building site characterization. Recently there has been consid erable progress in the direct inversion of magnetotelluric and other low-frequency induction data for multidimensional elec ABSTRACT A new method of the resistivity imaging based on trical conductivity structures (Berdichevsky and Zhdanov, frequency-domain electromagnetic migration is devel 1984; Smith and Booker, 19881991; Wannamaker et aI., 1989; oped. Electromagnetic (EM) migration involves down Eaton, 1989; Madden and Mackie, 1989; deGroot-Hedlin and ward diffusion of observed EM fields whose time flow
One of the most challenging problems in electromagnetic (EM) geophysical methods is developing fast and stable methods of imaging inhomogeneous underground structures using EM data. In our previous publications we developed a novel approach to this problem, using EM migration.In this paper we demonstrate that there is a very close connection between the method of EM migration and the solution of the conventional EM inverse problem. Actually, we show that migration is an approximate inversion. It realizes the first iteration in the inversion algorithm, based on the minimization of the residual field energy flow through the profile of observations. This new theoretical result opens a way for formulating a new imaging condition. We compare this new imaging condition with the traditional one, obtained for simplified geoelectrical models of the subsurface structures.This result also leads to the construction of a solution of the inverse EM problem, based on iterative EM migration in the frequency domain, and gradient (or conjugate gradient) search for the optimal geoelectrical model. However, the authors have found that in the framework of this method, even the first iteration, based on the migration of the residual field, generates a reasonable geoelectrical image of the subsurface structure.
IntroductionOne of the most challenging problems in electromagnetic (EM) geophysical methods is developing fast and stable methods of imaging inhomogeneous underground structures using EM data. Solution of this problem is important for many practical applications ranging from mineral exploration to waste and building site characterization. In the papers (Zhdanov et al., 1995) and (Zhdanov et al., 1996) and the references therein we have developed a novel approach to EM imaging based on the notion of EM migration. The method includes downward continuation of the observed field or one of its components in reverse time and application of the corresponding imaging conditions. However, until recently the relationship between EM migration imaging and traditional EM inversion have remained unexplored. The conventional EM inversion means a method which predicts the geoelectrical model generating the theoretical data closed to observations. The EM migration introduced in our previous publications constructed an image of subsurface geoelectrical structures, and there was no guarantee this image, if included in a geoelectrical model, would give rise to theoretical EM fields that matched those observed. Meanwhile, Tarantola (1987) demonstrated that seismic wave migration, which was the prototype for EM migration, can be treated exactly as the first iteration in some general wave inversion scheme. In this paper we formulate an important new result: EM migration, as the solution of the boundary value problem for the adjoint Maxwell's equation in frequency domain, can be clearly associated with the inverse problem solution. In other words, we prove that a geoelectrical model constructed on the basis of migration images would actually generat...
One of the most challenging problems of electrical geo physical methods is the interpretation of time domain electro magnetic (TDEM) sounding data in the areas with the horizontally inhomogeneous geoelectrical structures. This problem is of utmost importance in mining exploration and environmental study, in particular, in the case of sounding conducted in the transmitter offset or slingram mode. The conventional 10 EM inversion technique cannot solve this problem, because the observed data are strongly distorted by horizontal conductivity inhomogeneities. The multidimension al EM inversion techniques existing today can handle only simple models, require repetitive forward modeling solutions, and therefore are very time consuming.We developed a new approach to the interpretation of TDEM data over inhomogeneous structures based on downward extrapolation of the observed electromagnetic field in reverse time (the time domain electromagnetic migration). Numerical solution of this problem is provided by an electromagnetic analog of the Rayleigh integral. TDEM migration transforms EM data, observed on the surface of the Earth, into immed iate geoelectrical images of geological cross sections. This transformation is very fast (requiring only a few seconds of CPU time on PC) and stable to the random noise in the data.The numerical results of rapid inversion based on the time domain electromagnetic migration illustrate the property of migration described above. This method has also been applied to waste site characterisation. We have analysed the data obtained as a result of high density TDEM profiling survey with the Geonics EM47 along the set of profiles, intersecting Cold Test Pit waste site within the Radioactive Waste Management Complex (RWMC) at the Idaho National Engineering Laboratory (INEL). Time domain electromag netic migration and resistivity imaging made it possible to outline the conductive sections of the pit filled with the waste.
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