Recent developments in quantitative interpretation of VLF-EM data

Abstract

The VLF-EM method has not experienced the same degree of advance in instrumentation and interpretation technique that has taken place in other EM survey systems. One reason for the lack of effort is due to some of the basic limitations of the method: first, the relatively high operating frequency limits the depth penetration; second, it is basically a single-frequency system (in the range of 15–25 kHz). Hence, there are fewer measurement parameters than in EM systems with active sources (multifrequency and transient), which limits the interpretation of complex conductors. Third, the azimuths of the VLF primary fields are fixed. Hence, conductors that do not couple well with the primary field remain undetected. Fourth, the method suffers from a lack of reliable interpretation procedures.

The first two deficiencies are inherent in the method. However, the last two deficiencies can be corrected, at least to a degree, by using portable VLF transmitters, so that all conductors can be detected, and by the development of suitable interpretation techniques. The interpretation procedure outlined in this paper requires information about the host rock resistivity and inclination of the conductors, which may be obtained from a VLF resistivity survey and from filtering of the tilt angle response. The problem of interpretation is worse in the case of airborne VLF data, since the measuring systems currently in use are neither properly calibrated nor corrected for aircraft motion, which limits any quantitative interpretation of airborne VLF data. Efforts are under way to improve the airborne VLF hardware for obtaining reliable field data and develop suitable interpretation procedures.