the Geyser found at Allabeda may have reached to the surface by travelling through these fractures or faults. From the close correlation of gravity gradient and fault model solution, these features can be interpreted as buried craters and vents. 48

50 4.4.2 Magnetic Dyke/sill Model For source position estimates of a dyke/sill model in 3D Euler deconvolution method, a structral index of 1 was employed to invert Euler s homogeneity equation over a window size of 20 with a maximum accepted distance of 3000 meters. Dyke/sill solutions, obtained in this way, are plotted in 2 and 3 dimensional maps and these are given in figures 4.4.2a & 4.6.2b, respectively. As in the map of fault solution plots, horizontal positions, in figure 4.4.1a, are represented by circles and depths are indicated by zone colours and proportionality of sizes. When width is much greater than depth, a geological body is modeled as a sill, since double peaks will be detected magnetically. Otherwise, width to depth ratio is small and a single peaked is detected to model the geological body as a dyke. In this way it is possible to differentiate dyke from sill. Depths to sources below ground surface vary from 689m to 2840m with mean depth of 1336 m. As can be observed in figure 4.4.2a, most of the dykes/sills occur at depths between 1000 and 2000 meters below ground surface. In most parts of the two map pairs (fig a 4.4.1b and 4.4.2a), i.e., between maps of fault & dyke/sill models, similar horizontal positions, degrees of clustering & trends are observed. However, they vary in details and Dyke/sill solution depths are deeper than fault solution depths. Such correlations, in one case, could be attributed to lava flow through faults/fractures to form dykes. Euler deconvolution estimates epicentral positions of dykes using a structural index of 1. In other case, the correlation could be attributed to blocks at different depth levels formed by faults. Blocks can also be modeled as sills with the same structural index, since their edges can be detected by the inversion of Euler s homogeneity equation. Depth to width ratio Those features with small width to depth ratio, estimated to occur at shallower depth are most likely fissural basalts forming the dykes. As borehole logging shows basalt layers intercalated with sediments are at depths less than 1000 meters correlate well with these features interpreted as dykes. 49

51 Figure 4.4.2a: Solution Plot of Subsurface Dyke/sill with varying depth shown with different colour 50

52 From the deep drilling wells (TD1, TD2 & TD3), it is known that Afar stratoid basalt series occurs at depths greater than 1000 meters. Thus those features at deeper levels could be associated to blocks of Afar stratoid basalts with larger width than its depth of burial. Section maps shown in figure 4.4.3a and 4.4.3b clearly portray horizontal positional associations of faults and dykes/sills which occur at different vertical positions. Borehole geological loggings are superimposed on the section so that exact correlations can be made possible. The upper curve (Figures 4.4.3a & 4.4.3b) was constructed from peaks derived from estimates of structural index 0.5, and interpolated points between the peaks may define fault planes and blocks. The curves intersect the axes of the logs of the three deep wells (TD1, TD2 & TD3) at the locations of recent basalts, where the rocks might be magnetically sound. In figure 4.5.3a, two peaks separated by a distance of 7 km are at relatively higher levels with central depression between them (encompassing TD1 & TD2). The lower curve (Figures 4.4.3a & 4.4.3b) was constructed from peaks derived from estimates of structural index 1, and interpolated points between the peaks may outline blocks while block edges are delimited by the peaks themselves. The upper and lower curves have nearly one to one match. The lower curve intersects the axes of the logs of TD1, TD2, & TD3, respectively, at 1400, 1150, & 1375 meters below ground surface, where the top of Afar stratoid basalts occur. From figure 4.4.3a it can be observed that wells TD1 & TD2 cross a line which is tangent to both the upper & lower curves. Well TD2 intersected the various geological layers at upper levels than TD1 did. The tangent line then defines a fault which vertically displaced the geological layers by about 280 meters. Aquater (1996) observed a vertical difference of 300 meters and interpreted such difference due to a fault. This shows that the interpreted geophysical results are consistent with the subsurface geology known from drilling as compiled by Aquater (1996). 51

53 Figure 4.4.3a: Section map showing plots of multiple solution peaks and interpolated points constituting the curves; the upper and lower curves are derived from anomaly attenuation rates of 0.5 and 1. Tangent to slopes of the upper curves are indicated by a red line. The superimposed deep wells are TD1 & TD2. 52

54 Figure 4.4.3a: Section map showing plots of multiple solution peaks and interpolated points constituting the curves; the upper and lower curves are derived from anomaly attenuation rates of 0.5 and 1. Tangent to slopes of the upper curves are indicated by a red line. The superimposed deep well is TD3. 53

55 The geophysical section maps (Figures4.5.3a & 4.5.3b) defined by the curves portray the subsurface configurations of Tendaho rift. The axial region is probably delimited by the interpreted hinged highs separating two half grabens that face each other. 54

