Seismic noise modelling in the Dampier Sub-basin

Abstract

Exploration efforts in the Northwest Shelf of Australia are hampered by the presence of strong, coherent noise that overprints the reservoir section and deeper intervals. The limited success of attempts to improve seismic data quality in this region suggests that complicated noise waveforms are involved. We gained insight into the noise problem using realistic synthetic seismograms, upon which processing methods could be fully tested and the results judged objectively using known primaries. Full-waveform synthetics were generated using the reflectivity method at several wells in the Dampier Sub-Basin and tied to field records from nearby seismic surveys. In addition, we used a finite-difference modelling technique to separate individual multiple modes from primary events in the synthetic data. We identified interbed multiples as the most cumbersome of the various multiple modes. They have similar amplitudes to weak primary events and are generated within high reflectivity packages in the overburden. They have similar moveout to the primaries and are not suppressed by stacking over the available offsets. Other significant noise sources identified include both P- and S-wave guided waves and mode-converted arrivals generated between carbonates in the Tertiary section. Processing tests were applied to the synthetics and the results were assessed visually and by correlation with the known primaries. Routine processing largely suppressed the guided waves and mode-conversions and, to a lesser extent, the water bottom multiples. Gap deconvolution in the tau-p domain helped to suppress the interbed multiples, which are difficult to remove due to their lack of velocity discrimination from the primaries. Poor image quality and velocity uncertainty could remain at the target level as a result of weak primaries and imperfect noise elimination over the available offset range.