Joint 1D inversion of MT and TEM data from Eburru, Kenya, and processing of gravity data from Theistareykir, NE-Iceland

Abstract

The use of MT and TEM methods in geophysical exploration has been successful in studying geothermal fields. They have been used widely in Iceland and Kenya to investigate subsurface resistivity structures and to delineate the most favourable places to locate wells. In this report, electromagnetic methods, MT and TEM, are discussed and joint 1D inversion of MT and TEM data from the Eburru field in Kenya applied. In addition, the gravity method is briefly discussed, and data collection and processing applied to microgravity data from Theistareykir, NEIceland, for the purpose of monitoring mass depletion of the geothermal field prior to and during exploitation. The first part of the report discusses the resistivity methods and their application to Eburru and the second part the microgravity method and its application in Theistareykir. The MT and TEM data collected from Eburru are located around the prospect; they were processed and 1D inverted. The results are presented as 1D resistivity models for each individual sounding, as well as resistivity cross-sections and iso-resistivity maps. The TEM data were used to correct for the static shift of nearby MT soundings through joint 1D inversion. The results indicate a high resistivity at shallow depths caused by unaltered rocks. In some areas low resistivity appears close to the surface which is attributed to geothermal manifestations.

A conductive cap is seen below the high resistivity, presumably corresponding to the smectite zeolite zone and reflecting an alteration temperature of 100-220°C. Below the conductive cap a resistive core is found, probably consisting of a mixed-layer clay zone and a chlorite-epidote zone reflecting an alteration temperature above 240°C. These temperature values might have decreased due to cooling. The resistivity reflects alteration, not necessarily the temperature. Still further down, a very low resistivity is found which probably indicates the heat source. The correlation between resistivity and temperature in the wells is discussed. Data collection of microgravity data is discussed as well as the processing of the microgravity data. Emphasis, however, is laid on the field method, to ensure good quality data. The data are presented as reduced gravity values.