Title: Processing and inverting resistivity soundings from Mt. Longonot field, Kenya. An interpretation of the subsurface resistivity structure in the context of the geothermal field

Abstract

Exploration of geothermal resources has become an important practice in volcanic regions.
Geophysical methods play a major role in surface exploration studies of geothermal resources.
Among the geophysical methods used in exploration are resistivity methods as well as seismic,
gravity and magnetic methods. Electrical resistivity methods, Magnetotellurics (MT) and Transient
Electromagnetics (TEM), which are commonly used in mapping the subsurface resistivity variations,
are described in this work.
The MT and TEM data, which are used in this study were collected in the Mt. Longonot high
temperature geothermal field in Kenya, located within the Great East African Rift System. As a part
of this project the data were processed and 1-D inverted, and the results presented as resistivity
cross sections and depth slices.
Strong geothermal manifestations and recent eruptions are indicative of a heat source under the
caldera, which could be at a shallow depth beneath the summit crater.

From the results found using 1-D modelling of the MT/TEM data, the resistivity structure of Mt.
Longonot field has mainly four resistivity sections. A high resistivity zone of unaltered
formations close to the surface, a low-resistivity anomaly observed at shallow depth, a high
resistivity zone associated with the change in alteration minerals from low to high temperature
mineralogy, and a conductive zone that dominates to the southeast of the study area and just below
the summit crater. It could be associated with a heat source.

It can be inferred from the study that Mt. Longonot is a fracture controlled geothermal system.
This is because the conductive layer is aligned along the
line with the Tectono Volcanic Axis structure and the
numerous volcanic centers to the southeast of the field.