Enhancement of transient electromagnetic soundings: A metallic model study

Abstract

Transient electromagnetic (TEM) soundings can be enhanced by additions of matched filtered signals from different coil separations. Results of laboratory metallic model studies are used to demonstrate this technique. A short pulse of current is transmitted from one coil. For each of the signals received in the other coil, the half‐space response (surface wave) is subtracted from the total signal to determine the reflected signal. These reflected signals are passed through matched filters corresponding to the proper separation and a possible layer depth. All of the matched filtered signals corresponding to a particular possible depth are added, and the summed output is stored. The filtering and summing process is repeated for a set of possible interface depths. The sum of matched filtered signals for the correct depth has a peak value of 1 and is symmetric about the peak. We believe that short‐range early‐time values of the transient response can be used to estimate the conductivity of the first layer. This estimate is used for numerical computations of half‐space responses. We have used several magnetic dipoles to represent the finite‐sized coils to show good agreement between theoretical and experimental half‐spaces. Theoretical reflection signals for matched filters can be computed numerically. The stacking of matched filtered signals is an electromagnetic analog of the common‐depth‐point (CDP) seismic technique. It should reduce the effects of nearsurface inhomogeneities and improve the resolution. As an incidental experiment, we show comparisons of the reflected signal and the “image transmission.” The reflected and transmitted signals are essentially the same and support the concept of hybrid‐ray theory for electromagnetic sounding.