Electromagnetic profiling interpretation across vertical faults and dikes

Abstract

When a vertical fault or dike scatters a normally incident TE‐mode plane wave, the horizontal components of the electric and the magnetic fields vary along the direction perpendicular to the strike. This scattering is also responsible for the formation of a vertical component of the magnetic field, which also varies along the direction perpendicular to the strike. Analysis of the lateral variations of the field components permits the identification of the type of geological structure, either fault or dike, as well as an estimate of the position of the geological contacts and the electrical conductivity of each medium. Previous exact analytical solutions have shown that two Fourier cosine integrals can express each field component. A series of functions specified for each model represent the kernel of each integral. From the field components obtained from the first non‐zero term of each series, we calculated the following functions: the dip angle and the ellipticity of the vertical polarization ellipse in a plane perpendicular to the strike of the structure, and the azimuth and the ellipticity of the horizontal ellipse. We established master‐curves of these functions for the interpretation of vertical faults and dikes for the polarization ellipsoid, for instance VLF or audio‐frequency methods. This representation has as variable the induction number , and as parameters σ2/σ1 and , where a denotes the half‐thickness of the dike. The interpretation procedure using curve fitting is possible because the induction number is represented on a logarithmic scale. This procedure represents an important step before automatic interpretation is carried out. Two case histories using field data illustrate the effectiveness of the procedure and the type of quantitative interpretation obtained from it.

Interpretation of VLF Measurements Related to Hydrogeological Surveys

“…The advantages of the specific electromagnetic method are that (a) it is very quick in fieldwork, (b) light equipment is used and (c) the data collected are quit easy to be interpreted in a qualitative manner. It is a favourable tool for many professional geologists-geophysicists as well of many researchers who apply VLF methods in addition with other geophysical methods in order to study hydrogeological problems as well as geotechnical ones (Benson 1997, Sampaio and Dias 2001, Hautot et al 2002, Sharma and Baranwal 2005, Drahor and Berge 2006a, b, Zlotnicki et al 2006). …”

The Equivalence of Inclined Uniaxial and Biaxial Electrical Anisotropy in Inhomogeneous Two-Dimensional Media for Homogeneous TM-Type Plane Wave Propagation Problems

“…The exact one-dimensional surface impedance expression for homogeneous plane wave incidence above a horizontally stratified (vertically inhomogeneous) earth has been well documented. Exact and approximate solutions for the surface impedance anomaly above a horizontally inhomogeneous half space with isotropic conductivity have been previously investigated for both TE-and TM-type waves [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The issue of a sloping contact has been considered theoretically by Dmitriev and Zakharov [16] and Geyer [17], and numerically by Reddy and Rankin [18].…”