Geophysical Exploration for Geothermal Resources: Rwanda's Experience

Abstract

Karisimbi Volcano is located at the junction between three countries: Rwanda, Uganda and Democratic Republic of Congo. Karisimbi is one of the volcanoes making a series of geological features connected to the western African Rifting system; perpendicular to it by an accommodation zone whose geothermal manifestations come to surface. Geophysical methods used to survey the three prospects consisted of Transient Electromagnetic (TEM), Magnetotelluric (MT) and Micro-seismic study. This work discusses challenges encountered all along the surveys and solutions brought to them. The quality of data was evidence that these methods suit the surveys in North Western Rwanda Geothermal prospects. Geothermal Exploration started in Rwanda in 1982. The French bureau of geology and mines started the survey but the scope of work was limited to reconnaissance studies. In 2006, when Geothermal Energy Exploration in Rwanda became a priority for the Government; intensive geophysical survey began. The Germany Bureau of Geosciences (BGR) started the works in North-Western Rwanda with the sub-contract of KenGen (Kenya Electricity Generating Company). The first results of the survey sparked the geothermal resource exploration in Rwanda. Two more surveys followed with the aim of narrowing the area of geothermal resources and delineate them to a reduced area. Transient Electromagnetic (TEM) and Magnetotelluric (TM) among other geophysical direct methods (Heat flow) were used. The results are presented and discussed throughout this work. The final phase of Exploration was conducted by New Zealand Institute of Earth Science and Engineering (IESE). During that work, existing geophysics data were gathered and a desk top study was carried out to refine the existing reservoir conceptual model with the new data. The results were used to evaluate the heat source size, depth to the geothermal reservoir to form a working model on which geothermal wells were sited.