Influence of the off-axis position of the transmitter and receiver circuits on the results of differentially normalized electromagnetic sounding

Abstract

In the differentially normalized method of electromagnetic sounding (DNME), the transmitter and receiver are grounded electrical circuits. The conduction and polarization properties of a section are studied by measuring the electrical potential difference (Δ U( t)) and the second potential difference (Δ 2 U( t)); the latter characterizes the spatial inhomogeneity of the electromagnetic field. Measurements of Δ 2 U( t) are strongly influenced by three-dimensional inhomogeneities within the receiver spread. To reduce this effect, measurements are made in two positions (left and right) of the transmitter circuit relative to receiver with subsequent averaging of the measured data. Often in field studies, the transmitter and receiver circuits are at an angle to each other, and the use of two transmitters in measurements leads to the need to determine a generalized transmitter for one-dimensional forward numerical modeling of field data. The effect of the off-axis (diagonal) position of the transmitter and receiver circuits on the data of electromagnetic pulse sounding and their inversion for a one-dimensional polarizable conducting medium have been studied in real and numerical experiments. In modeling, the effect of induced polarization (IP) is taken into account by introducing the resistivity frequency dispersion (Cole-Cole equation). Validity of the calculation of the generalized transmitter is estimated for the solution of the one-dimensional forward problem with the inversion of field diagonal measurements. The effect of three-dimensional objects on the results of measurements using the above observation system is estimated by solving the 3D forward problem for a polarizable conducting medium.