Resistivity imaging by time domain electromagnetic migration (TDEMM)

One of the most challenging problems of electrical geophysical methods is the interpretation of time domain electromagnetic (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 multidimensional 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 immediate 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 electromagnetic migration and resistivity imaging made it possible to outline the conductive sections of the pit filled with the waste.