On recovering induced polarization information from airborne time domain EM data

Abstract

In this paper we propose a methodology to generate a 3D distribution of pseudo-chargeability from airborne time domain electromagnetic data. The processing flow is as follows: (a) Invert the TEM data to generate a background conductivity. This may involve omitting data that are obviously contaminated with IP signals, such as the existence of negative transients in coincident loop surveys. It is assumed that this is a background conductivity that is uncontaminated by IP signals. (b) Compute the EM response from the background conductivity and subtract it from the observations. This yields the d^IP data, and removes the EM coupling. (c) It is recognized that the background conductivity is likely not correct, but we assume that the major effects of this inaccuracy will lead to a large scale, smoothly varying perturbation to d^IP data. If this correct, then these are easily recognized and can be separated. This process is similar the problem faced in potential fields where a smooth background field needs to be removed. (d) The final data are linearly related to a pseudo-chargeability through a sensitivity function that is analogous to that employed in usual DC-IP ground surveys. (e) The d^IP at various time channels can be inverted individually. The pseudo-chargeability models may be useful in themselves or they may be further processed to estimate Cole-Cole, or equivalent, parameters. We demonstrate our procedure on synthetic data and on a field data set from Mt. Milligan. For the field example we are able to identify chargeable targets that showed no indication of negative transients in the raw data. From the images we could also make inferences about the relative strength and depths of the chargeable bodies.