Summary

We present a practical formulation for forward modeling and inverting time domain data arising from multiple transmitters. The underpinning of our procedure is the ability to factor the forward modeling matrix and then solve our system using direct methods. We formulate Maxwell''s equations in terms of the magnetic field, H and discretize the equations using a finite volume technique in space and a backward Euler in time. The MUMPS software package is used to carry out a decomposition of the forward operator, with the work distributed over an array of processors. The forward modeling is then quickly carried out using the factored operator. The factorization allows traditional Gauss-Newton inversion mthodologies to be implemented with greater efficiency than could be obtained from iterative techniques. As a demonstration we invert VTEM data at Mt. Milligan which is a Cu-Au porphyry deposit in British Columbia. 1D inversions produce a conductive artifact at depth that is inconsistent with geology. 3D inversions however, even from a limited number of stations, yield a more realistic result. Through the use of a synthetic model that emulates the geology at Mt. Milligan, we are able to show why the geologic artifacts arise from the 1D inversions. Lastly for the field data, we show how the resolution of the inversion result is further enhanced as progressively more transmitters are added.

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