EDWARDS, R.A.AND P.J. BARTON, Bullard Laboratories, Dept. of Earth Sciences, University of Cambridge, Madingley Road, Cambridge, C133 OEZ, U.K.

Abstract: Imaging Salt Diapirs Using High-Resolution Wideangle Seismic Data

Salt forms a high impedance barrier that obscures deeper structure on conventionally acquired seismic profiles. However, wide-angle energy, recorded by closely spaced seabed seismometers, may be used to image the flanks and bases of salt diapirs. Dense overlapping sampling using diving rays allows structures which form potential hydrocarbon traps on the flanks of the salt diapirs to be delineated.We present the results of a high-resolution 2-D wideangle seismic survey in an area characterised by salt diapirism. Acquisition involved a dense array of ocean bottom seismometers in conjunction with closely spaced shots. The data haive been modelled using two contrasting methods; conventional ray-tracing through a velocity model composed of discrete layers, and tomographic inversion over a smooth velocity grid. Both methods have delineated the size, overall shape and P-wave velocity of the salt features. Small (less than 1 km wide) structures on the flanks of the salt diapirs, which could not be resolved on a coincident reflection profile, have been well defined by the ray tracing method. Error analyses show that uncertainty in the location of the flanks of the diapirs is less than 50 m.To model these structures successfully, it is essential to record wide-angle refracted energy which has turned through the diapir flanks. The tomographic inversion reproduced the main features of the ray traced model, but could not delineate finer structures. This is a function both of modelling strategy - the smoothest possible velocity model is produced - and of limitations in grid node spacing and consequent computational time.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England