Tau-p velocity imaging of regolith structure

The tau-p velocity imaging method, first developed for obtaining the velocity field from marine multichannel seismic data, has been applied to refracted waves from the regolith in regional seismic reflection surveys on land. The technique converts travel time picks from the refracted wavefield into two-dimensional velocity models, by transforming from time-offset into the tau-p domain. Each arrival is mapped individually, and the 'true' velocity and position of the ray turning point is obtained by considering reversed raypaths. Thus the data are transformed directly into a depth or two-way time image of the subsurface displayed in seismic velocity. The method is extremely fast and involves no interpretive steps or iteration. Ideal datasets contain the refracted wavefield sampled densely and equally in the shot and receiver domains. It was therefore decided to test the application of the method for mapping the velocity structure of a portion of regolith and to compare the results with those obtained using more conventional methods. The area chosen for study was part of a regional seismic reflection line across the Lachlan River palaeo-valley in central NSW. The data set consisted of the first break picks for 240 channels with receivers spaced every 40 m and vibration points every 40 m. The velocity images were produced as both time and depth sections and compared with one and two layer refractor models from a refraction tomographic approach. A low velocity region on the image corresponds to the deepest part of the refractor model, interpreted as the thickest part of the palaeo-valley. Bedrock velocity variations are also mapped and agree with the changes along the (lowest) refractor velocity profile. While further tuning may be required for land work, the technique has the advantage that velocities can be directly imaged and potentially related to regolith physical properties.