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An Approximate Inversion Procedure For Lines/Grids/Collections Of Time-Domain Electromagnetic Soundings
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 11th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems, Mar 1998, cp-203-00015
Abstract
Time-domain electromagnetic surveys typically comprise numerous soundings. Any<br>useful interpretation procedure must be rapid enough to cope with these large amounts<br>of data. Even though the geological target invariably displays a degree of three-dimensionality,<br>it is often possible to obtain information about its structure from an Earth model<br>constructed from the results of one-dimensional inversions of each sounding. We derive<br>from this process an approximate inversion procedure. The observations from all soundings<br>are averaged to generate a representative data-set which is then inverted using a rigorous<br>one-dimensional algorithm to produce a layered background model. As by-products of the<br>inversion, the sensitivities for the background model are available, as well as the value of<br>the trade-off parameter in the objective function being minimized. Approximate inversion<br>steps, which do not involve re-calculating the full sensitivities, are then carried out for<br>each sounding. Each step requires only the solution of a small matrix equation, and a<br>few forward modellings. Two or three steps generally result in a marked decrease in the<br>objective function, and hence an improvement in the model for that sounding.<br>The technique is illustrated with synthetic data generated from a three-dimensional<br>model, and with field data collected in Venezuela. The inversion procedures are tailored<br>to produce piecewise-constant models, and use a robust measure of data misfit. For the<br>field example, rigorous one-dimensional inversions provide the model for comparison. The<br>approximate inversion is shown to provide much of the same information, but in a substantially<br>reduced amount of time.