Michel Cauterman

Numerical modeling for electromagnetic remote sensing of inhomogeneities in the ground

After a brief description of the mathematical formulation, we show some of the benefits that can be derived from numerical models. One of these benefits is to furnish an estimation of the suitability of electromagnetic prospecting methods with regard to specific sounding tasks. Using as an example the detection of a magma pocket with a line-source of current or an electric antenna as excitation, we also call attention to the limit of validity of two-dimensional models. Then, we consider the remote sensing, from the earths surface, of cavities embedded at small depths. We point out the disturbing effect of a conducting overburden on the efficiency of electromagnetic methods. A three-loop system is proposed and the various prospecting parameters of this system are conveniently optimized with the help of the numerical model. This three-loop system is then shown to be far superior to the standard two-loop method.

Shielding Performance of Triply Shielded Coaxial Cables

We determine the shielding effectiveness of a triply shielded coaxial cable and compare it to the performance of a singly shielded cable. Then we apply well-known transmission-line theory to make a parametric study of the shielding performance as a function of the geometrical and electrical parameters of the braids. We suppose that the three braids are homogeneous, in order to use exact expressions relating the transfer impedance to the shield characteristics. We study the influence of the intersheath space on screening effectiveness and on the intersheath resonances. To damp these resonances, a material having a low conductivity could be used between the braids. To improve the shielding at low frequencies, an intermediate copper braid can be replaced by a ferromagnetic material. We take into account the variation of the permeability as a function of frequency and we point out, in this case, the effect of the intersheath space.

Transient Response of a Deposit Excited by a Pulse of Current

Three-dimensional numerical models have been used to compute the transient response of an heterogeneity. The source is a pulse of current injected in the ground between two electrodes. In a first step the conductivity of the deposit is assumed to be a constant whatever the frequency. Then an induced polarization effect is taken into account by introducing a complex conductivity varying with ferquency. Changes in the late response of the anomaly are pointed out.

Simulation of the High-Frequency Response of a Heterogeneous Ground through the Finite Difference Technique

To simulate the propagation of an electromagnetic plane wave in an inhomogeneous ground, the finite difference approach can be used. One of the main problems in using this method is imposing the boundary conditions near the ground surface, especially at high frequency. Indeed, for the E polarization, the upper top of the numerical grid must be sufficiently far away from the air-ground interface in order to neglect the field due to the heterogeneities and the discretization of the atmosphere is necessary. For magneto-telluric modeling, improved boundary conditions have already been proposed. This paper deals with a new condition, valid everywhere in air and which can be applied for E and H polarization. Thus even at high frequency, as for radar applications, only one line is added to the grid discretizing the ground.