Abstract

Deep seismic exploration in the Gippsland Basin is hindered by strong noise below the Latrobe Group coal sequence. The reflectivity method provides a means for constructing detailed and accurate synthetic seismograms, often from little more than a partial sonic log. The noise contributions to the synthetics can then be interpreted using additional synthetics computed from variations upon the depth model and by exercising control over the wave types modeled. This approach revealed three types of persistent noise in progressively deeper parts of the subcoal image: (1) mode-converted interbed multiples (generated within the coal sequence), (2) S-wave reflections and long-period multiples (generated between the coal sequence and the Miocene carbonates), and (3) surface-related multiples. The noise interpretation can also be performed upon semblance analyses of the elastic synthetics to guide a velocity analysis away from a well. This procedure helped to avoid picking the interformation long-period multiples, whose stacking velocities were only 5 to 10% below those of the weak target zone primaries. An improved subcoal image was obtained by making full use of the versatile noise suppression offered by a tau -p domain processing stream. By separating the strong linear events at the far offsets, it is possible to stack a larger portion of the target zone reflections, provided hyperbolic velocity filtering (HVF) is applied to suppress the transform artifacts. Hyperbolic velocity filtering can be incorporated into a point-source tau -p transform to suppress S-wave reflections and guided waves while preserving plane-wave amplitudes to assist the subsequent deconvolution of the mode-converted interbed multiples. Stacking in the tau -p domain is achieved using an elliptical moveout correction that reduces wavelet stretch and approximates the exact reflection traveltime better than NMO. Two regional seismic lines were reprocessed in this manner and cointerpreted with the modeling studies performed at nearby wells to avoid the noise events that still remained. Several new events appeared in the immediate target zone, passing the low-frequency character expected following transmission through a coal sequence.

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