Fracture Detection and Determination Its Orientation Using Different Geoelectric Null-arrays

A comparative study of the behaviour of three null-arrays, midpoint null-array (MAN), Wenner-g null-array and Schlumberger null-array in response to a fracture, both in profiling and in azimuthal mode is presented. The term “geoelectric null-array” is used for direct current electrode configurations yielding a potential difference of zero above a homogeneous half-space.The main objective is to determine which array(s) best localise vertical structures or best identify their orientation. Forward modelling of the three null-arrays revealed that the Wenner-g and Schlumberger null-arrays localise vertical fractures the most accurately, while the midpoint null-array combined with the Schlumberger null-array allows accurate orientation of a fracture. Field test measurements were carried out to verify the theoretical results above a quarry in Les Breuleux with the three null arrays and many classical arrays. The results were cross-validated with quarry-wall geological mapping. In real field circumstances, the Wenner-g null-array showed to be the most efficient and accurate in localising fractures. The orientations of the fractures in accordance with the numerical results, were most efficiently determined with midpoint null-array, however Schlumberger null-array adds accuracy to the results. This study shows that geoelectrical null-arrays are better than classical arrays for the characterisation of fracture geometry.