Advances in electromagnetic geothermal exploration based on using of an indirect electromagnetic geothermometer

Abstract

Electrical conductivity of rocks could serve as a convenient proxy parameter to be used for indirect temperature estimation from the surface electromagnetic (EM) data, which is especially useful for electromagnetic sounding of geothermal areas. The paper presents a review of the recent advances in electromagnetic geothermal exploration based on using of an indirect EM geothermometer. It is shown that the temperature interpolation in the interwell space using EM sounding data results in a sharp increase of the estimation accuracy the latter being controlled by 4 factors: faulting in the space between the place where the temperature profile is estimated and related EM site, distance between them, meteoric and groundwater flows and lateral geological inhomogeneity. The accuracy of the EM temperature extrapolation in depth depends on the ratio between the borehole length and the extrapolation depth. In particular, when extrapolating to a depth twice as large as the well depth the relative error is 5-6%, and if the extrapolation depth is three times as large, the error is about 20%. This opens up the opportunity to use available temperature logs for estimating the temperatures at depths 3-10 km without extra drilling. Application of the indirect EM geothermometer in two geothermal areas (Soultz-sous-Forêts, France, and Hengill, Iceland) is discussed. It is shown that practical application of indirect electromagnetic geothermometer enables constraining the locations for borehole drilling, determining the heat sources and estimating the heat transfer dominating mechanisms.