Háskóli Íslands

Ph.D. student: Charles Muturia Lichoro

Dissertation title: Regional Geothermal Characterization Of Northern Kenya Rift From Resistivity And Gravity

Opponents: Dr. Egidio Armadillo, Professor at the University of Genova
Dr. Peter Omenda, Chief Manager of Energy Research and Development at GDC

Advisors: Knútur Árnason, ISOR Iceland GeoSurvey and William Cumming, Cumming Geoscience, USA,

Supervisor: Dr. Andri Stefansson, Professor at the Faculty of Earth Sciences at the University of Iceland

Other members of the doctoral committee:
Bryndís Brandsdóttir, Research Scientist at The Institute of Earth Sciences, University of Iceland
Lúðvík S. Georgsson, UNU-GTP, Orkustofnun

Chair of Ceremony: Dr. Magnús Tumi Guðmundsson, Professor and the Head of the Faculty of Earth Sciences at the University of Iceland.

Place and time:  December 13th at 13:00 in the Aula - main building of the University of Iceland

Abstract:

Korosi, Paka and Silali volcanoes in the northern Kenya rift was conducted in order to assess the occurrence of geothermal resources in the context of the geophysics of the major volcanic centers. Additional study has also included speculation of magma detectability below the volcanoes.

Resistivity data has been interpreted using 1D joint inversion of magnetotelluric (MT) and co-located transient electromagnetic (TEM) which 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 Ωm surficial layeroverlying a ∼ 10 Ωm low resistivity zone interpreted as the hydrothermally altered claycap of the system. Low resistivity is also correlated with clay-rich volcanoclastics deposited on the margins of the rift adjacent to the volcanoes. The cap overlies a higher resistivity zone of about 60 Ωm 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 data has been analysed through estimation of shallow density by testing the correlation of measured gravity with topography, reviewing results of earlier density measurements of surface rocks in the Kenya Rift, comparison to regional gravity surveys that constrained larger scale deeper density contrasts and 2D gravity modeling correlated with the MT resistivity pattern. Gravity models reveal a dominant 10 to 15 km wide gravity high of 8 mGal amplitude striking NNE along the inner rift corresponding to high resistivity outlined at 2 km depth by recent MT studies. 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-surfacepyroclastics or deeper tuffs altered at 60 to 180°C to hydrothermal smectite clay.

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. Away from the volcanoes on both East and West low density and low resistivity anomalies flanks the central gravity high. 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 tointrusions.

Surface fissure swarm correlates with both high gravity and resistivity south of Paka volcano but northwards between Paka and Silali volcanoes, there appears inconsistency where the fissure swarm strikes NE but both gravity and resistivity shows a feature west of the currently active fissure zone. This suggests that the rift has recently moved eastwards.

A preliminary review of geoscientific data has been made to evaluate the developed geothermal systems within the Kenya rift and use them as analogs for the geothermal prospects. The comparisons are based on thermal features, geological structures, gravity anomalies and resistivity signatures. Further, 2D synthetic modelling was performed to speculate on the size and depth of magma beneath the volcanoes and demonstrate what can be resolved at depth and what cannot. This has also been used to illustrate the limitations of 1D inversion and test the depth and size of magma that can be detected.

About the doctoral candidate:

Charles was born in Kenya in December 1973. He graduated from the Technical University of Mombasa with Diploma in Electronics Engineering in 1997 and later proceeded to Egerton University in Kenya where he graduated with a Bachelor of Technology (Engineering Option) in 2006. He then obtained a Diploma in Geophysical Exploration from the University of Iceland under UNUGTP program in 2009 and later was enrolled at the University of Iceland for a Master’s degree in geophysics. In 2015 he started his PhD work at the University of Iceland focusing on geothermal characterization of the Northern Kenya Rift using Resistivity and Gravity methods. Charles is married and has three sons and a daughter. He has been working for Geothermal Development Company (GDC) since 2009 and previously worked for KenGen in Kenya.

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