Multi-frequency electromagnetic method for inductive measurement of ground induced polarization and resistivity
D.A. Dias, H.K. Sato, M.V.B. Machado and O.A.L. de Lima
Journal name: Geophysical Prospecting
Issue: Vol 67, No 1, January 2019 pp. 210 - 225
Info: Article, PDF ( 26.73Mb )
A 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.