THE KIBIRO GEOTHERMAL PROSPECT

Transcription

1 THE KIBIRO GEOTHERMAL PROSPECT A Report on a Geophysical and Geological Survey Gestur Gíslason Knútur Árnason Hjálmar Eysteinsson Prepared for Icelandic International Development Agency and Ministry of Energy and Mineral Development, Uganda June 2004

2 2 A Report on a Geophysical and Geological Survey

3 The Kibiro Geothermal Prospect Executive Summary The report describes a geothermal survey, which was carried out in the Kibiro area, located in the Hoima district in western Uganda. The aim was to advance the geothermal pre-feasibility survey initiated in the area by the Ministry of Energy and Mineral Development. The work carried out under this phase was mainly geological and geophysical investigations where as earlier investigations at Kibiro had concentrated on the geochemistry, with the aim to study the characteristics of the subsurface fluid, including its temperature. The geological investigation was spread over an area of 40 km 2, covering a part of the rift floor of the western branch of the Eat African Rift System, the eastern escarpment of the rift, and the crystalline basement east of the rift. The hot springs of Kibiro emerge at the escarpment, which form a boundary between the old basement rocks to the east and the young sedimentary formation of the rift to the west. The main rock type east of the escarpment is granite or granitic gneiss, which has been heavily block faulted during the formation of the rift. Surface manifestations in the form of hot and warm springs, steaming ground, sulphur and calcite deposits is found at the escarpment and following the main fault lines of the block faulted crystalline rocks. Three geophysical methods were employed; the electrical resistivity of the subsurface layers was measured using the transient electro-magnetic method (TEM), a gravity survey was performed as well as a magnetic survey. The resistivity of subsurface rocks is the most diagnostic parameter of geothermal activity that can be measured from the surface, where a geothermal reservoir is usually reflected by anomalous resistivity behaviour, usually low resistivity coinciding with the geothermal resource. In order to record the resistivity characteristics of the Kibiro area, a total of 70 TEM soundings where collected. A low resistivity anomaly was located in the sediments below the Kibiro village, and a low resistivity trench was traced into the crystalline basement, following the fault lines of the block faulted granites, first to the SSW of the hot springs along R. Kachuru and then following East-West fault lines towards Kigorobya. At greater depth a NE-SW trending anomaly is seen parallel to the escarpment of the rift in the crystalline basement. The magnetic measurements do not show any convincing indications of intrusive bodies close to the surface. The anomalies seem to be mostly connected with beadrock topography and do not reveal any obvious linear structures. Similarly the gravity data do not show any distinct density variations, except for the big density contrast between the sediments in the Rift and the granites outside the rift. There is an indication of higher gravity field in an area roughly coinciding with the E-W lowresistivity anomaly. This might indicate deep higher density intrusives acting as a heat source for the geothermal activity producing the low-resistivity anomaly. The cause of these low-resistivity anomalies can, at the moment, not be stated with certainty, but the most likely explanation is conductive alteration minerals in fractures in the otherwise resistive base-rock. Saline water in fractures could also be a possible candidate, but the relatively low salinity of the hot springs in Kibiro and other springs in the area make this rather unlikely. The conclusion is that the Kibiro area is certainly worth further investigating. Some additional geological and geophysical studies are needed but the question whether an exploitable geothermal resource is present can only be answered by drilling. 3

4 A Report on a Geophysical and Geological Survey Table of Content Executive Summary...3 Table of Content...4 List of Figures...5 List of Tables Introduction The Location of the Study Area Previous Work Geology of the Kibiro Kigorobya Area Introduction Description of rock units Granite and gneiss Pyroxenites Soil cover Rift sediments Bedding and joints Tectonics Geothermal surface manifestation Hot and warm springs Steaming ground surface alteration Discharge in Lake Albert Carbonate springs Calcite deposits Geological summary TEM Resistivity Survey The role of resistivity surveying in geothermal exploration The central-loop TEM method Instrumentation and field procedure Data processing and interpretation Data acquisition Processing and interpretation of the Kibiro TEM data The resistivity structure of the Kibiro area Cross-sections Iso-resistivity maps Gravity Survey Data acquisition Data processing Results Magnetic Survey Instrumentation and field procedure Data acquisition Data processing Results

5 The Kibiro Geothermal Prospect 6. Summary and Conclusions Recommendations...62 Reference...63 Appendix I: TEM-sounding curves and their interpretation...64 Appendix II: Resistivity Cross-Sections Annex III: Kibiro Geothermal Survey Group List of Figures Figure 1.1. Geothermal prospects of Uganda... 7 Figure 2.1. Map of study area, showing main access roads... 9 Figure 2.2. Structural map of the Kibiro Kigorobya area Figure 2.3. A pyroxenite outcrop (location KL-64 on fig. 2.1) Figure 2.4. A rose diagram showing principal trend of pyroxenite outcrops Figure 2.5. Rose diagram for joints and bedding Figure 2.6. Banding, joints and veins in a granitic outcrop (Location KL-50 in fig. 2.1). The E-W trending joint appears to be the youngest Figure 2.7. Joints in pyroxenite outcrop (location KL-67 in fig. 2.2). The main joints form a 60 angle, and the main stress field is from NW-SE as per stress diagram to the right Figure 2.8. The escarpment at L. Albert. View to south from the Kachuru village Figure 2.9. The Kachuru Fault, a view to north. The steep-sided valley is more than 200 m deep Figure A view from the main hot spring area, Mukabiga. A spring emerges from a foot of a fault breccia outcrop Figure A view over the Muntere salt gardens. Water from warm springs and seepages channeled to the various gardens Figure Deposits of tar (black) and sulfur (yellow) on mylonite Figure Map of geothermal surface manifestations and calcite deposits Figure 3.1. A resistivity cross-section from the Nesjavellir geothermal field, Iceland, showing comparison with alteration mineralogy and temperature, determined in wells Figure 3.2. The principle of TEM soundings. The top part shows a schematic field layout and the induced fields. The bottom part shows the current waveform and the induced voltage recorded in the receiver coil Figure 3.3. Location of TEM-soundings and cross-sections Figure 3.4. Resistivity cross-section along line T Figure 3.5. Resistivity cross-section along line T Figure 3.6. Resistivity cross-section along line T Figure 3.7 Resistivity cross-section along line T Figure 3.8. Resistivity cross-section along line T Figure 3.9 Resistivity cross-section along line T Figure Resistivity cross-section along line T Figure Resistivity map 900 m above sea level Figure Resistivity map 800 m above sea-level Figure Resistivity map 700 m above sea-level Figure Resistivity map 600 m above sea-level Figure Resistivity map 500 m above sea level Figure Resistivity map 400 m above sea-level Figure Resistivity map 300 m above sea-level

6 A Report on a Geophysical and Geological Survey Figure Resistivity map 200 m above sea-level Figure Resistivity map at sea-level Figure 4.1. Location of gravity profiles Figure 4.2. Bouger gravity along profile G Figure 4.3. Bouger gravity along profile G Figure 4.4. Bouger gravity along profile G Figure 4.5. Bouger gravity along profile G Figure 4.6. Wiggle plot of Bouger gravity Figure 4.7. Bouger gravity map of the Kibiro area Figure 5.1. Location of magnetic profiles Figure 5.2. Wiggle plot of the variation of the magnetic field along profiles M1, M2, M3 and M4 around the background value of nt Figure 5.3. Wiggle plot of the magnetic field zoomed in on the area with E-W profiles Figure 5.4. Magnetic map Figure 6.1. Resistivity at 500 m a.s.l., faults and fractures (pink lines), geothermal surface manifestations (red stars) and an area of slightly higher gravity field List of Tables Table 3.1. Coordinates (UTM, Arc1960) and elevation of the TEM-soundings Table 4.1. Coordinates (UTM, Arc1960, m) and elevation of gravity reference stations Table 4.2. Statistic on gravity survey at Kibiro

7 The Kibiro Geothermal Prospect 1. Introduction Uganda is experiencing a rapid growth in electricity demand, approximately 2-3% per month in resent years. Despite its abundant hydro resources, the Government of Uganda recognises that it must diversify its energy sources, and has initiated studies on indigenous alternative energy sources, one being geothermal that is found in a number of locations in Uganda. This report describes a geothermal study carried out at Kibiro in western Uganda under an agreement between the Governments of Uganda and Iceland. The report is a result of a team effort, and the participants are listed in Appendix III. 1.1 The Location of the Study Area The Kibiro geothermal prospect is one of three geothermal areas, which have been studied by the Department of Geological Survey and Mines (DGSM), the other two being Katwe Kikorongo (Katwe) and Buranga in Kasese and Bundibugyo districts respectively (fig. 1.1). Kibiro is situated in Hoima district, on the eastern shore of Lake Albert. Kibiro is located at the foot of the escarpment of the western branch of the East African Rift System (EARS), and is connected by a rough but motorable track up the escarpment from Kigorobya township, which has road connection to Hoima in the south and Masindi to the north through Biiso. The escarpment, which rises over 300 m above Lake Albert, has a profound influence on land use in the area. Figure 1.1. Geothermal prospects of Uganda 7

