Recovering IP information in airborne-time domain electromagnetic data

Abstract

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) Apply 3D inversion to TEM data to restore a background conductivity. This might involve omitting responses that are obviously contaminated with IP signals, such as negative transients in coincident loop surveys. The recovered background conductivity is assumed to be uncontaminated by IP signals. (b) Compute the TEM response from the background conductivity and subtract it from the observations. This yields the dIP data, and reduces the EM coupling. (c) The background conductivity is likely not exactly the earth conductivity, but we assume that the major effects of this inaccuracy will lead to a large scale, smoothly varying perturbation to the dIP data. If this correct, then these can be recognized and 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 dIP data 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 a field data set from Mt. Milligan. In the field example, we identify chargeable targets that show no indication of negative transients in the raw data. From the images we can make inferences about the relative strength and geometries of the chargeable bodies.