Electromagnetic coupling in frequency-domain induced polarization data: a method for removal

Electromagnetic (EM) coupling is generally considered to be noise in induce d polarization (IP) data and interpretation is difficult when its contribut ion is large compared to the IP signal. The effect is exacerbated by conduc tive environments and large-array survey geometries designed to explore dee per targets. In this paper we present a methodology to remove EM coupling f rom frequency-domain TP data. We first investigate the effect of EM couplin g on the IP data and derive the necessary equations to represent the IP eff ect for both amplitude and phase responses of the signal. The separation of the inductive response from the total response in the low-frequency regime is derived using the electric field due to a horizontal electric dipole an d it is assumed that at low frequencies the interaction of EM effects and I P effects is negligible. The total electric field is then expressed as a pr oduct of a scalar function, which is due to IP effects, and an electric fie ld, which depends on the EM coupling response. It is this representation th at enables us to obtain the IP response from EM-coupling-contaminated data. To compute the EM coupling response we recognize that conductivity informa tion is necessary. We illustrate this with a synthetic example. The removal method developed in this work for the phase and the pur cent frequency eff ect (PFE) data are applicable to 1-D, 2-D and 3-D structures. The practical utility of the method is illustrated on a 2-D field example that is typica l of mineral exploration problems.