Electromagnetic geophysical tomography

An electromagnetic tomographic method for subsurface imaging is described. It reconstructs the electrical conductivity and dielectric permittivity profiles of the medium under investigation from phase-shift measurements of the received signals. Ray-optic approximations are made so that the problem reduces to solving two linear systems of equations. This is accomplished by a stable algebraic reconstruction algorithm. The algorithm is iterative, it can be applied to different data-collection arrangements and incomplete-data problems. The method is tested with the cross-hole geometry using synthetic data. This geometry leads to an incomplete-data problem because the region of interest is investigated only from its lateral boundaries. Computer simulations show that the features of the host medium can be reliably reconstructed even in the presence of noisy data. The tomographic algorithm described has the very desirable characteristic of being self-correcting. The adverse effects on the final solution caused by error propagation and accumulation, which often spoil the performance of most direct algorithms when high throughput is required, are therefore not significant. The geotomographic method is of an active nature and the characteristics of the signals can therefore be carefully chosen to suit the application in question.

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