Three-dimensional modelling of VLF data

The VLF technique is being increasingly applied to environmental and hydrogeological problems with a growing requirement for quantitative interpretation. One of the main difficulties with the VLF method stems from 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 main concern is the interpretation of target strike directions when the transmitters used are rotated from principal-mode (i.e., two-dimensional) directions. Results for transmitter/strike rotations of between 0 and 75° of the two principal modes are presented and discussed. For mapping purposes, the invariant apparent resistivity and phase provide important interpretational simplifications. For two-dimensional modelling/inversion, the field distortions introduced by directional complexity remain significant. The results also demonstrate the importance of apparent resistivity measurements in detecting and defining small-scale, isolated subsurface features