Three-dimensional modelling of VLF data

Abstract

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 smallscale, isolated subsurface features.2

Quantitative 2D VLF data interpretation

“…For resistivity mapping purposes the use of two azimuthally distinct transmitters is recommended (Beamish, 1998). The two joint data sets allow rotational invariants to be formed thus overcoming the dependence of anomaly response using single transmitter data.…”

The VLF-EM imaging of potential collapse on the LUSI embankment

“…The quantitative interpretation tool has been developed by several researchers (Baranwal et al, 2011;Beamish, 1994Beamish, , 1998Beamish, , 2000Kaikkonen and Sharma, 1998;Monteiro Santos et al, 2006;Oskooi and Pedersen, 2005). Generally the inversions use finite element method (FEM) or finite difference method (FDM) as forward modeling tool.…”

A VLF survey using current gathering phenomena for tracing buried faults of Fethiye–Burdur Fault Zone, Turkey

“…Although, some modeling and inversion, techniques for interpretation of VLF data tried by several authors (Beamish, 1998(Beamish, , 2000(Beamish, , 2002Pedersen and Oskooi, 2004;Pedersen, 2005, 2006), they pointed out that single frequency VLF measurements have restrictions to resolve vertical structures and the lateral positions of the deep conductors unless the aid of wide band frequency RMT method.…”