The role of in-field seismic processing in early evaluation in under-explored areas of the Darling Basin

Abstract

The 2008 Rankins Springs Seismic Survey was a joint initiative by Geoscience Australia and NSW Department of Primary Industries under the Onshore Energy Security Program (OESP) in the under-explored southeastern Darling Basin. Regional seismic acquisition parameters of 300 channels, 40 m group interval and 80 m vibration point interval nevertheless allowed detailed imaging of a 3 second (TWT) thick sedimentary sequence in the Yathong Trough. Use of three 12 second vari-sweeps from truck mounted Hemi 50 (50,000 lb) vibrators provided sufficient energy to image from immediately below regolith to the Moho. The sweep frequency ranges 6 - 64, 10 - 96 and 8 - 80 Hz were chosen both for deep penetration and high resolution in the sedimentary section. In-field processing produced a high quality preliminary section on a daily basis using an iterative process of automatic residual statics calculation on a deep gate and interactive stacking velocity analysis. Both automatic statics and stacking velocity were essential for successful imaging, but velocity was more important, as initial estimates based on first arrival velocities produced a degraded section. The field seismic section clearly shows a fault bounded trough, with evidence of compressional structures in the upper part and hints of underlying older sedimentary basins. The in-field stacking velocity analysis also provided immediate evaluation of the maximum depth of the trough, approximately 6 km, deeper than expected. Efficient in-field processing allowed early notification to project partners of a successful survey, facilitating future planning, and provided a sound basis for streamlined subsequent processing.