Tomographic Imaging of Cross-well EM Data

A 2-D iterative EM Born inversion method has been developed for estimating the Earth’s electrical conductivity stmcture between two boreholes. The theory is developed for a vertical magnetic dipole located in a bore hole in either a whole-space or half-space that exhibits a cyliidrical symmetry about the bore hole axis. The inversion scheme uses the first order Born approximation to invert for an initial model, and then employs higher order terms in the Born series to achieve more accurate images. The inversion routine has been applied to a cross-hole data set collected at the British Petroleum test site near Devine, Texas. The geology is predominantly 1-D with interbedded layers of sandstone, shale and limestone. The resulting image shows a resistive zone within a conductive medium which is consistent with both the well log and 1-D inversions. Continuing experiments are being conducted at the University of California’s Richmond Field Station test site. The goal of these experiments is to use cross-well EM to monitor the injection of salt water plumes. Inversion results of forward model calculations that simulate the experiment show that the iterative Born method does reconstruct the horizontal extent of the plume. Initial tests of the inversion using the field data support hese model results.