The application of electromagnetic frequency sounding to groundwater problems

Abstract The use of an electromagnetic frequency sounding method in groundwater investigations seems to be attractive for two reasons. First, a change in depth penetration of the current by changing the frequency is far more convenient than displacing the current electrode as is done in resistivity sounding. Secondly, in arid areas with a highly resistive surface layer the difficulties of introducing the current into the ground that are encountered in resistivity sounding are avoided. Our institute has carried out experimental electromagnetic frequency sounding surveys in two arid areas in South Tunisia. Our choice between the different varieties of this method was controlled mainly by practical considerations. Thus, we avoided the use of a mixed method in which either at the transmitting or at the receiving side conductive contact with the ground is used. We also avoided, because of the instrumental complications involved, methods in which the phase difference between the transmitted and the received signal is determined, and we restricted the measurements to the absolute value of the magnetic field. Finally, we used only horizontal transmitting coils because these allow a far greater strength of the transmitted field than vertical coils. The methods remaining after this elimination are the horizontal coils method and the perpendicular coils method. Both methods were used during the first months of the survey. It was found during this period, however, that the vertical coils system is far more sensitive to lateral changes in resistivity than the horizontal coils method. For this reason, the perpendicular coils method was abandoned. Using the horizontal coils system, in 85% of the cases response curves were obtained that permitted a good interpretation in terms of 2, 3 or 4 resistivity layers.