Application of an improved technique for interpreting transient electromagnetic data

Abstract

An improved conductivity-depth transform method for interpreting time-domain electromagnetic sounding data is amenable to a wide range of transmitter waveforms, source/receiver configurations, and geoelectrical situations. The method is based on a deconvolution scheme that can take into account an arbitrary transmitted current pulse to estimate the step response of the earth from traditional off-time transient electromagnetic (TEM) measurements. Then a rapid imaging scheme that makes use of a current-filament image of the source is applied to transform step response decay curves into resistivity-versus-depth profiles. No assumptions are made regarding the early-time or late-time behaviour of the measured transients. Because of the absence of this or other constraints, the method allows considerable flexibility in its application to real data. This interpretation technique is also fast enough to permit real- time processing of airborne data. Some new approximate velocity-conductivity functions associated with horizontal and vertical airborne loop sources are derived from a study of the behaviour of electromagnetic fields diffusing through various quasi-layered models. Furthermore, new software has been successfully applied to several ground and airborne data sets, including a GEOTEM data set from Nevada. Comparison of these results with DIGHEM data, ground TEM data, drill hole logs, and geological control indicates that this improved interpretation method produces results that are more accurate than those generated using a commercial conductivity-depth transform scheme that was applied as part of the original survey.