56 5 DC Resistivity Structure 5.1 Stacked Geoelectric Sections Figure 5.1 is a stacked geoelectric sections to present the resistivity structures in full view and to provide a more revealing insight into the resistivity structure of Tendaho than individual sections would have provided. As can be seen on the stacked DC geo electric sections, stepping resistivity structures are observed at the borders of Tendaho rift followed by a lowered and uplifted resistivity structures, reflecting the Tendaho subsurface configurations, as depth to the top of resistive basements vary along the resistivity profiles. As revealed by the geological mapping, stratoid basalt series are observed outcropping at the rift shoulders, while sedimentary sequences fill the Tendaho graben. Drilling encountered the intercalation of post stratoid recent basalts with sediments and Afar Stratoid series below them at greater depth than depths of the resistive basement. Thus the central elevated resistivity structure is not caused by uplifted Afar stratoid basalts. With more depth the recent basalt gets thicker and current cannot penetrate deeper as it is resistive. It may disguise the stratoid series due to such effect and can also be mistakenly considered as stratoid series. Configuration of the resistivity structure (shown in figure 5.1 & 5.2) together with the drilling result demands the existence of resistive dyke/sill structure composed of post stratoid recent basalt. The Stepping resistivity structures correspond to step fault blocks that bound the Tendaho rift. The depressed ones can be interpreted as grabens, since the stepping faults ends on them and drilling intersected Afar stratoid at greater depth. In between these grabens recent basalt, as mentioned above, forms the horst structure. Such interpretation is consistent with results obtained from residual gravity and horizontal gravity maps. 55

57 Figure 5.1: Stacked Geo electric Section of Tendaho rift (using individual resistivity section by Aquater, 1980) 56

58 It can be said that the resistive basement may not be composed of the same kind of rock unit, that is, the Afar stratoid series is not the only resistive basement. All the structures, namely, the interpreted stepping faults, grabens and the central horst can be easily recognized by the observed vertical resistivity discontinuities which could be lithological contacts and or normal faults, bounding these structures. The central part of Tendaho, containing resistive buried sill probably forms the axial zone of the rift. Such zone is then a buried ridge axis which could be a southward continuation of Manda Hararo. Thus both the geo electric sections and contour map of the resistive substratum (figure 5.1 & 5.2) have mapped the local subsurface geological setup. As a result, thicknesses of sediments deposited over the graben and the resisitivity ridge, on the averages, are 1200 and 800 meters, respectively. There exists a major resistivity discontinuity between profiles 13 and 14 (figure 5.1) or in the area encompassing Kurub & Gum Atmali (figure 5.2). It has terminated the horst structure, and the resistivity features, north of this discontinuity, are not similar since no horst structure is observed there. In the central part, southeast of TD 1 the horst is displaced by a NE trending right lateral fault and its intersections with the horst bounding normal faults may give rise to an excellent permeability. A NE trending magnetic structure found just at the position of GBH 2 in the residual magnetic map (figure 4.3) interrupts the NW trending structure and coincides with this right lateral fault. At this region, the NW trending anomaly of total field anomaly (fig. 4.1) and residual gravity anomalies Fig. 3.2) are displaced. Termination of a NW trending linear feature at this same area can also be observed in horizontal gravity gradient (fig. 3.3). Resistivity discontinuities trending in NW and NE and their crisscrossing with each other are similar to that observed in gravity and magnetic maps. There are distorted elliptical resistivity features (fig. 5.2) at Airobera which have positional coincidence with circular features observed in horizontal gradient map (Fig. 3.3) and position plot maps of magnetic fault or sill/dyke (fig 4.5.1a &4.5.2a). Data scarcity and large separation of VES points may result in the difference of shapes. 57

59 Figure 5.2: Map of Interpreted resistivity structures superimposed on resistivity contour of the bottom substratum (modified from Aquater, 1980) 58

60 5.2 Correlation of Resistivity Layers with Borehole Geological Logs Geophysical data from the 1979/80 survey were re examined in light of geologic data from the drill holes. As a result, relationships between the electrical resistivity, alteration and temperature due to the encountered geothermal fluids were examined. All the geothermal wells fall within the interpreted horst structure; however, no DC resistivity profile lines perpendicular to the general strike pass over the well area and as a result of this, correlation with the well geological log is not exact. In fact, wells TD 1, TD 2, TD 4, TD 5, and TD 6 are found between resistivity profiles 15 and 16 (figure5.2a & 5.2b). When geo electric sections of these profiles are correlated, the resistivity layers more or less have a one to one match, though not exact, with the layers observed in borehole logs. Furthermore, the interpolated depth contours of 800 and 900 between these profiles indicate the continuity of the layers across the profiles in the direction of the general strike. Such coherency stands to reason that it is possible to find correspondence between the geological logs and the resistivity layers, so that results of this correlation can be extrapolated to areas of geothermal interest where there are no wells. The resistivity layers with low high low high alternations, though not correlated exactly, correspond to sediment sequences intercalated with basalt layers as encountered by the geothermal wells (TD 1, TD 2, etc). With such correlation it turned out that post stratoid recent basalt is at depths of 800 to 900 meters; the drilling intersected Afar stratoid series at greater depths of 1397 m in well TD 1 & 1179 m in well TD 2. In similar manner TD 3 crossed the recent basalt at 612 meters depth coinciding more or less with depth contour of 700 m and the stratoid series is at 1251 meters. The post stratoid recent basalt is then the resistive sill forming the horst as has been discussed above. In the gravity section it was pointed out that difference of depths in well TD 1 and TD 2 is due to an east west trending fault as revealed by horizontal gravity gradient map (fig. 3.3). A fault which vertically displaced the geological layers by about 280 meters has been revealed by deconvolution of the magnetic field. 59