8 A Report on a Geophysical and Geological Survey Below the escarpment at Kibiro the main occupation is fishing and salt production, but the area around Kigorobya is a fertile agricultural land, where the main cash crop is tobacco and cotton. Closer to the escarpment there is less rainfall and cattle keeping is the main agricultural activity. 1.2 Previous Work Geothermal investigations at Kibiro were initiated by the Department of Geological Survey and Mines (DGSM) in 1993 in co-operation with UNDP under a project called Geothermal Exploration UGA/92/002 & UGA/92/E01, a project co-funded by the Government of Uganda, UNDP, the Government of Iceland and OPEC (Gislason, 1994). The project was primarily focused on geochemistry and geology, and the main conclusion was that although the surface manifestations were not extensive, the chemistry of the hot spring water indicated sub-surface temperatures in the range of 200 C (Ármannsson 1994). No clear indication was found on the nature of a heat source, but the main attention for a reservoir was directed towards the thick sediments of the tertiary rift system. A new DGSM project, Isotope Hydrology for Exploring Geothermal Resources, UGA/8/003, in co-operation with International Atomic Energy Agency (IAEA), was initiated in 1999 and completed in 2002 (IAEA, 2003). Under the project hydrological studies were carried out in the same three areas as the earlier project and the findings regarding the Kibiro area showed that the sources of recharge was meteoric water, and originated from higher ground than in the immediate escarpment area, pointing inland away from the rift. The isotope geothermometers indicated lower reservoir temperature (140 C) than conventional geothermometers (in the range of 200 C). The isotope research implies water-rock interaction, old age of the system, or low water/rock ration. The study suggests that the reservoir rock in Kibiro is granitic gneiss. In 2002 the Icelandic International Development Agency (ICEIDA) prepared a status report, where the current situation in the geothermal survey was reviewed, and recommended further actions to be taken to complete a pre-feasibility study in the three above-mentioned geothermal areas (Gislason 2002). Since then the Africa Development Fund (ADF) funded a project to complete the study in two of the areas, Katwe Kikorongo and Buranga, but ICEIDA agreed to assist the Ministry of Energy and Mineral Development (MEMD) to complete the study in Kibiro. A project agreement was signed in 2003 and a work plan agreed upon, based on the recommendation set forward in the status report. 8

9 The Kibiro Geothermal Prospect 2. Geology of the Kibiro Kigorobya Area 2.1 Introduction The study area is located on the eastern escarpment of the western branch of EARS. The mapped area covers the small peninsula below the escarpment where the villages of Kachuru and Kibiro are located, the escarpment itself as well as the land extending from the escarpment shoulder towards the east. The area under geological investigation covers about 40 km 2, and spans altitude difference from 620 m a.s.l. (Lake Albert) to 1,100 m a.s.l. Access can be arduous in the study area because of deep and steep valleys (fault lines), especially closer to the escarpment. Bushes, often dense, cover the land and a casual worker assisted the team by cutting bush to ease access. Four-wheel drive vehicles were used where possible to follow tracks or for bush tracking. During the field season (February, April and May 2004) the temperature was in the range of C. The fieldwork was limited to 21 days in three field visits. It is obvious that this limited time did not allow a detailed geological mapping, but was limited to identifying the main rock formations found in the area, to get an understanding of the complex tectonic environment, and to explain the relation of the geothermal occurrences to the local geology and tectonics. It is clear that further geological work is needed in the area if the project proceeds to the feasibility stage. Figure 2.1 shows the study area, main access roads and villages, as well as location of field observations. Figure 2.1. Map of study area, showing main access roads 9

10 A Report on a Geophysical and Geological Survey 2.2 Description of rock units The escarpment, which cuts through the field from SW to NE, divides the study area in two entirely different geological environments. To the east the geology is dominated by ancient crystalline basement, characterized by granites and granitic gneisses, where as in the rift itself are thick sequences of sediments. Although the rock description was not a major task, a short description of the various rock units will be given. Qualities of exposures vary within the study area. The best rock exposures are found in the escarpment face, where access to the rock is very good, but here the rock is usually highly transformed by the fault movements. Good outcrops are also found along the numerous fault-lines in the block-faulted rock, but on the eastern side of the mapped area they are restricted to a few outcrops on hills, and one has to bear in mind that these are the hardest rocks, and therefore resistive to weathering. The softer rocks have weathered more and form the depressions in the undulating landscape Granite and gneiss This ancient rock forms the entire basement rock east of the escarpment. It varies in appearance, from fine grained to coarse grained with quartz and feldspars being the main minerals, but biotite and amphibole are also present. The fine-grained granites have greyish colour but with increasing mineral size the rocks become light in colour or even with pink hue depending on the feldspar colour. Banding is common (although sometimes absent), and sometimes the rock can be classified as granitic Figure 2.2. Structural map of the Kibiro Kigorobya area 10

11 The Kibiro Geothermal Prospect gneiss. The banding is more pronounced the closer one gets to the escarpment, and usually the bedding is dipping very steeply (60 90 ), and the most common direction of the strike is close to N20 E although E-W direction is also common (figure 2.2). Close to the escarpment and in association with fault line the granites are mylonitic, pseudotrachylitic or brecciated, as is to be expected in highly faulted environment. The stage of weathering or alteration of the granites depends strongly on the structure and location of the rock. The massive, un-banded granites tend to be very fresh, and generally the rock is less altered the farther one goes from the escarpment. It has though to be remembered that there the outcrops are fewer, and therefore the more weathered rock (if it exists there) is covered by drift deposits and soils Pyroxenites Simmons (1921) is first to describe dike-forming intrusives within the present study area. Simmons refers to these rocks as pyroxenites as a field term, but later authors (Kakenga et al. 1994) have referred to it as diorites. This formation is most commonly recognized as very black, angular blocks on the surface, forming a small, elongated ridge in the landscape (figure 2.3). These outcrops are usually m wide, but can be as long as a few km. These formations follow a direction, which varies between directly N-S to N20 E as can be seen on the rose diagram on figure 2.4. Within the study area this formation is most common on the slopes and hilltops above Kibiro. In hand specimen the rock is fine to medium grained, dark grey in colour and usually very fresh looking. No phenocrysts are seen, but joints are common. Although no thorough petrographic study has been W N S 40% Figure 2.4. A rose diagram showing principal trend of pyroxenite outcrops 20% E Figure 2.3. A pyroxenite outcrop (location KL-64 on fig. 2.1) carried out on the various rock types under this study, a brief examination of these intrusives in thin section reveals that the rock is composed of quartz, pyroxene (orthopyroxene most common but clino-pyroxene also present), apatite and relatively large amount of an opaque mineral. But feldspars are absent, and the rock can therefore not be classified as diorite. A study in thin section of a contact between granite and the pyroxenites, and although the contact is very sharp (1-2 cm wide), it has no cooling margin as would be expected in an intrusive dike, but rather a gradual disappearance of feldspars as the grain size changes from course grained to fine grained. 11

12 A Report on a Geophysical and Geological Survey Based on the above description it can be doubted that this formation is actually intrusive, or if its occurrence is related to a stress-related transformation of the granites during the block faulting of the base rock. Volcanism in the western branch of the East African Rift System is not widespread but where it is found the volcanic rocks are of a peculiar composition (Holmes 1936), where carbonatites and ultrabasic pyroxenites have been described, and are considered not to be older than a few thousand years old (Holmes 1965). A thorough petrographic study is needed to decide if the pyroxenites of Kibiro are of intrusive origin, and could then be regarded as a possible heat source for the Kibiro geothermal system Soil cover The soil covering the base rock outside the main rift may influence the current study. For instance the varying thickness may affect the results from magnetic and gravity survey and thick layers rich in clay may cause low-resistivity at shallow level. Despite this the time frame for the present geological fieldwork did not allow for a detailed study on the loose cover. The soil cover appears to be thin close to the escarpment, and gets gradually thicker towards Kigorobya, as the landscape changes and the land is more flat. The soil is reddish-brown in colour, typical for the final stage of the weathering of granites, i.e. red lateritic soil. This type of soil is rich in aluminium and iron, but generally its clay content is low as it is composed of a mechanical mixture of fine grains of quartz with minute scales of hydrates of alumina. Although laterite soil is known to reach a thickness of tens or hundreds of meters, it is not likely to be the case in the present study area, as outcrops of granite are found on hilltops throughout the area not too far apart. It has been noted that closer to the escarpment, the soil is in some places more brownish than reddish, which may be the weathering product of the pyroxenites Rift sediments Considerable studies have been made on the sediments in the various parts of the EARS, mainly through research for possible oil deposits. Most of these studies are based on the use of geophysical methods, but a few wells have been drilled. The sediments are predominantly clastic sediments, commonly fluvial and lacustrine deposits (Morley et al., 1999). Little is known about porosity and permeability of the rift sediments. They span considerable time span, m.y. and although recent fluvial and lacustrine sediments usually have high porosity, with time these sediments become compact and cemented, which reduces the initial porosity and permeability. Reports on the few deeper wells, which have been drilled in the Albertine Rift, have not described any well testing or subsequent results. Wayland (1925) was first to describe the stratigraphy of the sediments of the Albertine Rift, which he divided into three units; the Epi-Kaiso gravels; the predominantly arenaceous Kaiso beds and the underlying argillaceous Kisegi beds. In the Kibiro region, the Epi-Kaiso beds are about 20 m, the Kaiso beds are 500 m thick, and the total thickness is more than 1200 m (Harris et al. 1956). The age of these two formations is uncertain, but Wayland (1925) considered the Kisegi to be possibly of Miocene age, and the upper part of the Kaiso beds to be late Pliocene or early Pleistocene. A 684 m deep well was drilled close to the oil seepage about 1.5 km 12

