Application of Transient Electromagnetics in Static Shift Correction for Magnetotellurics Data Case Study: Paka Geothermal Prospect in Kenya

Abstract

Paka geothermal prospect is associated with the Quaternary Paka volcano located along the Kenya rift, Although magnetotelluric (MT) method is prefered for sub-surface exploration for geothermal resource. The MT method is effective for delineating geothermal reservoirs which are characterized by high resistivity contrast between the reservoir and the cap rock (clay caps) that are located on top of the reservoir zone. Often, dc shift “static shift“ on the apparent resistivity on MT plots at all frequencies may be caused by shallow or near surface conductivity heterogeneity due to local geology and rugged topography often related with volcanic fields such as the Paka volcano. There are various techniques proposed to solve for static shift on MT data, which include the use of topographical data during data inversion process and the use of complimentary resistivity data from methods such as direct current (DC) method or the Transient electromagnetic method (TEM). In this study, we investigate for static shift on the MT data collected over the Paka geothermal field and the application of TEM data to correct for static shift on MT data caused by near surface conductivity heterogeneities at the MT station. The static shift values in this region were found to range from 0-2 an indication that the MT apparent resistivity curves were shifted up or down indicating either higher (shift up) or a lower (shift down) of the true apparent resistivity values. A look at the cross-sections reveal a shallow layer of low resistivity values which is probably the sub-surface heterogeinities responsible for the shift and if its not corrected leads to errors in the final interpretation of resistivity structure.