Magnetotellurics Static Shift Correction Using Transient Electromagnetics Data from Suswa Geothermal Prospect in Kenya

摘要

In this study, we investigate for static shift on the MT data collected over the Suswa geothermal prospect 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 heterogeneities responsible for the shift and if it's not corrected leads to errors in the final interpretation of resistivity structure. Suswa is the southernmost of the quaternary volcanoes in the axis of the Kenyan rift. Suswa is unique in having two calderas; the outer and the inner. Magneto telluric (MT) method is preferred 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 Suswa 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).