BARTON, P.J. and R.A. EDWARDS, Bullard Laboratires, Dept. of Earth Sciences, University of Cambridge, UK

Abstract: Rapid Velocity Images of Complex Structures

In areas where high impedance layers such as salt or basalt obscure deeper structures on conventional reflection profiles, the densely-sampled wide-angle technique has a proven record for mapping the geometry and seismic properties of buried sediments and basement. Wide-angle surveys are being acquired with progressively denser spatial sampling, either through the use of large numbers of ocean bottom seismometers, or by using two ships in a synthetic aperture profile. Conventional ray-based inversion methods are pushed to their limits by these massive datasets, and new tomographic, waveform inversion and migration techniques are being developed to exploit their full information content. However, the new techniques tend to build on the old ones rather than supersede them, and the initial step of building a high resolution velocity model requires considerable expertise and can be time-consuming. We present a method for transforming wide-angle (refracted) travel-time arrivals into a 2D tau-p domain, which produces a velocity map directly from the seismic data. The use of simple geometric relationships between reversed raypaths gives a robust framework for the incorporation of unreversed data. The method gives a very rapid 'brute-stack' image of subsurface velocity structure, and we present data from several examples, including images of structure beneath high impedance basalts. The image may be downward-continued into two-way-time or depth and used for pre-stack depth migration of conventional data. The next step will be to extend the technique to the use of tau-p inversions of the seismic data, so that traveltime picking becomes unnecessary.