Examples of data processed using a new technique for presentation of coincident- and in-loop impulse-response transient electromagnetic data

Abstract

Coincident- and in-loop transient electromagnetic (TEM) methods measure the response as a function of time after the current in a transmitter is switched off suddenly. Voltage measurements at each delay time are often normalized to an apparent conductivity so as to provide a crude indication of the subsurface conductivity structure. The first stage of the new processing technique is to employ a sensitivity analysis to associate each apparent conductivity with a depth. In cases when the actual conductivity structure varies rapidly with depth, the apparent conductivity curves tend to vary slowly as a function of depth. These slowly varying curves appear to be a smoothed version of the actual conductivity structures. The second stage of the processing algorithm is to calculate an approximate operator which is the inverse of the smoothing operator. Applying this inverse operator to the apparent conductivity curve yields a sharper, more rapidly varying estimate of the conductivity, termed the 'spiked conductivity'. The technique has been applied to field data in an area where the conductivity structure is essentially horizontally layered. The spiked conductivity shows greater contrast and resolution than the apparent-conductivity/depth curve and the results are consistent with those from a more sophisticated three-layer parametric inversion routine. The spiked conductivities are obtained easily and quickly and, unlike the inversion routine, do not require a priori knowledge. The processing technique can be installed on small computers, so results can be generated in the field and a first-order indication of the conductivity structure can be obtained quickly.