The Resistivity Structure of the Abaya Geothermal field,

Transcription

1 Geological Survey of Ethiopia Hydrogeology, Engineering geology and Geothermal Department The Resistivity Structure of the Abaya Geothermal field, Southern Main Ethiopian Rift, Revealed by Magneto-telluric Imaging. Mohammednur Desissa and Yohanes Lema December, 2006 Addis Ababa

2 Abstract To understand the crustal electric structure of the geothermal field around the Lake Abaya, 15 MT soundings have been carried out. Site spacing varies between 2.5 and 8 km based on access condition. Measurements were taken along four lines. The 1D inversion of the data confirms the presence of low resistivity zones related to structurally weak zone, hydrothermal alteration, different clayey units; extremely to highly altered ignimbrites or to magmatic brine (saline water). Very high resistivity zones observed below the surface are associated to volcanic intrusion while those continue from the surface are due to volcanic rocks. The foot of volcanic centers (magmatic centers) observed in the surface are not deeper than 2km. The dominant controlling structure is associated with NE-SW oriented structure which could be activated by Wonji fault. ii

3 Abstract...ii Table of contents...iii List of figures...iv 1. INTRODUCTION...iv 1.1. Back ground and location Basic concepts Review of previous work Geological and tectonic settings Objectives of the MT survey Methodology and data acquisition Data processing and presentation Modeling and interpretation Conclusions and Recommendations...16 Acknowledgement...18 References...19 iii

4 List of Figures Figure1. Location ma of survey area...2 Figure2. Geological map of the northern Lake Abaya area...5 Figure3. Distribution of MT survey lines in relation to geological map...7 Figure4. Maximum Electric Direction (MED), strike of 2D structure...9 Figure5. Examples of fitted model...10 Figure6. Geoelectric section along line Figure7. Geolelectric section along line Figure8. Geoelectric section along line Figure9. Geoelectric section along line iv

5 1. INTRODUCTION 1.1. Back ground and location Prominent geothermal systems of Africa are associated with Great Rift Valley, which is one of the major tectonic structures in the world. Geothermal energy is renewable, indigenous and environmentally friendly. It is a base load energy which is independent of weather conditions. Since Ethiopia is affected by prolonged draught, the reliability of hydropower during dry season is a basic power problem related hydro. Therefore, successful exploration and exploitation of geothermal energy is of paramount importance. The Ethiopian Rift (ER), which is part of the East African Rift System (EARS), covers about 150,000 sq.km (i.e, nearly 15% of the country). In the Ethiopian rift, geothermal explorations have been carried out since 1969 involving national and international survey organizations. Previous studies revealed more than 16 geothermal potential areas with an estimated energy potential of about 1000MWe. The present study area is located at the southern part of the Main Ethiopian Rift (MER), bounded between longitudes and latitudes (about 500km south of Addis Ababa). The field study was conducted from 4 th to 25 th April Basic concepts The MT method is a way of determining the electrical conductivity distribution of subsurface from measurements of natural transient electric and magnetic fields on the surface. Results interpreted from measurements at a single site are sometimes compared to an induction log. Results from a line of measuring stations are interpreted to give underlying conductivity distribution and structure. That picture of subsurface could in turn be related to porosity and salinity, temperature and hot magma, etc. The time variation of earth's electric and magnetic fields at a site are recorded simultaneously over a wide range of frequencies. The variations are analyzed to obtain their spectra and apparent resistivities as a function of frequency are computed from spectra. The MT method depends on the penetration of electromagnetic energy in to the earth. Depth 1

6 control comes as a natural consequence of the greater penetration of lower frequencies. The measurements are absolute. Their interpretation gives true resistivity values and depth, not just anomalies. Depth interpretation based on MT data is therefore much more definitive than that based on gravity or magnetic data. b) Within geothermal prospect area Figure1. Location ma of survey area 2

7 Table1. Coordinates of MT sites. Site No. Easting Northing elevation Remark Line 1 Line 2 Line 3 Line 4 2. Review of previous work Previously geological, geochemical and infrared surveys were carried out in the area and revealed occurrence of several hot springs and fumaroles while geothermal gas studies have indicated that the temperature of springs' water is around C. It was concluded that distribution of hydrothermal active zones is strongly controlled by tectonics and geological structures. Most of fumaroles and hot springs are related to volcanic complexes. Geophysical studies including gravity, magnetics and DC resistivity surveys were conducted in the northern part of the present MT survey. Strong thermal features were seen to associate with low values of gravity, magnetic and DC resistivity. Moreover, structures trending in NNE-SSW, N-S and NW-SE were delineated by geophysical methods. 3

