An approximate inversion algorithm for time-domain electromagnetic surveys

Time-domain electromagnetic surveys typically comprise numerous soundings, and any useful interpretation procedure must be rapid enough to cope with t he subsequent large amounts of data. Even though geological targets invaria bly display a degree of three-dimensionality, it is often possible to obtai n information about their structure from an Earth model constructed from th e results of one-dimensional inversions of each sounding. We derive from th is process an approximate inversion procedure. The observations from all so undings are averaged to generate a representative data-set which is then in verted using a rigorous one-dimensional algorithm to produce a layered back ground model. As by-products of the inversion, the sensitivities for the ba ckground model are available, as well as the value of the trade-off paramet er in the objective function being minimized. Model updates, which do not i nvolve re-calculating the full sensitivities, are then carried out for each sounding. Each update requires only the solution of a small matrix equatio n, and a few forward modellings. Two or three updates generally result in a marked decrease in the objective function, and hence an improvement in the model for that sounding. The technique is illustrated with synthetic data generated from a three-dimensional model, and with field data collected in Venezuela. The inversion procedure is tailored to produce piecewise-constan t models, and to use a robust measure of data misfit. For the field example , rigorous one-dimensional inversions provide the model for comparison. The approximate inversion is shown to provide much of the same information, bu t in a substantially reduced amount of time. (C) 1999 Elsevier Science B.V. All rights reserved.