The electrical resistivity method of prospecting and its application to geothermal exploration in the rift valley of Kenya
Bhogal, Pravin Singh
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In 1971 the East Afric3n Power and Lighting Co. Ltd. (EAPL) in collaboration with the United Nations DevelopmentProgramme (UNDP)initiated an investigation in the Rift Valley of Kenya to explore and evaluate the potential of the geothermal resources for the production of electric power. The regions' selected, Olkar-La., Eburru and Lake Bogoria, for carrying out detailed geophysical and geochemical surveys had been previously recognised irem ext ensi ve ground geological work,infra-red aerial photography, water well drilling CLTr1edconnaisance geophysical surveys carried out by various groups over the last ten years. This thesis reviews the previous work done and describes the work done by the author at the three prospects to outline the geothermal reservoirs using the electrical resis- .::.,..1. • tivity method. A ground magnetic ~vey was also carried out at the Olkaria prospect. At the Olkaria area various electrical configurations were tested along a profile and it wa..sfound that the Polar-dipole array was the rrost sui table fran the point of view of field operat Ions , depth of .iuvestigat.ton and resofut ion. This array was used to carr; out a detailed resistivity survey Of the Olkaria area, in order to ...... outline -i..Le boundaries of the geothermal system. A well defined resf.st.I vi ty low of ~O-:-30o' hm-mbounded by resisti vi ties of 100 ohm-mwas defined at a depth of 250m- a resistivity contrast of 1: 10 in going fran the geothermal. to the non-geothermal environ- rrent, At a depth of about 1 km the resisti vi ty contrast was found to be lower i.e. 1:5. Several zones of low resistivity of~les3 than 10 obrrHnwere also outlined at this depth. Boreholes sited in one of these low resistivity anomalies have been success- ful in producing a steam-water mixture under pressure, with a maximum temperature of 2800C at a depth of 1350 m. The magnetic survey outlined a large positive anomaly associated with the whole of the central part of the geothermal area, superimposed by a shallow negative anomaly. The positive anaTaly has been interpreted as representing a reversely magnetised source at a depth of 3-4 km, and the negative anomaly as representing hydro- thermally altered zones where the conversion of magnet ic minerals into non-IT~etic minerals have caused a local reduction in reversely magnetised rocks. The successful boreholes OLK. 2,3,4, are located within one of these magnetic 10Vv'8.The magnetic survey has shown that the technique offers considerable scope in mapping Int.rusf ves and hydrothermally altered zones when used in "~..t • con,duction with other geophysical methods. Electrical resistivity measurements made at the Eburru prospect indicate that the area is underlain by ~ extensive reservoir or flow of hot chloride water and that the steam supplying the fumaroles is channeled to the surface through faults frem .the boiling of this chloride water. The characteristic high resistivities of the surface rocks are attributed to the ....._ desaturation 'of :r.:.Qcbkys the escaping steam. Resistivity measure- , .- ments indicate that hot water fran Eburru f'Lowsto the north tov.~ds Lake El~enteita. A borehole at the summit of EbtITTU and another one at the northern fc~t of the volcano are suggested to test the vertical extent of the steam zone, ~~d the tdt~eratlITe and character of the underlying hot water body, vi The presence of hot water and steam around Lake Bogoria indicates that the heat flow in the immediate environs of the lake is large and that a large heat source must exist in the vicinity of the lake. Electrical resistivity and temperature measurerrents indicate that the source is in the southern bay, and that hot water flows from south-east to the west and north, and around the margins of the lake steam ascends through faults and mixes with the hot chloride water to produce springs. The most favourable site to test the geothermal potential is the south-eastern shore of the lake.