8 3. Geological and tectonic settings The collision between east and west Gondwanaland has defined the major geological and structural features of East Africa. The late Protrozoic Mozambique belt, representing Himalayan-scale orogeny, abuts the Archaen complexes of western Ethiopia. In Ethiopia, the belt is transected by numerous predominantly N-S to NNE-SSW suture zones and associated ophiolitic slivers and NW-SE trending shear zones. In the area, pre-rift basaltic rocks unconformably overlie the Precambrian basement. The prominent units in the area are: Volcanic rocks: these include the pre-rift basaltic rocks, ignimbrites, trachytes, pumiceous pyroclastics, rhyolites, and rhyo-obsidian flows. Sedimentary succession is represented by Pleistocene volcano-sedimentary rocks, Holocene lake sediments and recent alluvium and fluvial deposits Structural observation was revealed presence of series of intense normal faulting and quaternary volcanism in the area. Duguna-Fango volcanic complex is believed to represent the southern sector of Wonji Fault (WFB), which is the active spreading axis of extension in the Main Ethiopian Rift (MER). The centers of this belt are considered to be the heat sources for wide spread hydrothermal activity fed by deep-seated magma chambers. 4

9 Figure 2: Geological map of the northern Lake Abaya area. (from Chernet 2006) 5

10 4. Objectives of the MT survey Previously DC resistivity survey was conducted over limited area that covers only the northern part of the current area (i.e around lines 3 & 4). Deep subsurface information related to thermal sources could not be acquired due to the following reasons. To achieve great depth penetration it requires a long electrode separation, which is difficult under such terrain conditions. It needs equipment capable of generating high power. Depth penetration of the DC method is highly affected by the upper resistive and/or conductive layers. Using potential field methods (gravity and magnetic) only qualitative interpretation was made. On the other hand, geothermal resources are genetically related by water and heat. Since the electrical resistivity is affected by changes in temperature and permeability, electromagnetic method can provide a model of subsurface resistivity variation. Of all EM methods MT seems to be appropriate due to several kilometers of penetration and the upper resistive volcanic rocks (as Ethiopian case) couldn t mask the EM field propagation. Since the measurements of MT are absolute and the interpretation gives true resistivity values and depth, not just anomalies, depth interpretation based on MT data is therefore much more definitive than that based gravity or magnetic data. Therefore, the MT survey was intended to: Map the geo-electrical structure of the area; Determine lateral and vertical extent of active hydrothermal zones and possible heat sources with respective depths Map prominent geologic structures associated with WFB and volcanic centers in the area. 6

11 5. Methodology and data acquisition Fifteen (15) MT soundings were carried out along four profiles (fig. 3) with the Short Period Automatic Magnetotelluric (SPAM mark III) data acquisition system (Ritter et al., 1998). The horizontal electric and magnetic field time series were recorded in 3 and 7 period bands for high frequency and low frequency modes respectively in the magnetic north (x) and east (y) directions. The electric potentials were measured between two nonpolarizable Cl2 PbCl2 electrodes, with separations of 50 m using cross configuration. Magnetic field was recorded using metronix MFS05 induction coils. Four geo-electric sections along four lines oriented nearly perpendiculars to the rift axis are presented. The station spacing varies between 2.5 and 8 km while line spacing varies between 12 and 25km based on access. The recordings typically lasted 4 to 10 hours. Position of each station was obtained with handheld GPS and the one installed within the MT unit. Figure3. Distribution of MT survey lines in relation to geological map. 7

