Mapping geothermal and structural features of the Nothern Kenya Rift with MT and gravity measurements

Abstract

A geophysical survey comprising over 400 MT and 515 gravity stations on the Korosi, Paka and Silali volcanoes in the northern Kenya rift was conducted to assess the occurrence of geothermal resources in the three major volcanic centers. A 1D joint inversion of MagnetoTelluric (MT) and co-located Transient ElectroMagnetic (TEM) has revealed a resistivity pattern consistent with the existence of several geothermal systems within the study area. Each geothermal system is characterized by a relatively resistive 100 ohm-m surficial layer overlying an ∼10 ohm-m low resistivity zone interpreted as the hydrothermally altered clay cap of the system. The cap overlies a higher resistivity zone of about 60 ohm-m with a top at about 1000 m depth, interpreted as a potential high temperature alteration zone. The trend of moderate high resistivity at the depth of the potential reservoir corresponds to the zone of intense faulting and fracturing as imaged on the surface.

Gravity models are dominated by 10 to 15 km wide gravity high of 8 mGal amplitude striking NNE along the inner rift corresponding to high resistivity below 2 km depth. Gravity lows due to structures shallower than 2 km depth at the Paka and Korosi volcanoes have been interpreted as low density bodies within their edifices, likely to consist of either unaltered near-surface pyroclastics or deeper tuffs altered at 60 to 180 °C to hydrothermal smectite clay. MT resistivity models were used to further constrain the 2.5D gravity models. The high resistivity, low density near-surface rocks on the flanks are interpreted to represent unaltered pyroclastics above the water table, whereas low resistivity, low density bodies underneath the Paka and Korosi volcanos indicate low density tuffs, hydrothermally altered to hydrated smectite clay. The deeper high density zone below the volcanic inner rift is likely to be a combination of higher temperature, low porosity alteration associated with geothermal reservoirs and/or denser rocks related to intrusions. Mapped fissure swarm agrees with both gravity and resistivity south of Paka volcano but northwards between Paka and Silali volcanoes there is inconsistency where the fissure swarm goes straight NE but both gravity and resistivity show a feature west of the currently active fissure zone. This suggests that the rift has recently moved eastwards.