61 Figure 5.2a: Geoelectric section along profile 16 found south of the well area (Extracted from Aquater, 1980). 60

62 Figure 5.2b: Geoelectric section along profile 15 found north of the well area (Extracted from Aquater, 1980). 61

63 Because of the prevalence of sediment fills, overburden layer resistivities, in general, are sequences of lows occurring nearly throughout the upper part of the geo electric sections. When referring to the borehole geological logging, the low resistivity layers correspond to sedimentary sequence of siltstone and sandstone. The relative highs are attributed to the intercalated recent lavas. As can be observed in the geothermal well logs and resistivity sections, thickness of the relative high resistivity layer increases with depth, corresponding to post stratoid recent basalts. All the geothermal wells except TD 2 & TD 3 are very close to the western part of the horst where influence of the bounding fault of the horst is pervasive. Resistivities of the layers just above this fault is very low and are in sharp contact as observed in all the geo electric sections and this low decreases even more as we go to the south. Close inspection of the sections reveals that the sharp contact can be associated with Dubti fault. Dubti fault seems to be syn depositional and its connection with the horst bounding normal fault is not known. Hydrothermal fluids may have channeled through Dubti fault that have resulted in the formations of mud cones and weak fumaroles that are found in the area of geothermal wells. Interestingly, these geothermal manifestations are concentrated over the western side of the central horst structure at Dubti plantation. As geothermal fluids alter rock mineralogy and increase the salinity of water, resistivity of rocks depend, other than water saturation in pores & rock type, on alteration and temperature. In almost all the geothermal well logs, the top layers are unaltered and their resistivities are relatively higher than the underlying zeolite clay zone which corresponds to resistivity varying from 0.3 to 4.6. Temperature profiles in this alteration zone show values ranging between 150 & C. Below the zeolite clay zone, we have layers characterized by a relative high resistivity values associated with wairakite prehnite zone and minor sporadic epidote. These alteration minerals are high temperature minerals with high permeability (wairakite indicates its high permeability). The geothermal wells crossed a shallow geothermal reservoir in this zone with an average temperature of C. 62

64 Depth of occurrence and thickness of alteration zones vary from borehole to borehole as well as the degree of alterations of rocks with their corresponding resistivities. For a better correlation and more understanding of the deep subsurface structure of Dubti geothermal field, Aquater (1996) reexamined results of the pre drilled geophysical results. Profile 5, shown in figure 5.3, was selected by Aquqter as a good representative geelectric section for correlating the resistivity structure with subsurface geology for it encompasses the axis of wells TD 3 and TD 1 in the general strike direction. Figure 5.3 is a geo electric section reproduced from the report supplied by Aquater (1996). The following is a reinterpretation of profile 5 in terms of lithology, alteration and temperature and review of Aquater s (1980 & 1996) interpretation of this profile. VES 170 was correlated with TD 1 in order to calibrate the resistivity survey. Moreover, TD 3 is used to constraint the resistivity data interpretation since it is found in close proximity to VES 169 &144. At well TD 1 five resistivity layers are identified to a depth of 1400 meters. The bottom resistive layer with 100 ohm meters is the Afar stratoid basalt series. It is getting shallower beneath TD 3 until it reaches the resistivity discontinuity. Overlying Afar Stratoid is a resistivity layer with 10 ohm meter occurring below TD 1 and has 400 meters thickness. It corresponds to alternating sequences of siltstone, sandstone and thin basalt. According to the geoelectric section by aquater (1996) shown in figure 5.3, its continuation away from TD 1 is not shown. Towards the north in well TD 3, post stratoid recent basalt layers, below 600 m, are thick; while the intercalated sediments and pyroclasts are thin. The resistivity method may not detect such thin layers since its resolving power decreases with more depth. 63

