Application of Time Domain Electromagnetic Method to Monitor the Shallow Resistivity of a Geothermal Field

Abstract

TDEM is an electromagnetic technique applied during the exploration stage of geothermal fields and to keep monitor its evolution during the production and maintenance stage. Its fundamental base lays in the injection of an electrical current through a transmitter square loop laid on the ground which induce a magnetic field that travels through the ground layers. Measurements of the resulting magnetic field are made in a receiving coil in the center of the loop since this is related to the resistivity of the ground rock layers. Due to its easy deployment, low electromagnetic noise susceptibility and low cost TDEM is used to monitor the shallow resistivity instead other techniques. Since the prospection depths are not higher than 1000 m, TDEM is used to monitor the resistivity variations caused by the changes of the chemistry of the rock due to the interaction of geothermal fluid. This way monitor a geothermal field becomes necessary to anticipate problems that could affect the supply of fluid from the reservoir or endanger the geothermal powerhouse, surrounding buildings or houses inside the geothermal area as a result of the loss of the mechanical strength.