Time domain electromagnetic‐induced polarisation: extracting more induced polarisation information from grounded source time domain electromagnetic data

Electrical induced polarization (EIP) surveys are used to detect chargeable materials in the earth. For interpretation of time domain EIP data a common procedure is to first invert the DC data (electric current on-time) to recover conductivity and then invert the IP data (current off-time) to recover chargeability. This DC-IP procedure assumes that the off-time data are free of secondary EM induction effects. To comply with this, early time data are often discarded, or not recorded. For mid-time data, an EM-decoupling technique, which removes EM induction in the observations, needs to be implemented. Usually responses from a half-space or a layered earth are subtracted. Recent capability in 3D time domain EM (TEM) forward modelling and inversion allows to revisit these procedures. In a TEM-IP survey, a high sampling rate allows early time channels of the EM data to be recorded. The recovery of chargeability then follows a 3-step workflow: (a) invert early time channel TEM data to recover the 3D conductivity; (b) use that conductivity to compute the TEM response at later time channels and subtract this fundamental response from the observations to extract the IP responses, and (c) invert the IP responses to recover a 3D chargeability. This workflow effectively removes EM induction effects in the observations and produces better chargeability and conductivity models compared to conventional approaches. In a synthetic example involving a gradient array we show that the conductivity structure obtained from the early time channel data, which are usually discarded, is superior to that obtained from the steady state DC voltages. This adds a further reason to collect these EM data. This article is protected by copyright. All rights reserved