12 6. Data processing and presentation The data from high frequency band and in three shortest period bands of low frequency mode are of excellent quality, leading to precise MT impedance. The data in longer period bands (3 to 6) did not provide such good transfer functions because of limited recording time. The electric and magnetic field time series were transformed into the frequency domain. The 2 * 2 MT impedance tensor Z relating the horizontal electric (Ex, Ey) to the horizontal magnetic (Bx, By) fields was determined using the robust remote reference method of Chave and Thompson (1989). In a 2-D Earth, the electromagnetic induction equations separate into two modes, the transverse electric (TE) mode (currents parallel to structure and in the direction of constant conductivity, the geo-electrical strike) and the transverse magnetic (TM) mode (currents perpendicular to geo-electrical strike). Currents parallel to structure only induce magnetic fields perpendicular to it, and vice versa for the TM mode. Thus the diagonal elements of the impedance tensor vanish. The MT impedance tensor at each site was rotated to the direction that best-fits an offdiagonal tensor (xx & yy). The geo-electrical strike direction was determined by rotating the impedance tensor into the complex direction minimizing its diagonal elements. This was done choosing the rotation angle of the real axis of the electric or magnetic field that either maximizes or minimizes the impedance tensor. This angle defines, for each period, a maximum electric field direction (MED) or a minimum magnetic field direction. In 2-D geometry the MED corresponds to the strike of the 2-D structure. At most sites, the rotation angle was well defined and constant with period and indicates a structural direction. This is illustrated in Figures 4 where the MED are shown for periods of 0.5 and 8 seconds respectively. At 0.5 second period, representative of approximately the first kilometer below the surface, there are four directions, N, N, 40-80_N and 10 40_N of which the two main directions, _N and 10 40_N are dominant. The 8

13 90 120_N direction corresponds to the NW SE that well agrees with the transverse tectonic structures while the 10 40_N direction corresponds to the NNE SSW active WFB. At a longer period, (8 s) or at deeper depth the NE SW direction dominates, indicating that deeper structures are more influenced by the active Wonji fault direction. A correlation made between resistivity and geo-electric strike (MED) at each site revealed that low resistivity is associated with NE-SW direction, which indicates that thermal activity is controlled by this structure. a) at 0.5seconds b) at 8 seconds Degrees clockwise Number of sites Figureigure 4. Maximum Electric Direction (MED), strike of 2D structure 7. Modeling and interpretation The 1-D inversion method used is based on an iterative technique that minimizes a misfit function between the data and the 1-D model calculation. Modeling was considered up to a depth of about 3km, where best fit between the data and model parameters were observed for all sites. The geo-electrical models are presented at two different depth levels 500m and 2000m for each line. Examples of resistivity and fitted model for four sites from four lines are shown in figure 5. 9

14 Ρapp (Ωm) Ρapp (Ωm) Ρapp (Ωm) Y Evaluation C Evaluation Copy exy Evaluation Site 001 Site 014 Site C l 1 XY Evaluation Site 008 Ρapp (Ωm) Horizontal axis is period Observed resistivity model resistivity Figure5. Examples of fitted model 10

15 a). 500m depth b). 2000m depth Figure 6. Geoelectric section along line 1 Site001 is characterized by low resistivity starting from surface and getting wider at around 1200m depth. The site was observed between fumaroles and hot springs which are favored by structurally weak zone (low resistivity). Low resistivity could also be resulted from hydrothermal alteration, different clayey units; or it may be related to highly altered ignimbrites or to magmatic brine or, saline water. This very low vertical resistivity could 11

16 indicate the upflow zone through which hydrothermal fluids migrate to the surface. Site 002 shows relatively low resistivity (10-15Ωm) up to a depth of around m and then it gets very low up to 1500m. Resistivity signature below 1500m is relatively high, which could be due to volcanic intrusion. Sites 004 and 003 are characterized by very high resistivity starting from surface up to 500m depth, which could be due to extrusive volcanic rocks. Below this depth, resistivity under site 004 becomes relatively low while under site 003 it is very low, which might be related to extremely altered rocks or related to magmatic brine (saline water). Resistivity signature along line 2 shows different characteristics compared to line1. The shallower part is mapped by relatively high resistivity except at site 005, where low resistivity is observed up to 100 m. The conductive anomalous zone is mapped at central part of the line below 1 km depth under sites 005 and 007, which could be related to the deep seated heat source. It is noted that site 007 was located at a foot of mount Selwa ; one of the volcanic centers in the area with hot caldera lake on top. Site 006 is mapped by relatively low value between 500 and 1000m depth. 12

