Active fracture characterization by cross-hole low frequency electromagnetic measurements: a theoretical approach

Detection and characterization of active fractures are important aspects of the study of hot dry rock (H.D.R.) and other geothermal applications. The cross-hole low frequency electromagnetic technique presents two advantages, i) it may detect the filling of the fracture itself (and therefore, only the active fractures) and ii) it has a fair sensitivity to conductive targets over a large range of distances. Results presented here refer to a simulation study aimed at determining solutions for the effect of the different parameters of simple and multiple fractures: orientation, size, electrical conductivity and thickness. The case of a susceptibility contrast is also considered but corresponds to very small signals. The distances between the holes are taken to range between one to several hundred meters; the power source (which simulates a vertical electrical cable) is assimilated to a linear source.