A rapid inversion technique for transient electromagnetic soundings

Abstract Interpreting electromagnetic (EM) data is difficult because responses of three-dimensional (3-D) models of the Earth are complicated and expensive to calculate. To simplify interpretation we developed an approximate method based on the behavior of transient currents in a layered-Earth model to estimate the variation of resistivity with depth beneath a controlled-source EM survey. Responses measured in the vicinity of each source are approximately matched at each delay time by the magnetic field of a current filament in free space. This filament, which moves downward with time, is an image of the loop or grounded-wire transmitter used in the survey. A continuous resistivity profile with depth is estimated from the velocity of the filament. Our inverse solution is simple and rapid, and provides meaningful interpretations even when the measured responses are contaminated by random or geological noise. Although overshoots in the estimated resistivity profile and a spatial smearing of the zones of anomalous conductivity can occur, our technique seems to be less biased by 3-D effects than is computer-intensive, layered-Earth model fitting using constrained nonlinear optimization. This is because we do not artificially parameterize the Earth into a finite number of layers. In a direct comparison using the two techniques to interpret a set of field data from a geothermal site, we find that certain features of the interpreted models are similar, and thus probably essential, while others are not. Given the inherent ambiguities and ubiquitous bias in EM inversion, the smoothed resistivity profile produced by the approximate scheme often is adequate.