VLF-EM Data Research Articles

Mapping groundwater contamination around a landfill facility using the VLF-EM method — A case study

An electromagnetic survey to detect leachate flow and to map its spatial distribution using the VLF-EM method was carried out around a landfill that is operating since 1998. The survey comprised twelve, roughly E–W, VLF-EM profiles in the western part of the landfill where conductive anomalies had been detected in a previous resistivity survey (Wenner and dipole–dipole arrays). The VLF-EM data were interpreted qualitatively, using the Fraser and the Karous–Hjelt filters. The quantitative interpretation of the data was done with a 2-D code that performs the inversion of the tipper data, using the results of the previous resistivity survey to constrain the inversion. Fraser filtered data and relative current density pseudosections indicate the presence of shallow and deep conductive zones that cross the landfill facility along known fracturing directions. The 2-D resistivity models calculated from tipper data indicate that these zones may have resistivity lower than 400 Ω m that is in a good agreement with the results obtained from the previous resistivity data. Chemical analysis of both surface and groundwater collected inside and outside the landfill confirms the presence of a halo of water contamination around the landfill location. This enables to infer that the anomalies detected by VLF-EM data are due to contaminated groundwater flowing in connected fractures.

On: “Exploration of a Lignite‐Bearing Basin in Northern Ireland Using Ground and Magnetic VLF-EM Methods,” R. J. McCaffrey, John McElroy, and Graham Leslie (March‐April 1995 GEOPHYSICS, p. 408–412)

This paper presents an analysis of ground magnetic and VLF-EM data over a basin of several hundred acres which is an appendage to the larger Lough Neagh autogeosynclinal basin in northeastern Ireland. There is little to do with lignite and the VLM-EM of the title. There is quite a bit to do with the magnetic sector which is, however, not analyzed by time‐proven methods of depth determination but rather by running a synthetic, one dimensional first derivative over the traverses given the name MAGGRO.

New developments in resolving detail in aeromagnetic data

Stacked profiles of aeromagnetic data often fail to resolve and display detail as this can be obliterated by larger amplitude features on adjacent flight lines. Contours and images of interpolated data can also obscure detail because they are sensitive to survey levelling errors and can show erroneous distortions caused by the interpolation method. Mclntyre (1981) showed that shaded stacked profiles of the second horizontal differences of flight-line data are effective in detecting and displaying detail in aeromagnetic data. Mclntyre used a square-root function to control the dynamic range of the computed difference so that both small and large amplitude features could be displayed without being obliterated by larger amplitude features on adjacent flight-lines. The shaded profile preferentially enhances positive values. Improvements on Mclntyre's shaded stacked profiles of horizontal differences form the basis of a new method of presenting horizontal differences of aeromagnetic data: the bipole map. This is a series of stacked flight-line profiles of bar-graphs of the horizontal difference values. The heights of the bar-graphs are controlled by the amplitude of the difference values whilst polarity is depicted with colour: red for positive values and blue for negative values. The bar-graphs are centred and located precisely on the curved flight-line; the flight-line forming the centre-line of the profile. Wavelength dependent automatic gain control (AGC) filters are used to modify the dynamic range of the horizontal differences so that both small and large amplitude anomalies can be displayed. The bipole map visually enhances both negative and positive values, more precisely positions the data and does not depend upon dubious magnetic values interpolated between the flight-lines. Bipole maps of the first, third and fourth horizontal differences are effective in resolving detail in other parts of the frequency spectrum. They supplement contours and images of longer wavelength features. Magnetic models show the responses of the horizontal differences for various inclinations of the Earth's magnetic field and different widths of the magnetic source. In areas of steep inclination, the horizontal differences delineate the strike axis of thin bodies and the edges of thick bodies. Bipole maps of horizontal differences may find useful application in displaying other types of geophysical data such as VLF-EM and aeromagnetic gradiometer data. Bipole maps of higher order differences are an effective means of resolving high frequency (instrument) noise in geophysical data. For aeromagnetic data from the Mt Magnet goldfield, located in the Archean greenstone belt of Western Australia, bipole maps of the second horizontal differences revealed that an anomaly from a banded iron formation might be caused by several close-spaced banded iron bodies rather than a single body, as might first be interpreted from the contours and images of the gridded data. Bipole maps of the fourth horizontal differences revealed a series of NNE striking magnetic lineaments which are not apparent in the contours and images of the gridded data.