13 The Kibiro Geothermal Prospect north of the Kibiro hot springs. The borehole log shows alternating layers of gray sand and grits with sandy clay and blue-green shales (Harris et al. 1956). Down to 120 m the sediments are predominantly sand and gravel, but between 120 and 250 m depth layers of clayey sand and shales are dominant. Below 250 m the proportion of sand is high, until the hole hits the basement rock at 1222 m. W 10% N 20% E 2.3 Bedding and joints In its least altered form the granites are without any banding, but banding is commonly associated with increasing faulting of the granitic gneiss and mylonitic S Figure 2.5. Rose diagram for joints and bedding rocks. Several distinctive strike directions of banding have been observed (fig 2.5), where east-westerly and N20 E are the most prominent one. Less common strike direction is N130 E (figure 2.5). The dip is usually very steep, in the range from 60 to vertical. Closer to the escarpment the bedding tends to have near vertical dip, but farther inland the bedding dips towards east and south. Bedding is usually absent in the pyroxenites. Figure 2.6. Banding, joints and veins in a granitic outcrop (Location KL-50 in fig. 2.1). The E-W trending joint appears to be the youngest Joints can be found in all rock types within the study area, and have the same dominant direction as seen in the bedding (fig. 2.6), and generally they are near vertical (fig. 2.2). The joints and fissures may be open or filled with secondary minerals, commonly quartz is present, but in rare occasion calcite is also found. 13

14 A Report on a Geophysical and Geological Survey A study of joints can reveal the stress field, which caused the joints. Figure 2.7 shows an outcrop in the Kabatindule area, south-west of the Kigorobya village. It shows the two prominent directions of joints in pyroxenite outcrop, east-west and N20 E, intersecting at 60 angle. Also present is the less common joint with N130 E trend. A principal stress field with direction NW - SE, can explain this pattern of joints, more exactly with direction 140 E, which is exactly perpendicular to the direction of the rift in the area (N50 E). Joints 130 Joints N20 E Principal stress from NW-SE N 60 Joints W-E W E S Figure 2.7. Joints in pyroxenite outcrop (location KL-67 in fig. 2.2). The main joints form a 60 angle, and the main stress field is from NW-SE as per stress diagram to the right. 2.4 Tectonics The tectonic pattern within the study area is very complex, and this complexity may well be the main contributor to the existence of a potential geothermal reservoir. The main faulting within the study area is the eastern escarpment of the rift, which cuts straight through the study area in N50 E. The visible vertical displacement at Kibiro, from Lake Albert at 622 m a.s.l. to about 960 m a.s.l. on the shoulder of the escarpment above Kibiro, or some 340 m (Fig. 2.8). The land continues to rise towards the southeast, and at the village of Figure 2.8. The escarpment at L. Albert. View to south from the Kachuru village. Kigorobya the land elevation is about 1100 m a.s.l. Seismic refraction data in the rift shows that the total thickness of sediments at Kibiro Butiaba is about 5.5 km (PEPD, 2002), indicating a total displacement at Kibiro of at least 6 km. The visible part of the escarpment fault, the extension belt, is restricted to a narrow band, not more than 1 km, and often concentrated in one to two faults. Away from this narrow belt of rift faults no linear structures striking N50 E, i.e. the direction of the rift valley, have been found. The western branch of EARS is formed by series of half 14

15 The Kibiro Geothermal Prospect grabens, i.e. the main extension occurs on alternating sides of the rift, characterised by a dominant main boundary fault, but series of normal faults are hidden by lakes and sediments (Morley et al. 1999a). Drilling for oil north of Kibiro and at Butiaba showed that the escarpment dips at 65 E towards the rift (Harris et al, 1956). In the granites the most prominent landform within the study area is the valley of River Kachuru, which flows off the escarpment by the village of Kachuru, south of Kibiro. The valley cuts straight through the granites, very steep and narrow and 200 m deep or more (Fig. 2.9). Its direction is N20 E, and parallel with it are series of smaller valleys and linear alignments. This fault zone, Kachuru fault zone (KFZ), is oblique to the main rift fault, and intersects it at the villages of Kachuru and Kibiro. At this intersection several geothermal surface manifestations are located (see section 2.5). Outcrops along the valley show strongly mylonated rock, a typical fault rock, and the elongated fabric of the influenced rock is in the same direction as the fault. Mylonite is common in the vicinity of KFZ but Figure 2.9. The Kachuru Fault, a view to north. The steep-sided valley is more than 200 m deep is not found away from the fault zones. River Kachuru originates from a swampy area west of Kigorobya. It follows a direct course to the west until in flows in to the Kachuru valley where it turns to NNE until it reaches the escarpment and flows into Lake Albert. The east-west direction of the upper parts of River Kachuru is a dominant linear structure in the study area, first described by Simmons (1921), who described an east-west fault just south of the study area and he associates it with a bend in the Rift just south of Kaiso peninsula in Lake Albert. Simmons interpreted the two fault line directions (i.e. N20 E and E-W) in the granites as block faulting associated with the formation of the Rift. Simmons also mapped and described two East-West faults west of Kigorobya (fig. 2.2), and these have also been mapped by later authors (Kakenga et al. 1994). River Kitawe follows a straight alignment just north of the Kigorobya Kibiro road, and flows down the escarpment in a deep gully by the hot springs of Kibiro (fig. 2.2). The direction of this alignment is NW-SE (N120 E), intersecting the main escarpment at the Kibiro hot springs, but this is the only occurrence of this alignment, which is perpendicular to the escarpment. It has been difficult to determine the relative age of the fault systems, but there are some indications. Apparently the NNE-trending Kachuru fault is the youngest one in the study area, even cutting through the main rift escarpment, and can possibly be detected in the gravity survey as it continues west of the escarpment under the sediments of Kibiro. The east-west trending fault lines are apparently cut off by the Kachuru Fault, indicating that they are older (fig. 2.2). On the other hand then the pyroxenites, which have the same N-S trending lineament as the Kachuru Fault Zone, are clearly cut off by the E-W trending faults. 15

16 A Report on a Geophysical and Geological Survey 2.5 Geothermal surface manifestation Hot and warm springs The hot springs of Kibiro are well known and especially the salt works of Kibiro, where salt has been extracted from the geothermal water for centuries (Gestsdottir, 1999). Gislason et al. (1994) have described the numerous hot and warm springs at Kibiro in details: The main area of hot springs, Mukabiga, is located in a N-S trending ravine at the base of the main fault escarpment (figure 2.10). A number of small springs issue from the boulder and gravel. Large amount of gas escapes from the springs, and there is a strong smell of H 2 S in the air. There is little evidence for carbonate precipitation, but white thread-like algae are common in the stream. Figure A view from the main hot spring area, Mukabiga. A spring emerges from a foot of a fault breccia outcrop. Close to the stream the threads are coloured black with sulfides. The springs are located on an elongated area, slightly oblique to the main fault. On the western side of the ravine is breccia outcrop, apparently related to a secondary fault, oblique to the main rift fault, and the springs are most likely controlled by the intersection of the two faults. Most of the springs drain into a small pool at the foot of the breccia outcrop, and gas bubbles continuously from the bottom of the pool.the total flow measured from the Mukabiga centre was 4 l/s, and the temperature range was C. A second group of hot or warm springs are found downstream, in an area of salt gardens called Mwibanda. The flow rate of 2.5 l/s is low, and the temperature range is C. Some of the seepages are on a straight N-S trending lineament, in a small dugout trench. Other springs are within the nearby salt gardens. The largest salt garden is called Muntere, and is directly north of the Mukabiga area (fig. 2.11). Its eastern side may be controlled by the secondary fault. Here the ground has been lowered down to the ground Figure A view over the Muntere salt gardens. Water from warm springs and seepages channeled to the various gardens. water level for the salt production. A number of small channels drain the area, but the water temperature is lower than in Mukabiga and Mwibanda, and there are no well defined springs. The highest recorded temperature is 39.5 C. 16

17 The Kibiro Geothermal Prospect Steaming ground surface alteration In the lower slopes of the fault escarpment just above the school in Kibiro is an outcrop of highly brecciated fault rock. A faint smell of hydrogen sulphide (H 2 S) is in the air and a thin film of sulphur covers the rock. In cracks a well-formed sulphur crystals grow in clusters. Also present is a black tar-like substance (figure 2.12), and according to a preliminary analyses by the Petroleum Exploration and Production Department (PEPD, personal communication 2004) in Entebbe, it is composed of some hydrocarbons. No steam is seen rising, but the smell and fresh deposits of sulphur demonstrate the presence of some gas being released. Calcite is found in cracks Figure Deposits of tar (black) and sulfur (yellow) on mylonite. and fissures, indicating water discharge at earlier stages. A band of brecciated fault rock can be traced along the escarpment both to the north and south of this manifestation. About 175 m to the north, and about 50 m south of the track down the escarpment, the fault is well exposed in a shallow gully, and the rock is highly altered in a variety of colours. Sulphur and tar is present on rocks, but no smell is found. Another similar site is found on the same fault about 220 m south of the school, where both sulphur and tar deposits are found. The first published description of the Kibiro hot springs come from Emin Pasha, who visited the area in 1885 (See Wayland 1920 a). He described the springs as follows: "The floor of the ravine and the stones with which it is littered are so hot that one cannot bear the hand upon them; the heat even penetrates through the soles to the feet. On every side we heard the continuous bubbling and hissing of water, and the gurgling of gasses issuing from the hot mud. Hundreds of tiny springs burst from the overheated soil, and filled the air with sulphurous gases, with which is mingled a slight smell of bitumen. The atmosphere has such a high temperature that we felt almost stifled, and as if we were in a steam bath - and this idea was further strengthened by the little jets of steam which rose on all sides from the boiling water." Emin Pasha s description does not fit the Kibiro hot springs as they are today, but it may well be that the areas described above have been more active in the recent past. In his report, Wayland (1920 a) comments that there is evidence to shew that this spring used to gush out at a considerably higher level than it does at present Discharge in to Lake Albert From the salt works in the salt gardens of Kibiro it is evident that some portion of the water seeps through the sediments towards the lake. It is therefore not unreasonable to expect that some hot springs may occur in Lake Albert. On several occasion the local fishermen in Kibiro and Kachuru have been asked if they have any knowledge of any 17

