Seismic-Style Processing of Numerical Em Data
Neese, John W. 1952-
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Standard seismic-processing methods are applied to numerical simulations for several variations of a standard 1-D numerical CSEM model, calculated using seismic-style acquisition parameters: an impulsive source and many receivers, unaliased. The data are normalized to unit maximum amplitude at each offset, making the weak far-offset signal visible (seismic-style) without computation of apparent resistivity (EM-style). Hence, the amplitudes are not appropriate for inversion (EM-style), but the moveout is interpretable (seismic-style) for the presence of a reservoir layer. Plots of normalized data exhibit distinctive moveout, similar to seismic data, but with significant dispersion. The dominant moveout is shown to be linear at a given frequency, consistent with theoretical expectation for lateral (head) waves. Conventional (semblance) velocity analyses and stacking detects a 100 Ωm reservoir on the basis of its linear moveout, but does not appear to be useful for picking stacking velocities. Standard f-k transforming and filtering does not appear to be promising. Linear Radon transforms also detect the reservoir. Standard seismic processing methods are modified to account for the strong frequencydependence of the EM data, in order to focus on the resistivity. Moveout as a function of frequency is replaced by moveout as a function of resistivity. A resistivity moveout correction is defined that indicates the existence of the reservoir, and estimates its effective resistivity. Similarly, a modified Radon transform (“EM-Radon”) is defined which also indicates reservoir existence and effective resistivity. EM-Radon is shown to be robust to noise, and to decimation of the receivers, and of the time-sampling. EM-Radon’s sensitivity is explored to depth, thickness, and resistivity of the reservoir; T-equivalence is confirmed. These calculations demonstrate that, in simple cases, numerical CSEM data, appropriately acquired, and processed seismic-style, can be interpreted for subsurface effective resistivity.