65 Figure 5.3: Reinterpreted Geoelectric and Pseudo resistivity sections along Profile 5 (after Aquater, 1996) 64

66 Within the anomalous 10 ohm meters resistivity layer, drilling indicates the presence of permeable zone at depth level between 850 to 900 meters, and temperature profile shows that this layer is characterized by a maximum temperature of C. A thin resistive layer composed of recent basalt is characterized by a resistivity value of 127 ohm meters, comparable to the resistivity of Afar stratoid series. The layer exists throughout the section along strike, but occurs at a relatively shallower depth, less than 700 meters below the ground in well TD 3. From surface to the top of the resistive recent basalt, three layers are identified by the resistivity method. They are characterized by a low high low sequence of resistivity values. The surface layer has resistivity values ranging between 1.48 to 2.9 ohm meters. It is an unaltered sedimentary rock, from surface to depths of 95 m in TD 1, 50 m in TD 2, and 59 m in TD 3, below these, the layer is altered to zeolite clay. The effect of this alteration is to decrease the resistivity of the sediment. The second layer has higher relative values ranging from 8.7 through 9 & 10 to 11 ohmmeters. It corresponds to sandstone and fractured recent basalts. The third layer is a very low resistivity layer with varying values between 0.35 and 1.74 and thicker than the overlying layers and observed to outcrop in the southern segment of the section at VES 174, 175 & 176. In well TD 1, Laumonite + Epidote + wairakite are the alteration minerals; laumonite is dominant between 210 & 390 m, while epidote wairakite is from 310 to 410. Their distributions are not abundant but they occur commonly or sporadically in the second and third layers. Thus their effects in changing the resistivity of the rocks seem minimal. However, the geothermal wells intersected a shallow reservoir within the second layer which has a relatively higher resistivity value. In well TD 4 chloride is common and Zeolite is abundant in the second layer. The 3 rd low, resistivity layer below a geothermal reservoir is possible, only if the fluid in the reservoir is an out flow and very close to the up flow zone to attain such high temperature. 65

67 6 Magnetotelluric Resistivity Structure The employed DC resistivity method at Tendaho, as have been observed above, could not go deeper beyond the top of the Afar stratoid basalt. Thus for more depth penetrations below the stratoid series and to detect a possible heat source, magnetotelluric (MT) method was applied to reach a depth down to more than 10 km. Kalberkamp U., et al., (2010) applied one dimensional and two dimensional model inversions to the collected MT data based on the dimensionality of the resistivity structure as determined from polar diagram. As can be observed in the stacked resistivity sections (figure 6.1), the 2 D inversion method revealed a feature of low resistivity anomaly diapir. Depths to such anomalous feature under profile lines 01, 03 and 97 are 5, 6, & 7 km deep, respectively. The diapiric feature is not observed in the geo electric section of Profile line 02 (figure 6.1) and so it differs from the other MT profiles in its deeper part. It coincides with resistivity discontinuity that has been revealed by the DC resistivity method. When profile lines 97 & 01 are compared, the discontinuity seems to offset the resistivity diapir. The low resistivity diapiric feature is characterized by less than 2 ohm meters, and its upper part is enclosed by 4 ohm meters value. Kalberkamp (2010) attributed such anomaly to a magmatic melt. This interpretation is consistent with the analytical results of Aquater (1995) that gases from the geothermal fluids, which are dominated by CO 2, indicate a strong magmatic origin. Furthermore, the supply of gases from magma is supported by the relative He content and by the ratio of 3 He/ 4 He. In the central position of the sections, the top surface layers are defined by very low resistivity values. It is observed along the whole Profiles in Dubti area. As in the DC resistivity, the top resistivity layer has a variable thickness. The layers are dominantly a sedimentary sequence with inter layering of recent basalt. They are composed of an unaltered top surface and zeolite clay altered zone. The layers also partially include laumonite wairakite prehnite altered zone. 66

68 N Figure 6.1: Stacked MT Resistivity sections assembled from individual section prepared by Kalberkamp U. et al (2010) 67

69 Below the surface layers are undulating resistivity layers with values greater than the upper surface ranging from 4 to 8 ohm meters. Though not abundant, wairakite prehnite mixed zone occur in this layer and according to the geological logs, the frequency of recent basalt is high and gets thick as we go deeper. It has an intermediate layer resistivity and occurs throughout the sections of all profile lines. Its layout differs from profile to profile. Within the geoelectric section of profile line 02 the layer rests horizontally over a high resistivity layer. In Airobera, it undulates above the high resistivity layer. In sections of profile lines 01and 03 the intermediate layer undulates and overlies a high resistivity layer. In the central part of these sections, it cuts through the various high resistivity layers and encloses the low resistivity diapir. But in the Airobera section, it is the high resistivity layer that encloses the resistivity diapir. On both sides of the MT lines occur very high resistivity features, with very high values ranging from 84 to 2048 Ohm meters which are unaffected under profile line 02 and remained horizontally stratified, while in other profile lines they are observed to occur only at the margins, showing that they are affected from the heat below. Sharp lateral resistivity discontinuities in the subsurface that define graben horst structures are not observed in the MT resistivity sections. The large separation between MT sites smoothed out such lateral discontinuities and the geo electric sections show only the undulating resistivity layers. In order to reveal the deeper structures, the low frequency component of the data was emphasized in the 2 D inversions and stacked sections shown in figure 6.1 is a small scale map that cannot discern the thin upper resistivity layer. The high frequency components of the MT soundings, according to Kalberkamp U., et al (2010), show a one dimensional characteristic. Thus the 1 Dimensional inversion applied to the high frequency MT data discerned the thin resistivity layers. Details of the 1 D MT resistivity data interpretation is found in the report by Kalberkamp U., et al (2010). Here only a single MT line passing over the well area is considered for comparison with the DC resistivity interpretation. The Afar stratoid basalt characterized by 100 Ohm meters is located at 1397 m below ground in well TD 1 and at depth of 1500 meters below VES 170. In the MT section (shown 68

