INVERSION OF SURFACE AND BOREHOLE ELECTROMAGNETIC DATA FOR TWO‐DIMENSIONAL ELECTRICAL CONDUCTIVITY MODELS*

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Abstract
A method for inverting electromagnetic fields induced by a line source in an earth of two‐dimensional conductivity structure is developed. Certain unique features of the finite element method are used to construct an efficient algorithm for the accurate calculation of the Jacobian matrix of partial derivatives, and the resulting linearized equations are solved using the damped least squares method.Case studies of theoretical data generated from a simple model of interest in geophysical prospecting show that, in general, it is impossible to obtain, from surface data alone, accurate estimates of the conductivity of structures buried deeper than 0.2 skin depths under a conducting overburden. The addition of borehole data to the surface data is found to increase the resolving power of the electromagnetic method dramatically. In particular, the borehole data appear to stabilize the inverse when only a poor initial estimate of the likely structure is given.