VLF Ground Surveys, a Powerful Tool for the Study of Shallow Two‐dimensional Structures*

Fischer, Gaston; Quang, B. V. Le; Müller, Imré

doi:10.1111/j.1365-2478.1983.tb01100.x

Cited by 61 publications

(38 citation statements)

References 10 publications

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“…As discussed by Fischer et al (1983) and Beamish (1994) in order to ensure consistency with a 2-D approach, the directional VLF data must conform to one of the two principal modes of 2-D induction. The assumption of infinite strike (which defines the 2-D case) provides two decoupled modes involving separate combinations of the field components.…”

Section: D Modelling and Inversionmentioning

confidence: 99%

Quantitative 2D VLF data interpretation

Beamish

2000

Journal of Applied Geophysics

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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.

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Section: D Modelling and Inversionmentioning

confidence: 99%

Quantitative 2D VLF data interpretation

Beamish

2000

Journal of Applied Geophysics

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“…The way VLF interacts with 2D structure is discussed by Fischer et al (1983) and McNeill and Labson (1991). The assumption of infinite strike (which defines the 2D case) provides two decoupled modes involving separate combinations of the field components.…”

Section: Vlf In 2d and 3d Environmentsmentioning

confidence: 99%

“…Two electrodes are then positioned perpendicular to the direction of Hmax and are used to record Ex. As noted by Fischer et al (1983), the Ex electric field is simply measured at a right angle to the Hmax direction and no effort is made to find the direction for which this field would reach a maximum amplitude.…”

Section: Vlf In 2d and 3d Environmentsmentioning

confidence: 99%

“…The technique has since been extended to include a measure of the induced horizontal electric field component E perpendicular to H. This VLF-R measurement provides a surface impedance value (e.g., E/H), usually expressed as apparent resistivity and phase, using short (e.g., 5 to 10 m) electric dipoles. The VLF-R measurement contains only marginal information on the vertical resistivity structure (Fischer et al, 1983) because, in effect, only a single frequency is available (Mathieson and Crossley, 1982). These factors suggest that the strength of the VLF method lies predominantly in the definition of lateral gradients in the subsurface.…”

Section: Introductionmentioning

confidence: 99%

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Three-dimensional modelling of VLF data

Beamish

1998

Journal of Applied Geophysics

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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

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“…5 m) electric dipoles. The VLF-R measurement, although appropriate for a one-dimensional (l-D) application, contains only marginal information on the vertical resistivity structure (Fischer et al, 1983) because, in effect, only a single frequency is available. These factors suggest that the strength of VLF methods lies predominantly in the definition of lateral gradients in the subsurface resistivity structure.…”

Section: Introductionmentioning

confidence: 99%

Two-dimensional, regularised inversion of VLF data

1995

International Journal of Rock Mechanics and Mining Sciences &am

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Very low frequency electromagnetic (EM) methods using VLF transmitters have found many applications in subsurface geophysical investigations. Surface measurements involving both the vertical component of the magnetic field (VLF-EM or VLF-Z) and of the apparent resistivity (VLF-R) are increasingly common. Although extensive VLF data sets have been successfully used for mapping purposes, modelling and interpretation techniques which assess the third (i.e. depth) dimension appear limited. Given a profile of VLF-R measurements the main purpose of the present study is to demonstrate an automatic method for the construction of a resistivity cross-section. The technique used is one of a new generation of regularised inversion methods. These techniques attempt to overcome the problem of equivalence/non-uniqueness in EM sounding data by constructing the resistivity distribution with the minimum amount of structure that fits the data. VLF data represent a special case of plane-wave EM sounding in that they conform, in practice, to a single-frequency technique. This fact imposes a limitation in the amount of vertical resolution that we can expect using such data. In the case of two-dimensional modelling and inversion, resolution through the cross-section is a resultant attribute from both vertical and lateral resistivity gradients within the subsurface. In order to provide insight into the practical application of regularised inversion techniques to VLF data, both synthetic and field examples are considered. Both sets of examples are primarily concerned with VLF data applied to near-surface fault mapping where the main aim is to assess the location, dip and depth extent of conductive subsurface features.2

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VLF Ground Surveys, a Powerful Tool for the Study of Shallow Two‐dimensional Structures*

Cited by 61 publications

References 10 publications

Quantitative 2D VLF data interpretation

Quantitative 2D VLF data interpretation

Three-dimensional modelling of VLF data

Two-dimensional, regularised inversion of VLF data

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