18 A Report on a Geophysical and Geological Survey underwater hot springs or localized anomalies in flora and fauna in the lake, as is commonly associated with discharging springs in lakes or sea. Although the fishermen did not know of any springs, this has been discussed and they blame for instance inflow of hot water for the killing of large quantities of fish. This appears though to be seasonal, which indicates that the reason is rather biological rather than geological. The only written indication for underwater discharge comes from the annual report of the Geological Survey (Wayland, 1920 b): A [oil] seepage appears not only at Kibiro but, so the native fishermen say, also on the Congo side. More over the fishermen tell of explosions which sometimes occur well out at sea in the lake. They say a rumbling noise is accompanied by an uprush of water and oil. Figure Map of geothermal surface manifestations and calcite deposits Carbonate springs At three locations small perennial springs were located in the granitic zones away from the escarpment (fig. 2.13). In all cases these are on one of the fault lines in the block-faulted area. In all cases the flow is small (< 1 l/sec.) and the ph of the water is between 9 and 10. Calcite precipitates from two of the springs and forms few cm thick deposits on rock surface downstream from the springs. Access is relatively easy to the spring on the Kachuru fault line, and it is a popular watering spot for cows, and the herdsmen said that the cows preferred this water to other, possibly because of mineral content of the alkaline water. The springs issue from fissures in cross-jointed rock outcrops. Under the present project no provision was made for water or gas sampling or chemical analyzes, but it is recommended that these tasks will be carried out as 18

19 The Kibiro Geothermal Prospect soon as possible in order to determine if the source is geothermal or not. Isotope studies should also be carried out in order to determine the origin of the water Calcite deposits Calcite is not a primary mineral in granitic rock, and it is not expected to be present in the geological environment in the block-faulted granites in the study area. During an early stage of the fieldwork it became apparent that calcite was present at some locations as secondary mineral in cracks and veins. A simple test with hydrochloric acid was used from that time to confirm if calcite was present or not. Unfortunately this was not done in the first five field days, and therefore the map with calcite distribution (fig 2.13) is incomplete in the northern most part of the study area. Calcite is a common mineral in geothermal systems, especially in boiling zones and at the outer zones of the system. The calcite in the study area may be of geothermal origin, but this has still to be proven. An isotope study could assist in that determination. In the field it was possible to differentiate between three different formations of calcite. The first occurrence is calcite, which has formed from water flowing through joints and fissures in the rock, but the rock has now been brought to the surface due to fault movements and/or erosion. The second group is the calcite, which is currently being deposited at the carbonate springs. These two groups are here interpreted as geothermal manifestations, the former may be fossil but the latter is active, and these locations are shown in figure Apparently the water in the headwaters of River Kachuru is saturated with calcite, and a thin film forms on rocks in the river. This calcite forms the third group, and as its origin is less certain than the former two groups, this has not been mapped. From the distribution of calcite it is clear that it forms in two distinct environments. Firstly calcite is present at the active geothermal manifestations at the escarpment at Kibiro, and secondly it is found along the East-West fault lines in the block-faulted granites. If the calcites are geothermally related, it is apparent that the East-West faults are playing an active role in the geothermal system. 2.6 Geological summary It was outside the scope of the present study to map the origin and early metamorphosis of the base rock. All attention was directed towards the understanding of the geothermal occurrence and its relation to the recent geological history, i.e. the formation and development of the Albertine rift, possible reservoir rocks, heat source etc. The Albertine rift is located within the western branch of the East African Rift System, and is characterized by thick accumulation of sediments, at least 5.5 km, but absence of any volcanic rock on surface. The western rift is considered at an early stage in development, and is younger (late Miocene Recent) than the more mature eastern branch (Morley et al., 1999). The Albertine rift is seismically active, characterized by deep-seated (27 40 km) large earthquakes. The rift cuts straight through the study area at N50 E, but immediately as one moves out of the escarpment region, this direction cannot be seen, neither in macro-structures, such as fault zones or lineaments, nor in finer structures, such as joints and fractures. In the granitic rocks outside the escarpment the main lineaments are the same in fault structures and in 19

20 A Report on a Geophysical and Geological Survey joints and fractures, the principal directions are N20 E and E W, and a less prominent N E. In summary the tectonic pattern in the study area is governed by the formation of the rift. The same forces that are tearing the Archaean shield apart in SE NW direction and forming the Albertine Rift have block-faulted the semi-brittle granitic baserock east of the escarpment. The fault system is most likely of the same age as the rift itself and results from the same forces. One of the objectives of the present geological work has been to try to identify a possible heat source for the Kibiro hot springs and to identify if a potential reservoir rock existed in the area. A permeable reservoir rock is necessary for an exploitable geothermal system to form, where cold ground water can percolate to depth and have ample contact with the hot reservoir rock. The escarpment divides the study area in two very different geological environments. Thick, clastic sediments, as are found in the rift, generally have good permeability and high water/rock ratio. It is clear that the sediments are thick enough to host a geothermal system if a suitable heat source is stored at depth, or if the regional heat flow is high enough. Ancient granitic base rock, as is found east of the escarpment, has usually on the other hand very low porosity and permeability. For a geothermal reservoir to develop in such an environment a secondary permeability must have formed, i.e. open and interconnected fractures and faults enable the groundwater to reach deep enough and to come into contact with hot rock. Reservoirs that are controlled by faults tend to be more difficult to exploit, as the drilling target can be confined to narrow zones. The geophysical survey indicates that the source of the Kibiro hot springs may be found in the granitic basement, east of the escarpment. The geological survey has confirmed that this area is block faulted, probably during the formation of the rift. The distribution of springs and secondary minerals (calcite) indicate that the faults have at some stage been permeable, and the relatively seismically active rift may assist in maintaining that permeability. The present study cannot confirm that the block-faulted area is open enough to allow the development of an exploitable geothermal reservoir within the granitic base rock. Drilling into the potential reservoir, identified by the geophysical methods can only confirm the existence of such a resource. The occurrence of hot springs within block-faulted rock adjacent to the rift is well known in south-western Uganda and other countries in Africa (McNitt 1982), but these resources have not been investigated by drilling. 20

21 The Kibiro Geothermal Prospect 3. TEM Resistivity Survey 3.1 The role of resistivity surveying in geothermal exploration. Apart from direct surface manifestations, the resistivity of subsurface rocks is the most diagnostic parameter of geothermal activity that can be measured from the surface. Rock containing geothermal fluids have different resistivity than cold rocks. Most commonly the geothermal water lowers the resistivity, but in some cases the resistivity increases again at very high temperatures. There are mainly two reasons why the geothermal activity lowers the resistivity. Firstly, the geothermal fluid has higher concentration of dissolved ions (higher salinity) than cold groundwater and is therefore more conductive. Secondly the thermal water interacts with the host rocks, forming secondary alteration minerals. Some of these minerals, such as clay minerals (smectite) and zolites, are conductive and reduce the resistivity of the rock formation. The type of alteration minerals that are formed depends on the host rock, the salinity of the fluid and temperature. At relatively low temperatures (lower than C) the alteration minerals are commonly conductive, but at higher temperatures (higher than C) the minerals are dominantly resistive. The temperature where the transition from conductive to resistive minerals occurs depends on the rock type. In acidic rocks it occurs at about C, but in basic rocks it happens at about C (Árnason et.al, 2000). High-temperature geothermal systems (high-temperature systems have temperatures higher than 200 C above 1 km depth) in porous and permeable rocks, often have widespread resistivity anomalies characterised by a low resistivity cap at the outer margins which is underlain by higher resistivity in the core of the system. Nesjavellir m a.s.l NJ-11 NG-7 NG m 200 Temperature C Resistivity Alteration > 25 Ωm Ωm 2-10 Ωm low resistivity cap High resistivity core Unaltered rocks Smectite - zeolite zone Mixed layered clay zone Chlorite zone Chlorite-epidote zone Figure 3.1. A resistivity cross-section from the Nesjavellir geothermal field, Iceland, showing comparison with alteration mineralogy and temperature, determined in wells. 21