70 in figure 6.2), the resistive basement that is defined by a 100 Ohm meters should be the Afar stratoid basalt. It undulates along the section, indicating its corrugated nature which determines the configurations of the overlying layers. Below TD00106, where the upwelling of Afar stratoid is observed, several layers are discerned. The layers seem to be bounded by buried faults probably formed by the upwelling. The apex of gravity ridge defined by horizontal gravity gradient (also by residual gravity) and the NW trending total magnetic anomaly axis coincide with the upwelling of Afar stratoid series at the location of TD Figure 6.2: One Dimensional MT Resistivity section reproduced from Dissa M., and Lemma Y., in Kalberkamp U. et al (2010) Figure 6.3: presents a three dimensional view of a resistivity structure of Dubti, which is constructed by putting together a 9 km resistivity depth slice and resistivity section of profile line 01. It can serve as the lower part a geothermal conceptual model, showing the configuration of the heat source, that supplies the geothermal system. 69

71 N Figure 6.3: Three Dimensional View of Deep Resistivity structure assembled from depth slice of resistivity map and geo electric section of Profile line 01 (maps modified from Kalberkamp U. (2010)) 70

72 Basaltic dike injection into the uppermost crust is observed to take place along the axial zone of Manda Hararu range, and thus dikes of recent age are taken as the probable heat sources in these two prospect areas. High density of faulting and fracturing is observed in outcrops of the thick succession of Plio Pleistocene age Stratoid series basalts which underlie the late Pleistocene sedimentary and intercalated lava succession which occupies the graben. It is thus thought that secondary permeability should be sufficiently high to house a viable geothermal reservoir in the Stratoid series rocks at depth. 71

73 7 Temperature gradients Thermal studies involving surface and in hole temperature measurements and heat flow survey are direct geothermal exploration tools. Boreholes in various localities of Tendaho rift were drilled for groundwater supply. Water samples from these boreholes were taken for groundwater or hydro geochemical studies. In 1980, Aquater and geological survey under the Ministry of Mines drilled eight multipurpose geothermal boreholes (MGBH). The Geological survey of Ethiopia drilled the 9 th one. Recently surface temperature surveys were carried out over selected areas at Dubti plantation and Airobera. To the knowledge of the author, no heat flow survey has been conducted in any of Tendaho geothermal fields. This section summarizes temperature data and results presented in detail by previous reports (Aquater, 1980), and attempt has been made to extend this work. Table 5 lists temperature gradients, and in hole maximum temperatures along with behaviours of temperature profiles. As can be observed in the table, most measurements were affected by unstable temperature condition (warm up effect). Other effects are ground water circulations, convective air movement and water loss through fractures or contacts. Variations of temperature profiles are observed because of such effects. Despite all these effects, Aquater (1980) determined temperature gradient that can be extrapolated to the average surface temperature based on observed constant trend. In the present work, temperature gradients were estimated using least square method over certain depth intervals or the whole depth. Those gradients, which show constant rate of temperature increase and give surface temperature value when extrapolated, are taken as reliable gradient. All the differences at any depth between the observed and predicted by the gradient represent the perturbation caused by the effects pointed out above. In the table below, it can be readily observed that the computed temperature gradients for the shallow multipurpose gradient boreholes vary from to C/km. The average gradient based on the present estimate is C/ km. Conductive heat loss from the shallow reservoir to the surrounding rocks may distort conductive heat loss from a deeper 72

74 Table 4: Temperature gradient No Well Temperature Temperature Gradient Gradient Type or Maximum Remark Name Gradient from computed using least temperature profiles Temper Aquater (1980) square method (present ture study) 1 GBH C/1km C/km Variable gradient 51 0 C Measurements taken 1 month after drilling 2 GBH C/km C/km Constant gradient C Measurements taken 80 days after drilling 3 GBH C/km C/km Constant gradient C Measurements taken 2 months after drilling 4 GBH C/km C/km Isotherm below 75 m depth down to bottom hole C Measurements taken 2 months after drilling 5 GBH C/km 190 0C/km Variable gradient C Measurements taken 1 month after drilling 6 GBH C/km C Last measurements taken 14 years later 7 GBH C/km C/km Variable gradient C Measurement affected by temperature transient 8 GBH C/km Temperature reversal C Measurements taken 3 months after with depth drilling 9 GBH C/km Variable gradient including isotherm from 130 to 170m C Measurement Affected by transient temperature 10 TD C/km Conductive above 600 meters C Deep exploration well 11 TD C/km Conductive between 1350 & 1450 depth interval C Deep exploration well (after 36 days corrected Roux method (Aquater) 73

