Multi-electrode resistivity imaging for environmental applications

For a few years, the evolution of electronic components and of computer processing have permitted to develop field resistivity equipment (SYSCAL Switch and SYSCAL Pro Switch units) which includes a large number of electrodes located along a line at the same time, and which carries out an automatic switching of these electrodes for acquiring profiling data. The apparent resistivity pseudo sections measured with such a technique are processed by inversion software which gives interpreted resistivity and depth values for the anomalies detected along the profile. The multi-electrode resistivity technique consists in using a multi-core cable with as many conductors (24, 48, 72, 96,) as electrodes plugged into the ground at a fixed spacing, every 5m for instance. In the resistivitymeter itself are located the relays which ensure the switching of those electrodes according to a sequence of readings predefined and stored in the internal memory of the equipment. The various combinations of transmitting (A,B) and receiving (M,N) pairs of electrodes construct the mixed sounding / profiling section, with a maximum investigation depth which mainly depends on the total length of the cable. The 2D resistivity images obtained with such a multi-electrode technique are used for studying the shallow stuctures of the underground located a few tens of metres down to about one hundred metres depth; these images supply an information which complements the one obtained with the more traditionnal Vertical Electrical Sounding (VES) technique, which mainly aims at determining the depths of horizontal ID structures from the surface down to several hundreds metres depths. Several examples are presented for various types of applications: groundwater (intrusion of salt water in fresh water), geotechnics (detection of a fault in a granitic area.), environment (delineation of a waste disposal area) and archaeology (discovery of an ancient tomb).