22 A Report on a Geophysical and Geological Survey If the salinity of the geothermal water is not significantly higher than that of the surrounding groundwater, and provided that the alteration minerals are in equilibrium with the present temperature (i.e. the system has not been heated up or cooled down recently), the resistivity structure reviles the temperature distribution. An example of this is shown on Figure 3.1, which shows a resistivity cross-section across the Nesjavellir high-temperature geothermal field, SW Iceland. The figure shows how the resistivity correlates with alteration mineralogy and temperature. In impervious crystalline rocks, like in the Kibiro area, the geothermal fluid is confined to fractures and fracture zones with little penetration into the surrounding rocks. Consequently the resistivity anomalies have much lesser lateral extent than in permeable rocks. The geothermal activity is therefore expected to show up as very complex resistivity structure with localised low-resistivity anomalies in the otherwise resistive rocks. 3.2 The central-loop TEM method There exist several methods for measuring the resistivity of subsurface rocks. They can be divided into galvanic or direct-current (DC) methods and electromagnetic (EM) methods. Some decades ago, the DC methods (mainly Shlumberger soundings) were widely used. In recent times, the EM methods have gained more popularity, mainly the Transient Electro-Magnetic (TEM) method and the Magneto-Telluric (MT) method. The MT method is based on measuring currents induced in the ground by time variations in the Earths magnetic field. In MT, the time varying magnetic field and the electric field generated in the surface are measured simultaneously. The MT method has the greatest depth of exploration of the available EM methods (some tens or hundreds of kilometres) and is practically the only method for studying deep resistivity structures. It has, however, limited resolution at shallow depths (in the uppermost 1 km). The MT method also suffers the so-called telluric shift problem, which is commonly resolved by applying the central-looptem method along with MT to resolve the resistivity in the uppermost kilometre. In the central-loop TEM method (hereafter called TEM) currents are also induced in the ground by time varying magnetic field. A loop of wire is placed on the ground and a constant current transmitted into the loop. The current builds up a magnetic field of known strength. The current is then abruptly turned off and the decaying magnetic field induces currents in the ground. These currents decrease, due to Ohmic loss, and the magnetic field decays as time goes on. The decay rate of the magnetic field is measured by recording the induced voltage in a receiver coil (or a loop) at the centre of the source loop. The decay rate of the magnetic field is dependent on, and can be interpreted in terms of the subsurface resistivity structure. This is shown schematically in Figure 3.2. The top part shows a schematic field layout and the bottom part shows the current waveform and the induced voltage in the receiver coil. 22

23 The Kibiro Geothermal Prospect Figure 3.2. The principle of TEM soundings. The top part shows a schematic field layout and the induced fields. The bottom part shows the current waveform and the induced voltage recorded in the receiver coil Instrumentation and field procedure There exist several types of instruments for performing TEM soundings. The instruments used in the present study where the Protem-67 instruments from Geonics Ltd. in Canada. They comprise a PROTEM digital receiver, TEM67 transmitter with motor generator, a receiver coil with the effective area of 100 m2 and 800 m of cables for the source loop. The source loop used, was a single turn m2 square loop. The transmitter transmitted 23 A half duty square wave current (see figure 3.2), at the repetition rate of 2.5 Hz, with two 100 ms current off intervals in each cycle. The receiver and the transmitter are synchronised by connecting them by a signal cable such that the receiver records the induced voltage in the receiver coil, during the current off intervals, at 30 time gates, logarithmically spaced from 0.09 to 69.8 ms after the current turn-off. During recording, 75 to 150 transients were stacked in order to reduce external electromagnetic noise. Several tens of stacked transients were recorded and stored in an internal memory in the receiver, to be later downloaded to a PC computer. 23

24 A Report on a Geophysical and Geological Survey Data processing and interpretation The recorded raw data are downloaded to a PC computer, using a DOS based software provided by the manufacturer of the instruments. The data are then loaded into a UNIX/LINUX based interactive graphical programme for quality check, rejection of corrupt readings and stacking of individual voltage recordings. The programme then converts the voltages into late-time apparent resistivity and finally writes out a file containing data ready for interpretation in terms of the resistivity distribution beneath the sounding site. Interpretation of TEM data is commonly done by what is called one-dimensional (1D) inversion. In 1D inversion it is assumed that the earth consists of horizontal layers with different resistivity and thickness. The 1D interpretation consists of determining the layered model, which response best reproduces the measured response. The assumption that the earth is horizontally layered may seem bold and is in most cases not valid. It lies, however, in the nature of the central-loop configuration that the induced current is predominantly horizontal and since it is the vertical component of the magnetic field that is recorded, effects of non-horizontal current distribution are further reduced. It can therefore be argued that central-loop TEM data are particularly well suited for 1D interpretation and experience has shown that relatively complex three-dimensional (3D) resistivity structures can be mapped in some details by 1D interpretation of TEM soundings (Árnason and Flóvenz 1992). More sophisticated interpretation, such as 3D inversion, is about to be available but is outside the scope of the present work. As stated above, the resistivity structure of geothermal systems in impervious crystalline rocks is expected to be rather complex with localised low-resistivity anomalies, confined to fracture zones. In permeable rocks the geothermal water influences the much larger volumes and produce larger anomalies. The assumption of horizontal layering is therefore to be born in mind when drawing conclusions from the present TEM data. It will probably make low-resistivity anomalies appear more smeared out. The programme used for the 1D interpretation is a UNIX/LINUX based inversion programme developed at Iceland Geosurvey. It reads the measure data and an initial model. The programme then iteratively adjust the model parameters (reisistivity and thickness of layers) so that the calculated model response fits the measured data as well as possible. It is customary to interpret TEM data by two types of layered models, i.e. with as few resistivity layers as possible and with what is called minimum structure or Occam s inversion, using many thin layers and demanding that variation in resistivity between layers is as small as possible. The results of layered and Occam s of the TEM data from Kibiro are shown in Appendix II. When all the soundings have been interpreted by 1D inversion, a 3D resistivity model under the survey area is compiled from the 1D models beneath each sounding. The 3D model is then presented by vertical cross-sections and resistivity maps at different elevations above sea-level, using a GMT (Generic Mapping Tool) based graphical software developed at Iceland Geosurvdey. 24

25 The Kibiro Geothermal Prospect 3.3 Data acquisition The TEM fieldwork at Kibiro started on February 19 th and continued until March 26 th. Sundays were taken off so there were 26 effective working days. Large parts of the survey area are covered with dense bushes and some places the accessibility was further limited by deep gullies. Despite the difficult conditions a total of 75 soundings were made, amounting to an average performance of about three soundings a day. The field crew consisted of three experts from GSMD, one driver and two local helpers. ICEIDA expert supervised the fieldwork during the first week, but after that the field crew was properly trained and the ICEIDA expert could concentrate on data processing, interpretation and survey planning. The recorded data were downloaded to a PC computer at the end of each day and inspected and processed by the ICEIDA expert. Table 3.1. Coordinates (UTM, Arc1960) and elevation of the TEM-soundings Sounding E-coord. N-coord. m a.s.l E N N S N S E N S N S N S S N S S N S S N S S N N S N S S S N N N S S S N S S S S

26 A Report on a Geophysical and Geological Survey Table 3.1 (cont). Coordinates (UTM, Arc1960) and elevation of the TEM-soundings Sounding E-coord. N-coord. m a.s.l S S S S E E E N S S S S S S S S N S N S S N S S S S S S S S At the beginning of the survey a 10 km long profile was measured at the top of the escarpment, with about 1 km station spacing. The centre of the profile is above the hot-spring in Kibiro. After that a second profile, 2 km inland from the first one, was measured. It then became clear that low resistivity anomalies were present to the south of Kibiro and the focus was put on that area. Figure x.3 shows the location of the TEM soundings except for one close to Hoima, which was used as a test and reference place. Their UTM coordinates (Arc 1960 datum) and elevations are also given in table Processing and interpretation of the Kibiro TEM data As stated earlier, the data were downloaded from the receiver every night after fieldwork. The ICEIDA expert processed the data as described above. Some of the data were of very poor quality because of extremely high subsurface resistivity and three soundings were omitted from interpretation due to high noise. A preliminary 1D interpretation was performed so that the emerging resistivity structure could be continuously updated to facilitate detailed planning of the survey and filling in details. 26

27 The Kibiro Geothermal Prospect Figure 3.3. Location of TEM-soundings and cross-sections. At the end of the fieldwork the interpretation was reviewed and improved. In Appendix I the measured late-time apparent resistivity sounding curves for all the TEM soundings are shown (red circles with error bars) as well as the layered models (green histograms) and their calculated response (black curves). Note that both the vertical and horizontal scales are logarithmic and that the horizontal scale is used to represent both the square root of time (in micro seconds) for the TEM curves and the depth below surface (in metres) for the layered models. Appendix I shows both layered interpretation (as few layers as possible; left) and Occam s inversion (minimum structure; right). Most of the sounding curves could be fitted satisfactorily by the response of a layered earth model. In some cases this was not possible, indicating that the resistivity structure beneath these soundings is far from being one-dimensional. This was to be expected, as discussed earlier. 27

