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PROCEEDINGS, Kenya Geothermal Conference 2011

Kenya International conference Centre, Nairobi, November 21-23, 2011

RESISTIVITY STRUCTURE OF SILALI GEOTHERMAL PROSPECT IN KENYA

Deflorah Kangogo, Joseph Gichira, Anthony Wamalwa ,Cleophas Simiyu& Yusuf Noor

Geothermal Development Company Ltd,

P. O. Box 17700- 20100 Nakuru,Kenya

dkangogo@gdc.co.ke; jgichira@gdc.co.ke ;munikatony@yahoo.com;&csimiyu@gdc.co.ke;ynoor@gdc.co.ke

ABSTRACT

Electrical resistivity methods are widely applied in geothermal exploration andare the cheapest means of acquiring geophysical data. Further detailed surveyslike exploration drilling which is cost intensive in an area under study is always

based on accurate surface exploration results. This study focuses mainly onapplication of electromagnetic methods namely Transient Electromagnetic(TEM) and Magnetotelluric (MT) carried out in Silali geothermal prospectsituated in the Northern Kenya rift aimed at mapping out the subsurfaceresistivity, which is then interpreted so as to provide information such as fluidfilled fractures, the reservoir and the heat source. Resistivity results obtained

revealed three main resistivity zones; 1) A shallow high resistivity zone (> 100m) to about 300 m below the surface which is as a result of unaltered rocks, 2)

An intermediate low resistivity zone (10 m) to depths of about 1 km which is

as a result of hydrothermal alteration minerals indicating the top of thegeothermal reservoir, and 3) A deeper high resistivity (> 50 m), up to 3 -4 kmdepth indicating high temperature minerals occurring at depth and also definingthe top of the heat sources.

Keywords:Kenyan north rift, Resistivity structure,Electrical resistivity,Reservoir, Heat source.

INTRODUCTION

Electrical resistivity methods have proven to beuseful tools in geothermal exploration for a long timenow. This is because they relate directly to the

properties that characterize geothermal systems suchas permeability, porosity, salinity, temperature anddegree of hydrothermal alteration of the rocks (Hersirand Bjrnsson, 1991). Geoelectrical measurements

provide information on the distribution of thesubsurface electrical resistivity.In high temperature geothermal systems, electricalresistivity variations are often predominantly caused

by hydrothermal alteration zones (rnason, et. al,2000). The hot fluids of a geothermal system lead to

the formation of a sequence of hydrothermalalteration minerals depending on the temperature.Resistivity methods are used to determine variationsin electrical conductivity of the sub-surface bothlaterally and with depth. Among the methods usedare the natural-source methods (magneto telluric) andcontrolled-source induction methods. EM methodsare more sensitive to conductive (low-resistivity)

structures compared to direct current (DC)

techniques.Several resistivity methods have been applied ingeothermal resource assessment for several decades,In DC resistivity sounding an electrical current isinjected into the ground and the potential voltagegenerated by the current distribution in the earth ismeasured at the surface. DC method is more sensitiveto resistive structures hence it has been used toidentify and delineate high-temperature systems. Inthe central-loop Transient Electro-Magneticsoundings, current is induced by a time varyingmagnetic field generated by a current in a loop andthe decaying induced magnetic field is monitored atthe surface. For MT method the current in the groundis induced by the natural time varyingelectromagnetic field. MT soundings have thegreatest penetration depth of all the electricaltechniques. This paper concentrates on the resistivitywork that has been carried out in the SilaliGeothermal prospect.

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Figure. 1. The areal distribution of MT&TEM resistivity soundings from the Silali prospect area

Results of MT and TEM Survey

The data was processed, and produced in apparentresistivity plots in form of contour maps at variouselevations and cross-sections across major geologicalstructures. The aim was to locate a possible sub-surface resistivity anomaly on the basis of relevant

parameters such as apparent resistivity values, shapeand size. These approaches are qualitative and of firstorder interpretations. Resistivity profiles of Silaligeothermal prospect are presented by resistivitycross-sections across major geological structures aswell as by iso-resistivity maps at different elevationswith respect to sea level.

