Evaluation of Electromagnetic Methods for Geothermal Reservoir Detection

Abstract

The size of low resistivity of the clay cap associated with a geothermal system create a target well suited for electromagnetic (EM) methods, and also make electrical detection of the underlying geothermal reservoir a difficult exploration problem. Using three dimensional (3D) numerical models we evaluate four EM techniques - magnetotelluric (MT), controlled source audio magnetotelluric (CSAMT), a long offset time domain EM (LOTEM) and short offset time domain EM (TEM), for use in geothermal exploration. All of these techniques can delineate the clay cap, but none can be said to unequivocally detect the reservoir. Our results, however, do indicated that an anomaly from a deep, conductive cap is due to the presence of the free electric charge at conductivity reservoir overlain by a larger, more conductive cap is due to the presence of the free electric charge at conductivity boundaries rather than Em induction; hence methods which rely on electric field measurements are superior to those where only the magnetic field is measured. Among the techniques and interpretation tools analyzed, we deem 2D interpretation of MT data the best means of reservoir detection. However, the maximum expected anomaly of 0.25 log units apparent resistivity and roughly 7" in phase form the reservoir will be evident only if high quality data are collected at closely spaced measurement sites. LOTEM electric field measurement look promising, especially since multi dimensional tools are being developed from LOTEM interpretation. Although CSAMT employs electric field measurements, it is not recommended for reservoir detection because the reservoir anomaly can be obscured by transmitter effects that cannot be reliably isolated. A combination of CSAMT and TEM measurements appears most appropriate for delineation of the clay cap.