Electromagnetic coupling in ip measurements using some common electrode arrays over a uniform half-space

Abstract

Electromagnetic (EM) coupling between grounded wires gives rise to frequency dependence of resistivity and hence, spurious anomalies in induced polarisation (IP) surveys. A general expression for the EM Coupling thus involved is derived for gradient array over a homogeneous earth, and on assigning special conditions, it is reduced to give the coupling in other commonly used arrays. Computations are made to evaluate the percent frequency effect (PFE) as defined in the frequency domain method owing to EM coupling with gradient, Schlumberger, Wenner, collinear dipole—dipole, pole—dipole, three— and pole—pole arrays. A comparison of the EM coupling of various arrays is drawn from practical considerations of IP surveys. In arrays where the potential electrodes are placed in a space enclosed by the current electrodes, the EM coupling is negative in sign and where the potential electrodes fall outside the space covered by the current electrodes, the EM coupling is positive. On the central traverse in gradient, Schlumberger and Wenner arrays, the EM coupling remains indeterminate or tends to be very high. Gradient array can still be used in IP surveys by confining observations to traverses away from the central one. Dipole—dipole, pole—dipole, three— and pole—pole arrays introduce even less coupling compared to dipole—dipole array. Notwithstanding that the studies are confined to a homogeneous earth, the comparison does provide useful criteria to avoid EM coupling, or to keep it within limits while exploring under more complex conditions.