Multi‐frequency electromagnetic method for inductive measurement of ground induced polarization and resistivity

Abstract

ABSTRACTA geophysical electromagnetic method to inductively measure the ground electrical resistivity and induced polarization has recently been tested. Its basic characteristics involve three major differences from other methods: the two electrical ground parameters are obtained through measuring magnetic field. For this purpose, a transmitter–receiver (T, R) electromagnetic system is used that operates in the frequency domain and consists of a horizontal loop as the transmitter for the perpendicular loops configuration on the ground surface; the measured function is the (T, R) inductive coupling main variation produced due to the presence of the earth, that is the magnetic field radial component; the measurements are conducted at a large number of frequencies (139 in the more advanced prototype), and the measured function is explored in the frequency interval 0.2 Hz to 1 kHz, a much broader frequency range of the induced polarization effect spectrum, than the one conventionally used in field exploration. Three major aspects are emphasized: (1) the existence of a small ‘main zone’ interior to a half‐space, which is responsible for most of the magnetic energy that the receiver measures on the half‐space surface. This permits to substitute the entire half‐space by the ‘main zone’ and, in a second step, to substitute the ‘main zone’ by an equivalent homogeneous half‐space with the electrical characteristics of such ‘main zone’; (2) the existence of a closed solution for the fields that the (T, R) system generates on the surface of a homogeneous isotropic half‐space, which provides exact functions with the two electrical parameters of interest as the variables (the apparent resistivity and relative polarization parameter); (3) the values of the electrical parameters so determined can be attributed to the central point of the ‘main zone’. Three‐horizontal layers half‐space and a conductive sphere in the free‐space are discussed as models. Four field surveys are analysed as examples and show a satisfactory performance of the method for detection of on‐shore hydrocarbon reservoirs, description of induced reservoir variations and structural features mapping at depths up to 2.5 km.