Evaluation of electromagnetic methods in geothermal exploration

Abstract

Using three-dimensional (3-D) numerical models we evaluate four electromagnetic (EM) techniques magnetotellurics (MT), controlled-source audio magnetotellurics (CSAMT), longoffset ime-domain EM (LOTEM) and short-offset time-domain EM (TEM) for use in geothermal exploration. The size and low resistivity of a clay cap in a geothermal system make it an easy target for EM methods, whereas electrical detection of a deeper geothermal reservoir under the cap is a difficult exploration problem. Any of these techniques can delineate the clay cap, but none can be said to unequivocally detect the reservoir. Our results, however, do indicate that an anomaly from a deep, conductive reservoir overlain by a larger, more conductive cap is due to electric boundary charges; hence lectric field measurements are superior to those that employ only the magnetic field. Among the techniques and interpretation tools analyzed, we deem 2-D interpretation of MT data the best means of reservoir detection. However, the maximum expected anomaly of 0.25 log units in apparent resistivity and roughly 7O in phase from the r&eIvoir is &mpromised if & outflow regon is uresent. LOTEM electric field measurements look Dromising, esiecially since multi-dimensional tools are being deieloped f& LOTEM interpretation. Although CSAMT employs electric field measurements, it is not recommended for reservoir detection because the reservoir anomaly can be obscured by near-field effects that cannot be reliably isolated. A combination of CSAMT and TEM measurements appears most appropriate for delineation of the clay cap.