28 A Report on a Geophysical and Geological Survey 3.5 The resistivity structure of the Kibiro area The 1D models of individual soundings were used to compile a 3D model of the resistivity structure of the Kibiro area. This model is presented here both as vertical cross-sections and as iso-resistivity maps Cross-sections Figures 3.4 to 3.11 show seven vertical cross-sections through the survey area. The location the sections are shown on figure 3.3. Five of the sections are parallel to the rift escarpment. The north-westernmost one is under the escarpment, on alluvial plane at the villages Kibiro and Kachuru (L5) and the other four (L1, L4, L2 and L7) are up on the escarpment with about 1 km interval inland. Two sections are perpendicular to the escarpment, one (L3) is along the road from Kigorobya to Kibiro and the other one (L8) is 3-4 km SW of L3. It is customary to sections based both on the results of layered model inversion and Occam s inversion. The philosophy behind the layered inversion is to connect layers of similar resistivity between soundings in order to reveal geological strata or thermal regimes, like in figure 3.1. This is in most cases easily done in stratified and permeable rocks where the resistivity structure has significant horizontal trend. In impervious and fractured crystalline rocks, like in the Kibiro area, the resistivity can have very strong lateral variations, with near vertical low-resistivity anomalies in fractured zones. As a result of this, it can be very difficult do deduce any consistent layering from the sections based on layered models. This is the case in the Kibiro area and after a close inspection it was decided that trying to determine layered structure in the sections was of no use. Sections with the layered models, without connecting an colouring layers are shown in Appendix II. The smoothed Occam s models do on the other hand have consistently defined resistivity values (from the inversion) at many depth values and an automatic contouring and colouring of the resistivity can be applied. This has been done on figures 3.4 to It is actually the logarithm of the resistivity that is contoured so the colour scale is exponential but numbers at contour lines are resistivity values. The depth of exploration of the TEM soundings is much greater in resistive earth (about 1 km) than in conductive environments. This is because shallow conductive layers or bodies screen the deeper resistivity structure. The contouring and colouring of the sections is therefore cut at shallower depths under shallow conductive anomalies than in resistive areas. 28

29 The Kibiro Geothermal Prospect Figure 3.4. Resistivity cross-section along line T5. Cross-section T5, below the escarpment is shown on figure 3.4. It shows low resistivity in the uppermost m. This reflects most likely the alluvial sediments which contain salt and hence are conductive. At the NE end of the profile there is very high resistivity below the near surface low-resistivity. The resistivity at depth decreases towards SW and is low, about 10 Ωm or lower under the sounding furthest to the SW. It should be born in mind here that the resistivity structure under the profile is almost certainly dominantly two-dimensional so that the values obtained by 1D inversion are not the true values. The profile is on the sediments and parallel to the escarpment of crystalline rocs with resistivity much higher than that of the sediments. The thickness of the sediments under the profile is probably some hundreds of metres, because two old wells that were drilled at the northern end of the profile hit the crystalline basement at the depth of 682 and 261 m depth, respectively (Gíslason et. al, 1994). Even though the resistivity values displayed on figure x.4 are not the true resistivity values under the profile, the decrease in resistivity at depth, from NE to SW is probably significant. 29

30 A Report on a Geophysical and Geological Survey Figure 3.5. Resistivity cross-section along line T1. 30

31 The Kibiro Geothermal Prospect Cross-section T1 at the top of the escarpment is shown on figure 3.5. The profile is 9 km long and with the centre of the profile, zero on the horizontal scale, is right above Kibiro. On all the profiles parallel to the escarpment, distances are measured positive to the NE and negative to the SW. The resistivity is very high, some thousands of Ωm, in the SW end of the section. Between 3 and 6 km to the SW, a relatively low resistivity, below 100 Ωm, is seen and 2.5 km south resistivity lower than 10 Ωm is seen at 500 m depth. The resistivity is again very high in the uppermost kilometre at the centre of the profile. Between 1 and 3 km north, the section shows low resistivity at about 6 km depth. The data of the soundings indicating this are very noisy and it is not clear whether this is real or not. At the northernmost 5 km of the section a low resistivity appears and at progressively shallower depths towards the end of the section. The low resistivity anomalies on the section on figure 3.5 are localized and no obvious layering is seen. They are probably a smeared out response of near vertical anomalies. Figure 3.6. Resistivity cross-section along line T4. Cross-section T4, which is about one km SE of L1 is shown on figure 3.6. The zero on the horizontal scale of this section is by the road from Kigorobya to Kibiro, almost due SW of Kibiro. The section shows some sharp and confined low-resistivity 31

32 A Report on a Geophysical and Geological Survey anomalies in otherwise very resistive rocks. The sounding furthest to SW shows two localized low-resistivity anomalies bordered by high resistivity to the NE, one at about 200 m depth and the other at 600 m depth. In the central part of the section, between 0 and 8 km SW, there is low resistivity at about 800 m depth, but with two peaks extending towards the surface, one at 5.5 km to SW (up to 300 depth) and the other at 3.5 km SW (up to 150 m depth). The latter aligns almost due south of the sharp low-resistivity peak at 3 km SW in section L1, both being in a valley extending SSW from Kachuru village. Figure 3.7 Resistivity cross-section along line T2. 32

33 The Kibiro Geothermal Prospect Cross-section T2, which is about two km SE of L1 is shown on figure 3.7. The section shows very complex resistivity structure, with localized low-resistivity anomalies separated by high resistivity. There is a well defined low-resistivity at about 500 m depth under the SW end of the profile and little further to the NE there is a shallow isolated anomaly. At 6 km SW a high resistivity extends to the depth of exploration of the TEM soundings. At 4.5 km SW there is low-resistivity anomaly bordered by high resistivity on both sides. Between 3 km SW and 1.5 km NE the resistivity is low beneath m depth with two shallower peaks at 1 km SW and 1 km NE. This might be a smeared out picture of three near vertical anomalies at 3 km SW, 1 km SW and 1 km NE. The anomaly at 3 km SW aligns with anomalies on sections L1 and L4, all being under the valley extending SSW from the village Kachuru. Finally there is localised anomaly at 3 km NE. Figure 3.8. Resistivity cross-section along line T12. Cross-section T12, which is about three km SE of T1 is shown on figure 3.8. This section shows generally lower resistivity than the sections closer to the escarpment. Between 8 and 4 km SW, there is low-resistivity at about 500 m depth. Above this is an indication of layered structure. Between 4 and 2.5 km SW an anomaly of very low resistivity extends from depth and close to the surface. This anomaly is sharply bounded by high resistivity, as deep as the soundings explore. From 1 km SW and to the NE end of the profile, a low resistivity is observed, being at the depth of about

34 A Report on a Geophysical and Geological Survey m at 1 km NW and elevated sharply to m depth in the eastern most 2 km of the section. Figure 3.9 Resistivity cross-section along line T3. Cross-section T3, shown on figure 3.9, is NW-SE oriented, along the road from Kibiro to Kigorobya. The zero reference on the horizontal scale is where it intercepts the section T1. The section shows the shallow low-resistivity under the sedimentary plain below the escarpment. The low resistivity is probably due to saline shallow groundwater in the sediments and is underlain by much higher resistivity. Under the escarpment is very high, except that a low resistivity might be present at about one km depth under sounding 10500E. This is though highly uncertain because the measured data of that sounding was very noisy and of poor quality. From about 2 km SE and to the SE end of the section, the resistivity is low at depth. The depth to the low resistivity is variable from 300 to 600 m but with no clear peaks. 34

35 The Kibiro Geothermal Prospect Figure Resistivity cross-section along line T13. Cross-section T13, shown on figure 3.10, is NW-SE oriented about 3-4 km SW of T3. The zero reference on the horizontal scale is where it intercepts the section T1 at the top of the escarpment. The section shows high resistivity in the uppermost kilometre at the top of the escarpment. At about 1 km SE, where the section crosses T4, a well defined low-resistivity anomaly under the valley SSW of the village Kachuru. At about 2.5 km SE and as far as the section extends, a well defined anomaly of very low resistivity is seen at shallow depths ( m). 35

36 A Report on a Geophysical and Geological Survey Iso-resistivity maps Like for the vertical sections, gridding and automatic contouring of the Occam s models was done in horizontal planes to produce iso-resistivity maps at different elevations. The resistivity is contoured and coloured in the same way (exponential) as the cross-sections. The contouring and colouring is clipped at the distance of 1 km from soundings at the margin of the surveyed area. The elevation of the survey area above the escarpment is about 950 to 1100? m a.s.l. and the elevation of Lake Albert is at 622 m a.s.l. and the alluvial plane at Kibiro and Kachuru is generally about 2-10 m higher. Iso-resistivity maps were produced at 100 m depth intervals from 900 m a.s.l., and down to 100m above sea level, which is estimated to the maximum depth of penetration of the TEM soundings. Figures x.11 to x.19 show the iso-resistivity maps at increasing depth. The coordinates of the maps are UTM (Arc 1960 datum) in kilometres. Figure Resistivity map 900 m above sea level. Resistivity map at 900 m a.s.l., is shown on figure This elevation is about 50 to 150 m below the surface to the SE of the escarpment, but above surface below it. The map shows high resistivity (100 to above 1000 Ωm) at this level, except for three soundings in an area of km E and km N. 36

37 The Kibiro Geothermal Prospect Figure Resistivity map 800 m above sea-level. At 800 m a.s.l., figure 3.12, the resistivity is still high in most of the survey area. The resistivity anomaly seen 100 m higher is still present and shallow relatively low resistivity is seen in some sounding scattered over the area. 37

38 A Report on a Geophysical and Geological Survey Figure Resistivity map 700 m above sea-level. At 700 m a.s.l., figure 3.13, the low-resistivity anomaly seen above has extended to the west and low resistivity is seen in two soundings at the valley SSW of Kachuru. The resistivity is also decreasing under soundings in the ESE part of the survey area. 38

