Magnetotelluric Static Shift Correction Using Time Domain Electromagnetics Case Study Indonesian Geothermal Rough Fields

Abstract

Indonesian geothermal prospect areas are generally located in the forest area, hills and mountains wich has very rough topographical conditions and also extreme difference elevation levels from the deepest valley to the highest peak. Those conditions cause a static shift effect in magnetotelluric data on apparent resistivity curves. Magnetotelluric (MT) is the best passive geophysical method for geothermal exploration. This method is effective for delineating geothermal reservoirs wich are characterized by high resistivity contrast between the reservoir and the cap rock (clay caps) that are located on top of the reservoir zone. Since the magnetotelluric method has a static shift problem in rough topographical areas, it should be corrected using shallow electromagnetic geophysically active methods such as the Time Domain Electromagnetic (TDEM) method. TDEM data is not affected by the natural electrical topography effect since the method injects a big artifcial electric current (8-20 Ampere) to the earth. TDEM data has high resolution for shallow resistivity structures (effective for 50m to 500m below the surface) where MT can not see the shallow zone in as much detail as TDEM. Then, for all Indonesian Geothernal MT surveys, we always conduct a TDEM survey as a standard procedures for correcting MT static shift effect at all MT sites. This paper will explain the MT static shift correction method using TDEM data.