Seismic velocities in the North West Shelf region, Australia, from near-vertical and wide-angle reflection and refraction studies

A significant limitation of conventional seismic reflection technology is that it poorly constrains seismic velocities in the deep part of the section. These velocities are needed for seismic migration, to convert from time to depth, and to estimate the likely rock composition at depth. Seismic observations at large offsets can compensate for this limitation. The Australian Geological Survey Organisation recorded high-quality refraction and wideangle reflection seismic data using ocean-bottom seismographs (OBS) along 5 profiles on Australia's North West Shelf, to maximum offsets of 300 km. The OBS transects coincide with previously acquired deep-crustal seismic-reflection profiles. Consequently, velocity information can now be derived from two independent data sets: common-depth point (near-vertical) reflection data and refraction/wide-angle reflection data. Residual (difference between these two velocity data sets) velocity images can be produced and analysed. Even within the top 5 km of the section residuals between the OBS- and CDP-derived interval velocities vary from -0.5 to +3.5 km/s and increase further with depth. Consequently, the depth equivalent of 4 s two-way time will vary by up to 1.2 km or 10%. The practical implications of co-processing of seismic velocity information are: improved seismic velocity estimation, more accurate depth migration and depth conversion of seismic data and less distortion of amplitudes and shapes of geological structures after depth conversion.