39 The Kibiro Geothermal Prospect Figure Resistivity map 600 m above sea-level. At 600 m a.s.l., figure 3.14, the shallow anomaly in the area km E and km N has extended still further to the west and now looks like a well defined E-W trending anomaly. The eastern end of this anomaly is not defined, but in the west it ends at km. The sounding furthest to SE also shows low resistivity, which might relate to the anomaly, but further soundings are needed to confirm this. Now the section plane is below surface below the escarpment and the shallow lowresistivity in the sedimentary plain at Kibiro and Kachuru is seen. A medium to low resistivity anomaly is starting to show up along the valley of R. Kachuru. Finally an ENE-WNW trending anomaly of medium to low resistivity is starting to appear across the road from Kigorobya to Kibiro, about 4 km SE of Kibiro. 39

40 A Report on a Geophysical and Geological Survey Figure Resistivity map 500 m above sea level. At 500 m a.s.l., figure 3.15, the resistivity is similar to that at 600 m a.s.l., except that low resistivity is now appearing furthest to NE and the sounding furthest to the north, below the escarpment also shows increasing resistivity at depth. The linear anomalies discussed above persist and have become clearer. 40

41 The Kibiro Geothermal Prospect Figure Resistivity map 400 m above sea-level. 41

42 A Report on a Geophysical and Geological Survey Figure Resistivity map 300 m above sea-level. At 400 and 300 m a.s.l., figures 3.16 and 3.17, the resistivity does not change much from the shallower depths, except that low resistivity now appears at the SW margin of the survey area and the soundings below the escarpment show high resistivity, except the sounding furthest to the south. 42

43 The Kibiro Geothermal Prospect Figure Resistivity map 200 m above sea-level. 43

44 A Report on a Geophysical and Geological Survey Figure Resistivity map at sea-level. At 200 m a.s.l. and at sea-level, figures 3.18 and 3.19, the picture changes somewhat. The E-W and the SSW-NNE linear anomalies have smeared out and in the southern part of the survey area a SW-NE trending anomaly, about 2.5 km to the SE and parallel to the escarpment. A low resistivity also appears under the escarpment northeast of Kibiro, but as mentioned while discussing the cross-sections, this should be considered uncertain because of the poor quality of the TEM data in this area. Finally the low-resistivity area at the NE margin of the survey area has extended to the SW. 44

45 The Kibiro Geothermal Prospect 4. Gravity Survey Geological mapping of the Kibiro area has indicated that mafic intrusives are found in the granitic gneiss bead rock, bordering the rift to the SE. Such intrusives, if present, could act as heat sources for the geothermal system at Kibiro. Basic intrusives do normally have higher density than the gneiss host rocks, producing gravity anomalies. Variable sedimentary thickness and displacements at faults can also produce gravity anomalies. To investigate if considerable density and structural variations are to be found in the Kibiro area, a gravity survey was performed. The gravity survey was performed by a field crew of geophysicist from the Petroleum Exploration and Production Department (PEPD) of the Ministry of Energy and Mineral Development (MEMD). Gravity measurements were taken using Scintrex CG-3 Model Autograv gravimeter that was operating at a resolution of 0.005mGal. Positioning and determination of the elevation was done by differential GPS technique, using a pair (base and a rower) of Magellan Promarc x-cm (10 channel, single frequency, carrier phase data) GPS system. The instruments were supplied by the PEPD. To achieve centimetre accuracy in establishing gravity stations, the rower GPS was used to collect data at each station for a minimum of 20 minutes. This time was increasing depending on the distance of the survey point from the base station. The rower and base data were then post processed using the MSTAR program to obtain station coordinates to 10 cm accuracy. 4.1 Data acquisition. The gravity fieldwork started on 16 th of February and was continued until March 26 th. During the first days two new base stations were established one at Kigorobya Health Centre and the other at Kabiribwa at the top of the escarpment above Kibiro. The gravity values at these reference stations were determined by connecting to an already established base station in Hoima. The new base stations were used for the GPS measurements and time dependent drift corrections for the gravimeter. They were also used for calculation of absolute and relative values of every surveyed station. The coordinates and elevation of the reference stations are given in table 4.1. Table 4.1 Coordinates (UTM, Arc1960, m) and elevation of gravity reference stations. Station East-coordinate North-coordinate Elevation Kigorobya Kabiribwa Data dumping from the gravimeter to the computer was done using the IDUMP program of Scintrex. Data posting was carried out using the CG3 program of Scintrex, which matches gravity measurements with survey data. 45

46 A Report on a Geophysical and Geological Survey Gravity measurements were made along four profiles (see figure 4.1). The first profile, G1, was a 19.7 km long with 100 m station spacing along survey line 1 at the top of the escarpment (the same profile as the TEM resistivity profile T1). The second profile, G5, is 3.2 km long and was run along survey line 5, on the alluvial plane below the escarpment with 100 m station spacing. The third profile, G2 is 8.4 km long with 250 m station spacing along the SW part of survey line 2. The last profile, G3, is along the road from Kibiro to Kigorobya (survey line 3). It is 5 km long and with 250 m station spacing. The locations of the gravity profiles are shown on figure 4.1 and table 4.2 summarises the statistics. Table 4.2. Statistic on gravity survey at Kibiro Profile Direction Length Stat. spacing No. of stations G1 N50 E 19.7 km 100 m 198 G2 N50 E 9.25 km 250 m 38 G3 N140 E 5.0 km 250 m 21 G5 N50 E 3.2 km 100 m 33 Total km 290 Figure 4.1. Location of gravity profiles. 46

47 The Kibiro Geothermal Prospect 4.2 Data processing To ensure compatibility with previous gravity data from the area, geophysicists from the PEPD, the same as processed existing data, performed the processing of the data. Near-zone topographic correction was done in the field (up to 100 m radius). Further away, topographic correction was done using digital maps. To get gravity reduced data the program GRAVRED was used (the following constants were used in the processing: Projection = UTM 36; Density = ; Gravity formula = 1967; GMT difference = -3; Units = m, m, m). GRAVRED calculates free air values, makes tide corrections and finally calculates absolute gravity and Bouger values with reference to existing base station at the Hoima post office. 4.3 Results Figures 4.2, and 4.5 show the Bouger gravity values along profiles G1, G2, G3 and G5 respectively. The gravity profile G1, figure 4.2, shows that the Bouger gravity is generally increasing towards NE, from 130 mgal in the SW and to about 125 mgal in the NE. Superimposed on this general trend are some short wavelength ( m) anomalies of 1-2 mgal. 47

48 A Report on a Geophysical and Geological Survey Figure 4.2. Bouger gravity along profile G1. 48

49 The Kibiro Geothermal Prospect The most prominent of these are between 2.5 and 5 km in the NW (the reference point on the horizontal axis is the same as for the resistivity cross-section, i.e. at the top of the escarpment, at the road to Kibiro). Between 2 km and 4 km SW, the gravity values are somewhat irregular. This is where the profile crosses the valley SSW of the village Kachuru. There is a gully at the bottom of the valley with steep hills and the irregularities are most likely due to too simplified topographic correction in the complicated topography rather than density variations. Figure 4.3. Bouger gravity along profile G2. Figure 4.3 shows the Bouger gravity values for the profile G2, along survey line 2, 2 km inland from G1. The zero on the profile is on the road from Kigorobya to Kibiro and the profile was only run to SW from the reference point. The profile G2 shows, like G1, a general decrease in gravity from NE to SW, but the Bouger values are generally about 7 m gal higher than for the profile G1, showing that the gravity increases inland. Like for the profile G1, short wavelength anomalies, of about 1 mgal, are superimposed on the general trend. Profile G3, figure 4.4, is perpendicular to G1 and G2, along the road from Kibiro to Kigorobya. The zero on the horizontal scale is 250 m NW of the reference point of the profile G1. The figure shows very sharp decrease in gravity towards the rift sediment filled graben as was to be expected. After the sharp rise, the gravity increases gradually to about 2.5 km to the SE and stays almost constant after that, to the SE end of the profile. 49

50 A Report on a Geophysical and Geological Survey Figure 4.4. Bouger gravity along profile G3. Figure 4.5. Bouger gravity along profile G5. Profile G5, figure 4.5, is from SW to NE, on the alluvial plane below the escarpment. The reference point (zero) on the horizontal scale is where the road down the escarpment enters Kibiro. G5 shows much lower gravity values than the SW-NE 50

51 The Kibiro Geothermal Prospect profiles upon the escarpment. This is due to the lower density sediments in the rift graben. Like for the SW-NE profiles G1 and G2 above the escarpment, G5 shows increasing gravity from SW to NE. The increase is, however, not as gradual as in G1 and G2, but rather as a relatively sharp rise between 0.5 and 1.25 km SW, from about 150 to 149 mgal in the SW and to about 148 to 147 mgal in the NE. This might indicate a step in the crystalline basement below the sedimentary plane. Figure 4.6. Wiggle plot of Bouger gravity. Figure 4.6 shows a wiggle plot of the Bouger gravity along the profiles. In these plots the Bouger gravity is plotted as positive (filled with red colour) and negative (purple lines) deviations from the mean value for each profile. The mean values are different for the profiles, as indicated in the figure. The figure shows well the gradual increase in gravity from SW to NE along the above and parallel to the escarpment as well as the increase of gravity towards SE from the escarpment. It also shows well the step like increase in gravity along the profile below the escarpment. 51