17 a). 500m b). 2000m Figure7. Geoelectric section along line2 13

18 a). 500m b). 2000m Figure 8. Geoelectric section along line3. 14

19 Line 3 is characterized by very low resistivity below 200 m and 800 m depth under sites 015 and 009 respectively while high resistivity is observed below site 008 up to nearly 2 km depth, which could be attributed to the presence of thick extrusive volcanic rocks under site 008. Site 009 is close to Bilate River where massive ignimbrite is observed. The MT survey result revealed that the thickness of ignimbrite is about 1km at site 009 and 2km at site 008. The same explanation discussed above applies to very low resistivity values under sites 015 and 009. a). 500m b). 2000m Figure9. Geoelectric section along line4 15

20 Line 4 is characterized by low resistivity zone under site 011 (similar to site 001) while resistive in the eastern side, which may indicate the upflow zone. The high resistivity under site 012 continues up to the depth of about 1 km and replaced by very low resistivity below this depth. The conductive zone observed at site 011 may serve as conduit through which geothermal heat flows to the surface. The hot spring observed lose to this site (Dimtu village) could be a manifestation of the presence of deep seated heat source. 8. Conclusions and Recommendations Experiments made by different researchers have proved that resistivity and temperature are inversely related ( ρ 1 T ). Temperature parameter influences the resistivity of rocks. During geodynamic activity, the crustal materials melt at high temperature and pressure conditions and thereby increasing the conductivity of crustal materials. In such conditions, high temperature values are associated with low resistivity (high conductivity) regions. In volcanic and geothermal areas, the causes of high conductivity (low resistivity) at depth may indicate the presence of hot magma chamber. In the present survey area, the presence of high temperature was revealed by revious studies. There are a number of hot springs and fumaroles smoking all the time. Moreover, direct rrespondence was seen between very low resistivity and NNE-SSW structure, which could be due to WFB. The result of MT survey in Abaya geothermal prospect area allowed haracterizing the electrical structure of the area. The result shows lateral and vertical variations of electrical resistivity. It can be concluded that: Surface manifestations of thermal features are strongly related to low resistivity (high conductivity) zones Low resistivity zones are wide and open towards depth, which could indicate that the area is potentially promising for geothermal energy resource Low resistivity zones are caused by a combined effect of hydrothermal alteration, saline fluids and structures 16

21 The low resistivity observed t deeper depth could be associated with conductive hydrothermal fluids injected from hot magma or highly fractured and weathered volcanic rocks or conductive sedimentary unit. The foot of volcanic centers (magmatic centers) observed on the surface are no more deeper than 2km The dominant controlling structure is associated with NE-SW oriented structure which could be activated by Wonji fault.though the current MT survey was bserved for short period of time, the results allowed characterizing the electrical structure of the area and determine lateral and vertical variations of electrical resistivity. Continues observation and use of MT method will enable us to explore and map geothermal potential areas, which could secure sustainable energy supply and utilizing alternative energy sources (petroleum, geothermal, coal etc). Therefore, systematically girded MT survey observing for long period of time is highly recommended. 17

22 Acknowledgement We would like to thank the National Environment Research Council (NERC) geophysical Equipment facility of the University of Edinburgh (UK), for permission to use the MT equipment. Our thank also goes to Prof. Kathy A. Whaler and Dr. Sophie Hautot for their help all the way through. At last but not least, our heart felt thanks goes to H.E. Ato Alemayehu Tegenu, minister of Mines and Energy and Dr. Meseret Teklemariam for facilitating the field work 18

23 References 1. Cagniard L Basic Theory of Magneto-telluric Method of Geophysical Prospecting: Geophysics, Kunetz G Processing and Interpretation of Magnetotelluric Soundings Geophysics, Vozoff K The magnetotelluric method in the exploration of sedimentary basins: Geophysics Hautot S.,P. Tarits and Kathy Whaler, Deep strcture of Baringo Bogoria Rift Basin(central Kenya) from Three-Dimensional magnetotelluric imaging: Implications for rift evolution. J.Geophy.R ,493-23, T. Harinarayana et al, Exploration of exothermal structure in Puga geothermal field, Ladakh Himalayas, India by magnetotelluric studies. Journal of Applied Geophysics, A. Hisafumi et al, 2006.Magnetoteluric resistivity modeling for 3D characterization of geothermal reservoir in western side of Mt. Aso, SW Japan. J.Geophy. Res