From two way time to depth and pressure for interpretation of seismic velocities offshore: Methodology and examples from the Wallaby Plateau on the West Australian margin

Abstract

The effect of water loading can account for sub-seafloor seismic P-wave velocity increases exceeding 1km/s from shallow to deep water, but it is commonly ignored in offshore studies. Direct comparative analysis of interval velocity patterns between areas of significantly different water depths requires various water pressure related changes in velocity to be accounted for. This apparently simple task is not easy to implement in practical terms. One approach is to try to predict velocity increase at any given depth due to the pressure effect of water layer above, and then to reduce the observed interval velocity at that depth by subtraction of the predicted velocity increase. Interval velocities at different locations thus reduced can be compared and conclusions about rock lithology can subsequently be made. The application of water depth adjustments to seismic velocities, and the methods used to apply them, remain controversial. The presentation of velocity models as a function of pressure rather than two-way time, or depth, emerges as a possible solution.Analysis of appropriately adjusted seismic interval velocities from Geoscience Australia's 2008/09 seismic survey GA 310 in conjunction with seismic reflection interpretation provides new insights into the geology of the Wallaby Plateau, offshore of Western Australia. Seismically distinctive divergent dipping reflector sequences (DDRS), identified in the area, are similar in seismic character to seaward dipping reflector sequences (SDRS) of inferred volcanic composition.The water depth adjustment of seismic velocities analysed in our study reduces the distinction between SDRS, DDRS and sedimentary strata such that discrimination between volcanic and sedimentary strata in DDRS or SDRS is equivocal. A major uncertainty of this interpretation is caused by a lack of global reference velocity models of SDRS and DDRS.