75 source (hot intrusion), and temperature gradients from deeper wells were not included in computing the average. Geothermal gradients can be used to predict crustal temperature at any depth and delineate areas with high surface heat flow. For vertical heat conduction under steady state condition with constant thermal conductivity and with no crustal heat production, the solution to one dimensional heat conduction equation (K 2 T/ 2 Z = 0) is then given by: T= Z * G + T 0, where T is temperature at any depth, Z is depth below ground, G temperature gradient, and T 0 is surface temperature. At Curie depth magnetic property of magnetic minerals vanish due to high heat in the ground. As determined in section 4, subsection 4.2 (spectral analysis of magnetic data), the ensemble average curie point depth for Tendaho is 5.25 km and the annual average surface temperature is 40 0 C. Applying the above relation and using C/km for the gradient value, the temperature at this depth is then estimated to be C. This could be a curie temperature of magnetite (Fe 3 O 4 ) which fits the standard assumption in curie point depth analysis. Figure 7.1 portrays the distribution of temperature gradients. At the northeast part open high gradient is readily observed. The central high gradient anomaly stands out very clearly. The center of this anomaly is due to the shallow geothermal reservoir heat transfer by conduction to the surrounding rocks as well as from deeper heat source (magma chamber). The two high gradient anomalous areas are separated by intermediate gradient which may bear correlation with the DC resistivity discontinuity. At this location the low resistivity diaper is not observed in the MT section, but shows up at deeper level (9 km) in plan view (figure 6.3). Unlike the low gradient anomalous areas, these high anomalous regions could be thermally active, owing to their occurrence in the axial region, where extension of the Tendaho rift induces high heat flow. 74

76 Figure 7.1: Map showing temperature gradients Isolated open gradient anomaly, at the southwest could be related to Allaobeda geothermal manifestations (hot springs and geyser). The high temperature gradient anomalies could be caused by the superposition of heat flows from magmatic intrusion, shallow and deep geothermal reservoirs. As a first approximation, contour closure of C/km may have delineated the geothermal reservoir. 75

77 8 Summary and Discussions of Geophysical Results At this stage it is possible to summarize results and synthesize the information obtained from the review of each geophysical method employed in Tendaho geothermal fields so far. At deeper level reaching 4.5 to 7 km the 2 D inverted MT data located a low resistivity diapir, which is interpreted as a magmatic melt. The 1 D inverted MT data revealed undulated resistivity structure with a central up lift in the region of the rift axis at depth 700 m below ground directly above the diapir. It is outlined by a 10 ohm m contour line which delineates the upper low resistivity layers. A contour line of 100 ohm m with similar undulation outlines the top part of the high resistivity layer, and the central uplift is at about 1200 m depth below ground. Such undulations along the sections indicate that such configuration has controlled the depositional style of the overlaying sedimentary sequences. Recalling solutions plots, which constitute the lower and upper curves, as obtained from the deconvolution of magnetic data bear correlation to the 10 and 100 Ohm m contour lines. In such case, the resistive layer (bounded by 100 Ohm m) is Afar stratoid basalt, while the 10 Ohm m is related to basalt of post Afar Stratoid. The central up lift mapped by the resistivity method corresponds to the magnetically mapped two hinged high blocks. The difference arises due to the large separation of the MT sites. The interpreted resistivity discontinuities of the bottom resistivity substratum (Aquater, 1980) shown in plan view by figure 5.2, correlate well with gravity gradient features and fault solution plots. As the DC resistivity survey was carried out from one end of the rift margin to the other, step fault blocks and graben structures were mapped and the relatively narrow spaced VES points enabled to resolve more layers and structures than the widely spaced MT Survey. The MT survey, however, complemented the DC resistivity survey in that the central up lift was not reached by the DC method, since current couldn t go beyond the deepest recent basalts, occurring at 800 to 1000 m depth. Furthermore, the capability of the MT method to 76

78 penetrate deeper depth enabled us to identify the heat source and extends the previous survey to the next level. The central gravity ridge observed in the residual gravity map (figure 2.4.2) is caused by a shallow dense mass, forming a subsurface horst structure. This is consistent with the northwest trending shallow resistivity basement occurring at the central part and the central magnetic lows which define the axial region of Tendaho rift. Flanking the central horst structure on either of its sides are gravity depressions which could be attributed to graben structures. The rift margin which bound the gravity depressions is expressed by gravity highs, owing to the occurrence of the stratoid basalt at shallow level or at the surface. All these features corroborate well with configurations revealed by the DC resistivity methods. The occurrences of circular, nearly circular or elliptical features within the gravity depressions or gravity ridge revealed by residual gravity may indicate the existence of craters or doming. These are more clearly observed in the gravity gradient map. When gravity gradient, fault solution plots of magnetic deconvolution and geological layer for craters are brought together, interesting results can be readily observed on the integration map shown in figure 8.1. Euler 3D deconvolution of the magnetic data revealed similar subsurface structures to that of gravity gradient. Several circular and elliptical features are observed; some are large and some are small, occurring within the larger ones. Most are closed features, while few of them are open. The features outlined by gravity gradients occur within features outlined by solution plots of magnetic deconvolution. With the assumption that the gravity features and the magnetic features are at different depth levels, two possibilities arise. When features due to the gravity are at deeper level, the bottom of the feature is narrow and get wider and wider at shallower levels. In the other case when the magnetic features are at deeper levels, the bottom of the feature is wide and get narrower and narrower at shallower levels. Upright funnel for the former or inverted funnel for the later is a good first approximation to 77