Iso-Resistivity maps Resistivity at 600m.asl

This elevation is about 500-600 m below the surface(Figure 2 The map shows a low resistivity anomallyof about 3.2 ohm-m in the caldera and on the easternand western sides of the prospect area. Thisconductivity as seen above could be related to thehydrothermal products present at this depth. Alsoseen in the map is a high resistivity anomally ofabout 56 ohm-m on the Southern side of the surveyarea trending in the NE-SW direction due tounaltered formations and it is along the major faultline cutting through the caldera in the western portionof the region.

Legend

TEM Soundings MT Soundings Cross-section lines

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Figure. 2. Resistivity anomaly map at 600 m above Sea level (masl)

Resistivity at Sea level

The map shows relatively high resistivity anomaliesof about 56 ohm-m in the central portion of the mapand trending in the NW-SE directions, and seems to

be extending to the east out of the caldera (Figure 3.).This higher resistivity is probably defining the steamdominated reservoir for this sector of the

prospect . Low resistivities of about 5.6 ohm-m arealso evident in the southeast and northeast of the map

bounding the high resistivity zone.These lowresistivities are probably due to the presence of lowtemperature alteration minerals.

Figure. 3. Resistivity anomaly map at sea level

Legend

MT Soundings Fumaroles Structures

Legend

MT Soundings Fumaroles Structures

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Resistivity at 3000 mbsl

At 3000 m below sea level (Figure 4), the deeperconductors are evident from the eastern part of the

prospect area and extending towards the central partof the caldera. These conductors could be definingthe top of the heat sources for this sector. A linear

high resistivity is also evident aligning along thewestern rim of the caldera which could be a boundary(structure) separating the resource inside the calderaand the one towards Kapedo.

Figure.4. Resistivity anomaly map at 3000 m below sea level

Resistivity at 5000 m. bsl

A low resistivity anomaly spreads almost in the entiresurvey area with more prominence in the eastern partof the prospect area from the centre of the caldera

(Figure.5). This probably is a heat source extendingtowards the Eastern part of the prospect area. A linear

high resistivity described in figure 4. is still evident atthis level.

Legend

MT Soundings Fumaroles Structures

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Figure.5. Resistivity anomaly map at 5000 m below sea level

Cross-sections

MT Cross-section NW-SE

In the central portion below the Silali prospect a highresistivity anomaly is evident which is probably amajor structure on the western part of the prospect at

depth (Figure. 6). A low resistivity anomally isevident at about 2 km depth and which seems to be

extending towards the eastern side of the caldera andwhich is interpreted as the heat source for thisgeothermal system. It is overlayed by a highresistivity anomaly at around 1km below sea leveland extending eastwards and this represents thereservoir level for this geothermal system.

Figure 6: 2D Resistivity cross-section along NW-SE

Legend

MT Soundings Fumaroles Structures

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MT Cross-section SW-NE

The section ( 7) shows two distinct low resistivityanomalies from a depth of 3 km. One is in the central

part of the caldera and the other is extending towardsthe eastern side of the caldera. The

deeper conductors are probably the heat sources forthis prospect. The anomaly extends further Eastoutside the caldera towards Akwichatis and Nasorotiareas which is the heat source for this geothermalsystem.

Figure. 7. 2-D Resistivity cross-section along SW-NE

DISCUSSIONS AND CONCLUSION

The low resistivity anomaly at shallow depth couldbe as a result of conductive alteration minerals whileat the deeper level highly conductive zone isinterpreted to be the heat source. A highly resistive

base-rock at depth is seen from the cross-sections.The orientation of the anomalies found in this surveyarea agrees with the major geological feature namelythe linearment and the NNW-SSE faluts.

The resource area based on the resistivity map at3000 m bsl is aligned to the eastern half of the Silali

prospect defined by the relatively high resistance (>50 m) due to steam dominated reservoir.

The Low resistivity (10m) anomally below the

depth of 6000m could be the heat source for thisprospect area. However, at shallow depth, the thinconductive layer may mean hydrothermally alteredlithology as well as low temperature alterationmineral.

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