Interpretation of ground VLF-EM data in terms of vertical conductor models : Sinha, A K GeoexplorationV26, N3, April 1990, P213–231

Interpretation of ground VLF-EM data in terms of vertical conductor models : Sinha, A K GeoexplorationV26, N3, April 1990, P213–231

Interpretation of ground VLF-EM data in terms of inclined sheet-like conductor models

The response of two-dimensional, inclined, sheet-like conductors with, low conductance values to plane wave electromagnetic fields in the very low frequency (VLF) range has been evaluated by using a numerical technique. The conductance values of the conductors considered are appropriate for those produced by water and/or clay-filled fracture and shear zones in the Precambrian crystalline rocks of the Canadian Shield. The surrounding host rock was assumed to be, resistive with resistivities in the 1–10 kΩ.m range to reflect the high resistivities over the shield areas. No overburden was assumed in this analysis.

Model studies of topographic noise in vlf-em data: Accounting for the direction of morphological strike relative to survey line and magnetic field directions

The effect of a sloping ground surface on electromagnetic-field measurements have been studied in models and verified in field studies. Slopes up to 30° have been simulated with plane conductors varying from vertical to 30° dip. A simple linear relationship has been established between ground slope and the amplitude of the real component of the resultant EM field due to a buried conductor. The use of this simple topographic noise correction is demonstrated in two field examples. The significance of the orientation of the conducting body in relation to the primary EM field and to the ground slope is demonstrated.

A method of reducing terrain relief effects from VLF-EM data

In VLF-observations uneven terrain relief causes distinct anomalies that originate from elliptic field polarization. This slope rotation field is produced by the interference of the radiated horizontal magnetic field with a component reflected by the earth's inclined surface. A diagram has been developed to provide the tilt and eccentricity (ratio of half axes) of the slope rotation ellipse, if the dip of the slope along the direction of the radiated magnetic field is known. It turns out to be possible to separate the influence of relief from that of local conductive bodies. The application is demonstrated on field data gathered in areas of hilly terrain.

Uranium Exploration Systems Case Study--Copper Mountain, Wyoming: ABSTRACT

End_Page 679------------------------------Regional surveys, including studies of rock chips and aerial radiometrics and magnetics, focused within an area of 400 sq km on the Canning stock, found enrichment in U, Th, and K. Stream-sediment data indicated an anomalous block adjacent to and including the North Canning and Fuller deposits. A subregional soil survey noted overlapping anomalies of eU, xU, Pb, Cu, and Ba in the vicinity of the North Canning deposit. This was confirmed by overlapping anomalies of eU, xU, Pb, Cu, and Ba in the rock-chip survey. Integrated radon and soil helium data were ineffective in delineating drilling targets at the subregional scale. The subregional magnetic, VLF-EM, and resistivity data confirmed the presence of a fractured and crushed zone within the host granite of the North Canning deposit. T is interpretation is based on the presence of an overlapping low resistivity zone and a magnetic depression of about 40 gammas. Core holes were sited within an area of 25 sq km that included the North Canning deposit, the leachable-uranium-in-rock-chip anomaly, the aerial radiometric anomaly, and overlapping geochemical (xU, eU, Ba, He, Rn, As, Ni, and Pb) anomalies in the detailed soil survey. These holes, logged with a multispectral (K-U-Th) probe, show zonation of K, eU, and eTh in the monzonite and granitic host rock and indicate both directional and genetic information related to the origin of the uranium deposit. End_of_Article - Last_Page 680------------