79 explain such possible situations. The first possibility is more likely because faults revealed by solution plots of magnetics are estimated to occur at shallow level within the sediment fills and gravity samples the effects of density variations of sound rocks at deeper level. In this case the magnetic features may have outlined ring fractures and faults while the gravity gradient mapped volcanic plug underlying collapse structures. The occurrence of geologically observed craters supports the idea that these features outlined buried calderas, craters and vents. Daorre crater falls at the edge or rim of a buried crater which itself is within a buried caldera. When magnetic sources positions were estimated using inversion of Euler homogenous equations of the magnetic field based on fault model, it was found out that there exist positional correlation of Fault solution plots (features) with the geothermal manifestation in Tendaho geothermal field. This is also true for gravity gradient features. Steaming ground, at Airobera, with the circular and east west trending linear features, mud cones alignment, in the area of deep & shallow geothermal wells, with the northwest striking feature, and hot springs and geyser, at Allalobeda, with linear and curved features, all bear positional correlations. Faults, fractures and lithological contacts are paths for fluid migrations towards the surface for the formation of geothermal manifestations. Thus the positional association of the features and manifestation confirms that these features are faults, showing the application of correct structural index in deconvolving the magnetic data. Close inspection of figure 8.1 shows the pattern and distribution of the linear features. Different trends and crisscrosses of these features are readily observed in the figure. In the residual magnetic field, gravity gradient, magnetic fault & sill solution plots, and in the integrated map of figure 8.1, four alignment directions of linear features can be easily distinguished. These are, namely, northeast, northwest east west and north south trending lineament features. Close inspection shows that these features crisscross each other as can be observed more in all of the maps. At the rift margins the Afar stratoid basalt is fractured 78

80 Figure 8.1: Map showing Superposition of Horizontal gradient, Fault locations and craters 79

81 by grid faults and those observed in all the maps are subsurface continuations of such faults. The Tendaho graben is then severely affected by four fault systems, trending NW, NE, EW, and NS. Other than this, ring fractures and faults have also severely affected the graben. Subsidence due to fault extension can be computed using McKenzie s (1978a) mechanical stretching model. It is assumed that volcanism in Tendaho has a negligible effect in rising up or uplifting the graben, and also assumed the crustal and lithospheric extension to be the same (uniform stretching). The quantitative model of uniform stretching is given by: Ys= yl {(ρm ρc).yc/yl (1 αv.tm.yc/yl) αv.tm. ρm /2}.(1 1/β)/[ρm (1 αv.tm) ρs ] Where Y s is fault controlled subsidence; ρ m = 3200 kg/m 3, is mantle density; ρ c = 2700kg/m 3, is density of crust; ρ s =2060 kg/m 3, is density of sediment; y l =125 km, is lithospheric thickness; T m = C; is mantle temperature α v =3.3X C, is volumetric heat capacity; y c = 25 km crustal thickness for Tendaho is taken from the estimation of wide angle refraction (Berckhemer H., et al, 1975); and β is stretching factor. All the constants except the crustal thickness are taken from published literature (e.g., Allen, P. A., & Allen, J. R., 1970). The calculated Fault controlled subsidence, Y s = 1.37 for β = 1.74, and 1.6 for β = 2. Depths to Afar stratoid basalts vary due to fault controlled subsidence, and these variations have been mapped by the resistivity methods and fault solution plots. Depth estimates from the magnetic, and resistivity as well as from deep boreholes are compiled in table 5 to compare the different results. 80

82 Table 5: Depth estimates as obtained from different methods Drilling (meters) Magnetic Sill/dyke Model(meters) Magnetic spectral analysis (meters) Resistivity basement (meters) Subsidence(meters) Depth TD for β= TD From 1000 to for β= TD for β=2.0 More or less the various results in the above table are similar. Drilling depth to Afar stratoid basalt and depth to sill/dyke are well correlated, with a little overestimation by the model. The slight difference may arise probably due to the effect of weathering of the upper surface of the Afar stratoid that could result in the loss of the magnetic property. The estimated subsidence, with a stretching factor of 1.75, is equal to the average of the drilling depth and closerto other estimates than with the depths estimated using β = 1.5 and 2. In general, such result tells that the assumptions and the constant values used in calculating the subsidence depth are reasonable and likely to be true. Taking β=1.75 is then enables us to estimate the lithospheric thickness to be equals to 71 km (y l /β =125/1.75). At this stage it is possible to construct fault blocks using the magnetic fault and sill models by drawing lines from solution peaks. The line inferred in such a way form fault blocks as shown in upper section of figure 8.2. As pointed out in the previous section, two half grabens made up of synthetic fault blocks face each other. They are separated by hinged high blocks with a central wedge shaped depression. 81

