Electromagnetic profiling interpretation across vertical faults and dikes

When a vertical fault or dike scatters a normally incident TE-mode plane wa ve, the horizontal components of the electric and the magnetic fields vary along the direction perpendicular to the strike. This scattering is also re sponsible 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 identificati on 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 con ductivity of each medium. Previous exact analytical solutions have shown th at 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 s eries, we calculated the following functions: the dip angle and the ellipti city of the vertical polarization ellipse in a plane perpendicular to the s trike of the structure, and the azimuth and the ellipticity of the horizont al ellipse. We established master-curves of these functions for the interpr etation of vertical faults and dikes for the polarization ellipsoid, for in stance VLF or audio-frequency methods. This representation has as variable the induction number theta (2) = root omega mu (0)sigma (2)x, and as parame ters sigma (2)/sigma (1) and theta (a) = root omega mu (0)sigma (2)a, where a denotes the half-thickness of the dike. The interpretation procedure usi ng curve fitting is possible because the induction number is represented on a logarithmic scale. This procedure represents an important step before au tomatic 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.