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2.5-D dc resistivity modeling which allows for subsurface current and potential electrodes, numerical errors depend on the coarseness of discretization and increase with spatial wave numbers. In this regard, the exponential extrapolation of Fourier transformed potentials is useful for the evaluation of inverse Fourier transform integral in the outmost range of wavenumbers. This paper presents an accurate integral scheme in the outmost range using an exponential function of which coefficient is represented by a ratio between the modified Bessel functions of order 1 and 0. The effectiveness of this scheme has been confirmed using a homogenous half-space model and a vertical fault model.

Borehole electromagnetic (EM) measurements, using fiberglass-cased boreholes, have proven useful in oil field reservoir characterization and process monitoring (Wilt et al., 1995). It has been presumed that these measurements would be impossible in steel-cased wells due to the very large EM attenuation and phase shifts. Recent laboratory and field studies have indicated that detection of EM signals through steel casing should be possible at low frequencies, and that these data provide a reasonable conductivity image at a useful scale. Thus, we see an increased application of this technique to mature oilfields, and an immediate extension to geothermal industry as well. Along with the field experiments numerical model studies have been carried out for analyzing the effect of steel casing to the EM fields. The model used to be an infinitely long uniform casing embedded in a homogeneous whole space. Nevertheless, the results indicated that the formation signal could be accurately recovered if the casing characteristics were independently known (Becker et al., 1998; Lee el al., 1998). Real steel-cased wells are much more complex than the simple laboratory models used in work to date. The purpose of this study is to develop efficient numerical methods for analyzing EM fields in realistic settings, and to evaluate the potential application of EM technologies to cross-borehole and single-hole environment for reservoir characterization and monitoring.