52 A Report on a Geophysical and Geological Survey Figure 4.7. Bouger gravity map of the Kibiro area. The PEPD has performed gravity surveys of the Hoima basin (PEPD 2002). Part of their date is from the present survey area and can be included in this study. The combined datasets have been used to produce the Bouger gravity map presented in figure 4.7. The gravity stations are far from uniformly distributed and large gaps exists so the map could be aliased, especially inland, but it does show some general features of the gravity field. The main feature is the gravity low below the escarpment, reflecting the lower density sedimentary filling in the graben. Above the escarpment the gravity field increases sharply the first two kilometres inland, but is rather flat after that. It is worth noting that the increase in the gravity field seems a little oblique to the escarpment. In the NE part of the survey area, the Bouger gravity rises above 130 mgal just at the escarpment, but towards SW the 130 mgal contour lines is progressively more inland. The reason for this is as yet not clear. Despite the uneven station spacing figure 4.7 seems to indicate that there is a weak gravity high in the area roughly bounded between 301 to 310 km east and 178 to 182 km north. This anomaly coincides with the area where the TEM soundings show the lowest resistivity. 52

53 The Kibiro Geothermal Prospect 5. Magnetic Survey As stated earlier, geological mapping indicted possible intrusives in the Kibiro area. Such intrusions often have different magnetisation from the host rocks. In order to investigate this, the geothermal survey included magnetic measurements. 5.1 Instrumentation and field procedure The instrument used were two portable Invimag proton precession total filed magnetometers from Scintrex. One of the magnetometers was used as a reference station to correct for diurnal variations. The reference station was placed in the survey area at the coordinates E and N (UTM, Arc 1960). The rover was used to measure the total field at 5 m interval along profiles. The positioning was performed by a hand held GPS instrument and a measuring tape. Pre-calculated coordinates of stations at 50 m intervals along the profiles were loaded into the GPS instrument, which was used to navigate to these points. The absolute accuracy of these locations is of the order of +/-5 m. Intermediate stations were positioned by stretching a 50 m measuring tape between the GPS positioned stations and readings taken at 5 m intervals. The coordinates of the intermediate stations were calculated by linear interpolation between the GPS stations. The relatively short station spacing of 5 m turned out to be necessary to avoid aliasing because bolder on and base rock outcrops on the surface produced high frequency nose. The readings were stored in internal memories of the instruments and at the end of each day, internal software performed the diurnal correction and the data were downloaded to a PC computer. 5.2 Data acquisition The magnetic measurements started on February 16 th and ended on March 26 th. The measurements were made along 11 profiles as shown on figure 5.1. Four of the profiles are parallel to the escarpment; M1, M2 and M4 above the escarpment and M5 blow the escarpment. The profile M3 is along the road from Kibiro and Kigorobya. As the TEM soundings were starting to reveal a N-S trending low-resistively anomaly to the south of Kibiro emphasis was put on that region and six relatively densely spaced E-W profiles (M6 to M11) were measured in order to see if a magnetic anomaly was associated with the resistively anomaly. A total of about 80 km were traversed amounting to over magnetic stations. 5.3 Data processing At the end of each day the diurnal corrected data were downloaded to PC computer. As stated earlier, the data contained, in some places, some high frequency noise and spikes. The high frequency noise vas mainly due to boulders and outcrops on the surface. The spikes are of unknown origin, but could be due to malfunction in the instruments or incorrect orientation of the magnetic sensor. After deleting obvious spikes, the data were smoothed by gaussian filter with the half amplitude of 100 m to reduce high frequency noise and finally re-sampled at 20 m intervals. 53

54 A Report on a Geophysical and Geological Survey Figure 5.1. Location of magnetic profiles. 5.4 Results The edited and smoothed magnetic measurements are presented here as wiggle plots and a magnetic map. The wiggle plots show variations (positive as filled red and negative as purple line) around the background value of nt. Figure 5.2 shows wiggle plots along the magnetic profiles parallel to the rift and along the road from Kibiro and to Kigorobya. The profile below the escarpment shows decreasing magnetic field from MW to SE. 54

55 The Kibiro Geothermal Prospect Figure 5.2. Wiggle plot of the variation of the magnetic field along profiles M1, M2, M3 and M4 around the background value of nt The profiles above and parallel to the escarpment show relatively quite and slightly positive field (relative to the background) in the SW part. Profiles M1 and M4 show short wavelength anomalies of nt in the northern parts. M2 also shows considerable variations in the middle of the profile, but again quite field in the NW part. The profile M3, along the road between Kibiro and Kigorobya shows increasing magnetic field inland. Even though some prominent anomalies are seen in the profiles parallel to the rift, they are not easily connected between profiles. As the TEM resistivity survey progressed, a low resistively anomaly started to emerge to the south of Kibiro. This anomaly was suspected to be in relation to N-S trending fractures. In order to investigate if this anomaly also had magnetic anomaly associated with it, six relatively densely spaced E-W profiles were measured in this area. Figure 5.3 zooms in on this area and shows wiggle plot of the E-W profiles and the other profiles in the area. 55

56 A Report on a Geophysical and Geological Survey Figure 5.3. Wiggle plot of the magnetic field zoomed in on the area with E-W profiles. The two northernmost E-W profiles, M6 and M10, show small anomalies and the magnetic field is also fairly quite along profile M8. The two profiles north of M8, M9 and M7 and the profile M11, south of M8, show some pronounced negative anomalies. Some of these anomalies seem to line up in N-S direction, just east of 306 km east. This anomaly is however absent in M8 so it is most likely that this is a coincidence. Other anomalies cannot be convincingly connected between profiles. Figure 5.4 shows magnetic map of the area around the E-W profiles where the profiles are most dense. The short wavelength anomalies seen on the wiggle plots show that this map is almost certainly aliased, except along the profiles. It should therefore be taken with a grain of salt. The map seems, however, to show generally lower magnetic field in an E-W elongated area around 183 km north and between 304 and 308 km east. The N-S directed anomaly of low magnetic field discussed above, just east of 306 km east, and between and north, also stands out. Due south of this, the magnetic field is also low in the southernmost E-W profile, but separated from the anomaly to the north by higher field. As stated earlier, this line up might well be in coincidence. If the negative anomalies seen on figure 5.4 are real, they reflect positively magnetised rocks and could be consistent with normally magnetised intrusions. 56

57 The Kibiro Geothermal Prospect Figure 5.4. Magnetic map 57

58 A Report on a Geophysical and Geological Survey 6. Summary and Conclusions The eastern escarpment of the Albertine Rift divides the study area in two entirely different geographical, geological, environmental and demographic areas. In the Rift, west of the escarpment, most of the area is occupied by Lake Albert, which limits the research there to a narrow shoreline below the escarpment. The geological environment within the Rift is one of a sedimentary basin, with accumulation of clastic sediments with thickness of more than 5.5 km. Boreholes near Kibiro show that the sediments are alternating layers of sand and shales, the former a good environment for water reservoir, the latter may restrict water movements. Granites and granitic gneiss of the pre-cambrian shield form the geological environment east of the escarpment. The basement rocks are covered by laterite soil of varying thickness. The granites are expected to have very low primary porosity and permeability, and if this rock unit is to host a potential geothermal reservoir, it must depend on secondary permeability. The faults of the escarpment are the main structures in the tectonic environment, running N50 E through the study area. Only the main fault of the escarpment zone is visible, but other faults are predicted buried under the sedimentary cover of the rift valley. The forces causing the rifting of the archean plate have put stress on the granitic rocks in the eastern part of the study area, causing block faulting of the entire area, with two main direction, N20 E and N90 E. The block faulting is of the same age as the main escarpment fault (late Miocene Recent), and as the area is known to be fairly seismically active the block faults may be permeable to water. This is confirmed at Kibiro where the hot springs emerge where a large N20 fault zone intersects the main escarpment. No direct geological evidence has yet been confirmed which shows any volcanic activity of the same age as the formation of the rift. The origin of a dike-like formation found in the granites above Kibiro has still to be determined, if it is an intrusive dike of a metamorphic origin, associated with the block faulting of the granites. Lake Albert controls the hydrological environment within the Albertine Rift. The rainfall in the Hoima Masindi area and the permeability of the granitic rock, on the other hand controls the hydrology east of the escarpment. The regional water level there is believed some 200 m above the lake, causing considerable hydrostatic gradient across the escarpment, maintained by the low permeability of the granites. Indication of secondary porosity and permeability in the granites is presented by the occurrence of springs and precipitation along the block faults etc. The Kibiro hot springs rise at a location m higher than the lake level of Lake Albert, indication that it is more likely to be related to a hydrological system with higher gradient, i.e. the region east of the escarpment. Isotope studies indicate that the origin of the geothermal components of the Kibiro hot springs is not from the hydrological environment of Lake Albert, but associated with higher ground areas east of the escarpment (IAEA 2003). 58

59 The Kibiro Geothermal Prospect The TEM resistively survey shows that the granitic basement rocks in the Kibiro area have generally very high resistivity. Superimposed on this high background resistivity are localised and near vertical and in most cases linear anomalies with sometimes very low resistivity. At shallow depths, above 300 m a.s.l., the most prominent anomalies are an E-W trending anomaly at about 179 km north and extending eastwards from about 303 km east. The limited time of the survey did not allow the survey to fully delineate the boundaries of this anomaly to the south and east. Another anomaly is found in the valley SSW of Kachuru village. This anomaly aligns about N20 E. At greater depth (200 m a.s.l. and below) a NE-SW trending anomaly is also seen parallel to the escarpment of the rift. Figure 6.1. Resistivity at 500 m a.s.l., faults and fractures (pink lines), geothermal surface manifestations (red stars) and an area of slightly higher gravity field. 59