Subsurface geoelectrical structure from 3-d inversion of magnetotelluric data of Gisenyi geothermal field, western part of Rwanda

Abstract

In this paper, a set of Magnetotelluric (MT) data collected from 69 stations at the Gisenyi geothermal field in the western part of Rwanda were used to derive a 3D electrical resistivity distribution model to investigate deep geologic structures related to the geothermal resources. The MT method is a geophysical technique commonly used in geothermal exploration projects. The aim of this study is to investigate subsurface electrical resistivity distribution in the Gisenyi geothermal area to gain an understanding of the process related to geothermal activities and to model deep crustal structures. MT data analysis using phase tensors revealed the nature of the subsurface structures of the geothermal area prior to performing a 3D MT inversion. The dimensionality analysis results revealed a 3D conductivity structure in the deeper portion. The ModEM 3D MT inversion code was used to infer the subsurface electrical distribution from the MT responses. The recovered 3D resistivity distribution model showed a conductive region (<10 Ω.m) at deeper depths beneath a thick high-resistivity zone with a resistivity more than 100 Ω.m. The 3D model indicates patches of high and low resistivity at shallow depths related the presence of recent volcanic lavas and basement rocks from the surface to depths of a few hundred meters. The MT model revels important subsurface structures that can be related to the geothermal resource in the area. The deep-seated conductive body is presumably associated to the heat source of the Gisenyi geothermal system.