83 As cited by Connard et al (1983), Haggerty (1978) cautioned that assuming a single curiepoint temperature for continental crust may not be right, since curie temperature as low as C may exist in the crust owing to the low temperature of oxidation of titanomagnetite. Also, according to Byerly P. E., et al (1977), the most common magnetic mineral in igneous rocks is titanomagnetite (Fe 3 O 4.TiFe 2 O 4, magnetite ulvospinel) with curie temperatures of less than about C and an increase in titanomagnetite causes a reduction in Curie temperature. Nevertheless, in the Tendaho case, the corresponding magnetic mineral to the predicted temperature of C could be magnetite, probably with insignificant Ulvospinel (TiFe 2 O 4.). Berktold et al (1972) compiled (from Khitarov, 1970 and Schult, 1974) experimental resistivity temperature curves for some rock types, assumed to exist below Afar. Figure 8.2 is a reproduction of such curves in order to estimate possible temperature which corresponds to the measured MT resistivity discussed in the previous section. From the MT sections, as portrayed in figures 6.1, 6.2., & 6.3, low resistivity diapirs occur at depths varying from 4.5 to 7.75 km with an average depth of 6.17 km below the ground surface. For resistivity values of 1 to 2 Ohm m, at an average depth of about 6.17 km, the corresponding temperatures as read off from curves 3 & 4 (see figure below) are C. Temperature in the order of C is a melting point for most igneous rocks. This is consistent with the interpretation of the low resistivity to be caused by a magmatic melt. 82

84 Figure 8.2: Graph showing relationship between Resistivity & Temperature from laboratory experiment (Khitarov, 1970 & Schult, 1974) as compiled by Berktold, et al.,

85 The layer of the hot rock below the base of the magnetic basement caps the magmatic melt and has an average thickness of 0.92 km with temperature range from 607 to C. Such results imply a high gradient of C/km. Table 6, given below, summarizes the results obtained in this and previous (spectral analysis, temperature gradient & MT resistivity) sections. The data in this table is used for constructing the lower section of figure 8.3. Table 6: Summary of results for temperature & depth ranges, and temperature gradient No Depth range Temperature (range) Average gradient Transfer of Heat Remark in km in 0 C 0C /km 1 0 to to conductive Igneous rocks capped with sediments to to to to conductive Hot rock capping the magma below 4 > 6.17 >1300 convective Melt rocks The central bottom part of figure 8.3 portrays a magmatic melt defined by resistivity lows and high temperature of C. As pointed above, it is capped by a hot rock whose top part is defined by a curie isotherm. The overlying rock is a magmatic basement, probably of basaltic (Adoli or Dahla) origin. The hot rock is bottom part of the magnetic basement and because of high temperature plastic deformation may contain the rise of magma. At this depth lithostatic pressure due to the overlying rock alone might not be sufficient to prevent magmatic rise. 84

86 Figure 8.3: Interpretative schematic cross sections portraying possible geometries 85

87 9 conclusion and Recommendation 9.1 Conclusion The applied geophysical methods give a revealing insight into the subsurface geological formations and rift configuration of Tendaho geothermal fields. This provides a sound basis to draw the following conclusions. The residual gravity & magnetic fields, and the DC & MT resistivity sections clearly define the rift configurations of Tendaho; the rift margins are characterized by step fault blocks and the central up lift, which is flanked by graben structures, coincides with the axial region of the Tendaho rift. Several linear features that bound the grabens and the ridges are revealed by high gravity gradient. Daorre crater occurs within circular gravity gradient. By the same token other curvilinear features are interpreted as buried caldera, craters, and volcanic vents. The NW SE and NE SW structural systems crisscross each other at the rift margins, showing that both are active faults. Their interactions at the ridge axis have geothermal significance in that they are active sites of the present day extensions, continuously generating fractures to counteract the sealing effect of hydrothermal activity. Southeast of Dubti well area and Aerobera could be sites of deep reservoirs. The Tendaho graben is severely affected by four fault systems, trending NW, NE, EW, and NS. Other than this, ring fractures and faults have also severely affected the graben. This review work revealed the existence of east west trending structures, subsurface continuations of northeast trending structures associated with buried caldera, craters, and vents 86 and ring fractures Estimated temperature of the low resistivity diapiric feature from temperatureresistivity relation is nearly C. Mantle temperature from various literatures is identical to such temperature probably indicating continues magmatic supply to attain temperature equilibrium Temperature gradient map outlined areas of high heat flow, where the geothermal reservoirs occur. The axial rift region coincides with the high heat flow area and the magmatic melt occurs just below it at an average depth of 6.17 km. As a first

88 approximation, contour closure of C/km may have delineated the geothermal reservoir. 9.2 Recommendation It is highly recommended to drill at Airobera where faults and ring fractures intersect and coincide with steaming grounds. Other highly recommended drilling site is at southeast position of the mud pools. This specifically is at the intersection of rift axis line and Dubti fault. Detailed MT survey and compiled results of all the available geoscientific studies will have to confirm the selected sites for drilling. Thermal surveys consisting of temperature gradient, heat flow and temperature measurements are recommended to assess the geothermal resource potential, delineate high heat flow area and quantify the magnitude of conductive and convective heat flows. Microseismic survey within Tendaho graben and high resolution gravity survey restricted within the axial region of the rift are recommended to map fractures and monitor mass change, fluid movement and subsidence during power production. 87

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94 Appendix 1a: Regional Gravity Field Map of Tendaho 93

95 Appendix 1b: Regional Total Field Map of Tendaho 94

96 Appendix 2a: Three Dimensional Solution plot of Dyke/Sill 95