Fax: 1-604-822-5485. A novel shape reconstruction method for 3D electromagnetic imaging Most of the techniques which are used for the electromagnetic imaging of the subsurface have in common that they mathematically define nonlinear inverse problems which are moreover severely ill-posed. This makes it extremely difficult to derive reconstruction algorithms which have the potential of being applied successfully not only to synthetically created data, but also to real data which are usually

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... Moreover, in many situations the objects of interest have a high contrast to the background parameter distribution. Due to the resulting strong nonlinearity of the inverse problem, the application of straightforward linearization techniques, as for example Born-type methods, is therefore often not justified. Due to the ill-posedness of these problems, even a small amount of noise in the data can cause artifacts in the reconstructions that might render them useless for practical purposes. There are several established techniques for addressing this inherent ill-posedness of the inverse problem, which are usually referred to as 'regularization techniques'. A well-known regularization technique is for example the Tychonov-Phillips regularization, which imposes some smoothness constraint on the images which are reconstructed. However, since in many applications the structures which are sought are not necessarily smoothly varying and might have a high contrast to the background parameters, the reconstruction of 'blocky' or discontinuous images might sometimes become more interesting. We will present a novel shape reconstruction method which is intended to be applied in situations where approximate values of the parameters inside these non-smooth, high-contrast structures are available, but the sizes, shapes, locations and geometry of these structures is unknown. This new shape reconstruction algorithm is a nonlinear inversion technique. It combines the advantages of the so-called adjoint-field inversion technique [2, 3, 8] , the level set technique for modelling propagating interfaces [9, 11] , and the fast forward modelling scheme EH3D [5, 6] for achieving fast, fully 3D inversions from relatively few electromagnetic surface-toborehole data.