3D Electromagnetic Inversion For Environmental Site Characterization

A 3-D non-linear electromagnetic inversion scheme has been developed to produce images<br>of subsurface conductivity structure from electromagnetic geophysical data. The solution is<br>obtained by successive linearized model updates where full forward modeling is employed<br>at each iteration to compute model sensitivities and predicted data. Regularization is applied<br>to the problem to provide stability. Because the inverse part of the problem requires the<br>solution of 10’s to 100’s of thousands of unknowns, and because each inverse iteration<br>requires many forward models to be computed, the code has been implemented on<br>massively parallel computer platforms. The use of the inversion code to image<br>environmental sites is demonstrated on a data set collected with the Apex Parametrics<br>‘MaxMin 1-8s’ over a section of stacked barrels and metal filled boxes at the Idaho<br>National Laboratory’s ‘Cold Test Pit’. The MaxMin is a loop-loop frequency domain<br>system which operates from 440 Hz up to 56 kHz using various coil separations; for this<br>survey coil separations of 15, 30 and 60 feet were employed. The out-of phase data are<br>shown to be of very good quality while the in-phase am rather noisy due to slight<br>mispositioning errors which cause improper cancellation of the primary free space field in<br>the receiver. Weighting the data appropriately by the estimated noise and applying the<br>inversion scheme is demonstrated to better define the structure of the pit. In addition,<br>comparisons are given for single coil separations and multiple separations to show the<br>benefits of using multiple offset data.