Untitled - Orkustofnun

NorthingsBw’Obuya and Omenda21Development of conceptual model of OlkariaThough it was possible to model vertical conductive features, possibly relating fault zones, it wasdifficult to isolate the cause of particular head-on resistivity anomalies to provide the resolutionrequired to locate the fault zones for the siting of wells.5.2.5 Transient electromagnetic methods - TEMUse of TEM method started at Olkaria in the 1990’s and our experience is that it gives betterresolution with depth than the Schlumberger method. However, for the Olkaria situation the depth ofpenetration of TEM is limited to about 700 m depths depending on the resistivity structure of thelocation. This is similar to that of Schlumberger soundings with a maximum distance of 3 km betweenthe current electrodes. Over one hundred TEM sounding stations have been covered in the greaterOlkaria area using an ungrounded loop of wire measuring 300 m x 300 m square. The data collected,processed and plotted in apparent resistivity maps in form of contours at various elevations (Figure 4).The data shows that the low resistivity anomalies are controlled by linear structures in the NE-SW andNW-SE directions and thatthe geothermal resource isOhm-m9908000confined within areas withresistivity value of less than10015 m at an elevation ofOlkaria N-E75 1400 m a.s.l. (Figure 8).99040009900000989600098920005.2.6 Magnetotellurics - MTThe resistivity is loweraround Olkaria West Field(OWF) than the area aroundEast (EPF) and North EastFields (NEF). The nearsurface difference inresistivity between the areasis caused by contrasts in thesubsurface geology. Analtered thick surficial layer ofpyroclastics occurring in theOlkaria West field is thecause of the near surface lowresistivity in the field(Omenda, 1994, 1998).The Magnetolluric resistivity technique is the latest method that has been acquired for geothermalexploration in Kenya. The method has gained favour than other resistivity techniques since it probesdeeper. The method is usually employed together with TEM, which provides shallow depthcomponent and serves to assist with MT static shift correction. Analysis of MT data from Olkariaindicates the presence of significantly enhanced conductivities below ODF, OWF and OEF (Figure 9).The deep anomalies have been associated with possible heat sources for the geothermal systems.5.3 GravityOlkaria West188000 192000 196000 200000 204000 208000EastingsOlkaria EastOlkaria DomesFIGURE 8: Resistivity distribution at 1400 m a.s.l.from TEM measurementsGravity survey of the shallow crust beneath Olkaria indicated a volcanic zone of three layers thatappears down-faulted in the Olkaria West area and showing low density (Ndombi, 1981). Gravityfurther revealed the presence of dense dike material along the Ololbutot fault zone. However, it is nowknown from geology that the N-S Olkaria Hill fault marks a major east-dipping fault that hasdownthown the Mau Formation to more than 3-km in the eastern area (Omenda, 1994, 1998).50302015100