Quantitative 2D VLF data interpretation

Abstract

The ability of the VLF-R (Resistivity) method to provide quantitative subsurface resistivity information is examined. The frequencies used in conventional VLF (15 to 30 kHz) provide the deepest penetrations of the multi-frequency, extended method of RadioMT. Both methods are considered. VLF data, being effectively single frequency, are insufficient to resolve 1D (vertical) structure in any detail. At the site investigation scale, however, it is the departures from the background (vertically uniform) structure that are of interest. Improved methodologies for the quantitative assessment of VLF data derive from advances in regularised inversion techniques. Hydrogeological and waste-site examples of VLF-R survey data, aided by wide-band (VLF/RadioMT) synthetic modelling and inversion studies, are used to illustrate their shallow (0 to 20 m) resolution capabilities in conductive environments.

Comparative study between filtering and inversion of VLF-EM profile data

“…The quantitative interpretation of single frequency VLF-EM data has been investigated by several authors (Beamish 1994(Beamish , 2000Chouteau et al 1996;Kaikkonen and Sharma 1998), who showed that quite detailed information about the subsurface resistivity distribution can be obtained from regularized inversion.…”

Application of the VLF-EM method with EEMD to the study of a mud volcano in southern Taiwan

“…Fortunately, a renovation of VLF-EM interpretation method was developed in the last two decades by adopting new inversion modeling techniques; therefore quantitative interpretation turned out to be possible nowadays. As a result, the VLF-EM survey is capable of yielding more accurate and reliable model of the subsurface structures (Beamish, 1994(Beamish, , 2000Monteiro Santos et al, 2006). However, the success of quantitative interpretation and inversion depends on the quality of the data (Jeng et al, 2007).…”

“…The reason for this is simply that the regularized inversion scheme attempts to solve the problem of 'equivalence but non-uniqueness' in EM sounding data by constructing a resistivity distribution with the minimum amount of structures that fit the data. In practical application, Beamish (2000) also indicated that single frequency VLF-EM data should be collected at a high lateral density with the intervals of 1 to 5 m to make the inversion more effective, and the degree of smoothing in the inversion should be decreased to reduce the global misfit of the inversion. Moreover, Jeng et al (2007) suggested that the S/N ratio is crucial in the inversion process, and an improperly filtered VLF-EM data may result in false models.…”

Landform characterization using geophysics—Recent advances, applications, and emerging tools

“…Signal penetration is a function of the frequency and material conductivity. For the purpose of landform studies, anthropogenic sources that operate at radio frequencies (radiomagnetotellurics, RMT) or only in the very low frequency (VLF) range of radio signals (Beamish, 2000), are most relevant. At these frequencies depths of penetration are typically on the order of meters to several 10s of meters.…”