3-DIMENSIONAL MODELING OF VLF DATA

The VLF technique is being increasingly applied to environmental and h ydrogeological problems with a growing requirement for quantitative in terpretation. One of the main difficulties with the VLF method stems f rom the directional polarisation of the transmitted field. To clarify VLF data interpretation when subsurface targets are complex and three dimensional, a modelling study was undertaken at VLF frequencies. A ma in concern is the interpretation of target strike directions when the transmitters used are rotated from principal-mode (i.e., two-dimension al) directions. Results for transmitter/strike rotations of between 0 and 75 degrees of the two principal modes are presented and discussed. For mapping purposes, the invariant apparent resistivity and phase pr ovide important interpretational simplifications. For two-dimensional modelling/inversion, the field distortions introduced by directional c omplexity remain significant. The results also demonstrate the importa nce of apparent resistivity measurements in detecting and defining sma ll-scale, isolated subsurface features (C) 1998 Elsevier Science B.V. All rights reserved.