The use of 3-D modeling in the design of a new measurement technique for near-surface surveys

A new technique for near-surface electromagnetic measurements is currently being developed Radic, 1994], which is based on purely magnetic measurements up to the MHz frequency range. Up to a factor the vertical derivatives of the horizontal magnetical elds can be used to replace the electrical elds in plane-wave methods. These derivatives are diicult to determine at low frequencies, but are large enough to be measured in the frequency range considered. Three-dimensional modeling-mainly based on the well-known code of Mackie et al. 1993]-was used to answer several questions concerning this technique. As the derivative is to be determined in a vertical hole, the measuring procedure itself may introduce a distortion of the elds. It was shown that this eeect is signiicant and has to be taken into account in the design of the sensor and/or the measuring procedure. Other modeling projects included: (1) the resolution of the method as compared with more traditional techniques of resistivity determination (2) the innuence of small-scale inhomogeneities, and (3) the use of tensorial measurements in complicated areas. Examples from these projects will be given. Under realistic conditions the derivatives to be determined are never larger than a few percent of the total eld. Therefore the modeling requires very high accuracy which can only be reached with very dense discretizations and an optimized iteration strategy.