Characterisation of Active Fractures by Low Frequency Single Hole and Cross Hole Electromagnetic Measurements

Abstract

Water circulation in is the dominant process for thermal exchanges and in rocks,the detection and the characterisation of active is thus an important element in geothermal studies. Low Electromagnetic methods can be used to characterize fractures when injected fluids are significantly more conductive or magnetic than the medium. They provide information over metric, plurimetric and ranges of distance from the wall so completing information supplied by wall imagery techniques. We propose here the principle of an apparatus which realizes both single and cross hole In the first application three orthogonal transmitters and orthogonal receivers 1.2 apart are used. The same sensors are used for the signal transmitted in by vertical Although in complex situations, where several discordant families are present, the orientation is to be restitute, the sensitivity of measurements, in both modes, to single fracture orientation is good. In cross hole, the responses are approximately proportional to the fracture area and conductance.