Untitled - Orkustofnun

NorthingsBw’Obuya and Omenda23Development of conceptual model of Olkaria5.4 MagneticsIn Olkaria, both ground and aeromagnetic data have been used to investigate the presence of ageothermal resource in combination with gravity. From the aeromagnetic maps several of theanomalies can be clearly correlated with surface expressions of volcanism such as craters, domes orcones, localised basaltic lavas or plugs. From these maps most of the volcanic centres tend to lie inareas with magnetic highs (positives). Sometimes a superimposed magnetic low (negative) exist; butthis is generally weak or zero.9910000990500099000009895000Olkaria WestOlkaria N-EOlkaria EastOlkaria DomesBhogal and Skinner (1971)analysed residual drapedaeromagnetic data flown at300m above ground surfacewithin the Olkaria area. Theirresults showed that thecentral geothermal area had apositive magnetic anomalytrending NW-SE (Figure 11).The anomaly is superimposedon a broad regionalnegative anomaly that coversthe entire southern LakeNaivasha region.Whereas, Mwangi and185000 190000 195000 200000 205000Bromley (1986) interpretedEastingsthe positive anomaly toFIGURE 11: Total magnetic intensity maprepresent de-magnetizedrocks due to alteration bychemical and thermalprocesses at reservoir depth, it is currently thought that the anomaly is related to the NW-SEgeological and structural development of the segment of the rift than demagnetisation by hydrothermalprocesses. The NW-SE trend is that of the main rift structural trend at Olkaria. A minor trend in themagnetic anomaly is in a NE-SW direction corresponding to the Olkaria fault zone.nT3400033875337503365033500333506. DISCUSSIONThe Olkaria geothermal system is closely associated with the Quaternary silicic volcanism in thesegment of the rift, which was active from late Pleistocene to Holocene epoch. The occurrence of latephase rhyolites (comendite) lavas and pyroclastics indicates the presence of shallow magma bodiessince it has been established that they are products of protracted fractional crystallization and crustalanatexis (Omenda, 2000; Macdonald et al.; 1987; Black et al., 1997). Such processes have thepotential to transfer large quantities of heat to the upper crust via the shallow crustal bodies. Resultsfrom seismics and magnetics indicate the presence of attenuating bodies at 6-18 km depth in OlkariaWest, NE and Domes fields (Simiyu et al., 1998a). The bodies also occur within regions of positivemagnetic anomaly (Mwangi and Bromley, 1986). The seismic data is also in agreement with recentgeological models that indicate that the bodies are discrete; fault controlled and experienced differentevolutionary histories (Black et al., 1997; Macdonald et al., 1987).The gravity survey of the shallow crust beneath Olkaria shows a general gravity high trending NNWand in line with the regional geological structure in the area. However, there are local highs that trendNE and inline with the recent fault trends (Figure 10). These local gravity highs are interpreted as dike