Combined structure and velocity stacks via the tau±p transform G.D. Jones, 1 * P.J. Barton 1 and S.C. Singh 2 1 Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK, and 2 Laboratoire de Geosciences Marines-case 89, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France Received June 2002, revision accepted August 2002 ABSTRACT Reflection and refraction data are normally processed with tools designed to deal specifically with either near- or far-offset data. Furthermore, the refraction data normally require the picking of traveltimes prior to analysis. Here, an automatic processing algorithm has been developed to analyse wide-angle multichannel streamer data without resorting to manual picking or traveltime tomography. Time±offset gathers are transformed to the tau±p domain and the resulting wavefield is downward continued to the depth±p domain from which a velocity model and stacked section are obtained. The algorithm inputs common-depth-point (CDP) gathers and produces a depth-converted stacked section that includes velocity infor- mation. The inclusion of long-offset multichannel streamer data within the tau±p transformation enhances the signal from high-velocity refracted basalt arrivals. Downward continuation of the tau±p transformed wavefield to the depth±p domain allows the reflection and refraction components of the wavefield to be treated simultaneously. The high-slowness depth±p wavefield provides the velocity model and the low-slowness depth±p wavefield may be stacked to give structural infor- mation. The method is applied to data from the Faeroe Basin from which sub-basalt velocity images are obtained that correlate with an independently derived P-wave model from the line. INTRODUCTION A method of automatically obtaining velocity and structural information simultaneously from the same dataset would be a valuable processing tool in any geological setting. However, such a technique may be particularly beneficial in geologically complex areas where it is required to sample the subsurface in two dimensions using refracted and diving rays, for example sediments overlain by a high-velocity layer such as basalt. Refraction and reflection data are generally treated separ- ately, one part of the dataset being discarded whilst the other part is analysed. Near-offset reflection data containing amp- litude and phase information are processed with tools such as normal-moveout correction, velocity analysis and migration, to obtain a depth image of the subsurface that may be interpreted in terms of geological structure. Conversely, re- fraction data are often used as the input to tomographic inversion processes designed to obtain a velocity model of the subsurface. The use of wide-angle refraction data in this way has historically been due to the low sampling density associated with refraction-data acquisition methods, with refraction data obtained using ocean-bottom seismometers (OBS) and reflection data with a conventional streamer. Additionally, the first step of analysing refraction data is usually to pick the traveltimes, a technique that may lead to an element of subjectivity entering the processing procedure. Early work on sub-basalt imaging used large explosive sources and a large offset to overcome the difficulties of conventional profiling (Jarchow, Catchings and Lutter 1994). Since sub-basalt regions have become the subject of hydrocarbon exploration, acquisition techniques have developed from using OBS data to acquiring densely sampled ß 2003 European Association of Geoscientists & Engineers 205 Geophysical Prospecting, 2003, 51, 205±213 *E-mail: gjones@esc.cam.ac.uk