Acoustic structure and seismic velocities in the Carnarvon Basin, Australian North West Shelf: towards an integrated study

Abstract

High quality refraction and wide-angle reflection seismic data recorded by ocean-bottom seismographs (OBSs) deployed by the Australian Geological Survey Organisation along the 700 km long transect in the Carnarvon Basin effectively supplement results obtained by means of the conventional reflection technology. Velocity information can now be derived from both CDP (near-vertical reflection) and OBS (refraction/wide-angle reflection) data. Generally, CDP-derived average velocities are lower than OBS-derived velocities and this deviation increases with depth: from ~0.1 km/s at 8 s two way time (TWT) to 0.8?1.6 km/s at 16 s TWT. If the CDP-derived velocities are used to depth convert reflection data, then depth to these TWTs would be underestimated by 0.4 to 6.4?12.8 km respectively. Some local anomalies (at ~6s TWT CDP-derived velocities may be more than 0.1 km/s higher than the OBS-derived velocities) distort this general trend. These would result in ~0.3 km local overestimates of the depth equivalent of 6s TWT. Co-analysis of the interval velocity field reconstructed from the travel time-based interpretation of the OBS data and the conventional reflection image of the crust in some cases shows their poor correlation. Prominent reflectivity seen in the conventional reflection data at depth 13?17 km does not correspond to any significant velocity increase imaged by refraction/wide-angle techniques. These reflections were taken in our pre-OBS interpretation to mark the top of the underplated layer in the crust. Velocity below that level estimated from the OBS data is in the range 5.7?6.4 km/s. This velocity range is considerably lower than anticipated for mafic rocks under appropriate PT-conditions. Quite significant local velocity increase in the depth range 15?23 km does not produce high-amplitude near-vertical reflections. Only a combination of reflection and refraction/wide-angle techniques provides a clue to a consistent geological interpretation of seismic data. Improved velocity estimation in the crust is crucial for accurate depth conversion of near-vertical reflection data.