IMAGING BENEATH AN OPAQUE BASALTIC LAYER USING DENSELY SAMPLED WIDE-ANGLE OBS DATA

A combined reflection/refraction (wide-angle) seismic survey was condu cted on the continental shelf north-west of Britain, using a conventio nal streamer with an airgun source, and static ocean-bottom seismomete rs (OBS) to record wide-angle energy. The shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection se ction. The basalts, however, proved to be opaque to the conventional r eflection method and prevented the imaging of deeper horizons, where a n important velocity inversion was anticipated. This paper reports on the processing, modelling and interpretation of the densely sampled wi de-angle OBS data that were coincident with the reflection profile. El even OBS instruments were deployed along a 75 km line and recorded sig nal from a powerful 149 litre (9100 in.(3)) airgun array fired every 5 0 m. Data processing was performed using a standard industrial reflect ion seismic software package prior to first-arrival picking. Processin g steps included geometry definition, trace summation and display of t he data using various scaling algorithms. An initial model was constru cted from 1D velocity-time profiles digitized every 4 km along the sta cked section. First arrival traveltime modelling rapidly converged to a detailed model of the structure of the top 5 km of the crust. Modell ing revealed the existence of a buried low-velocity Mesozoic sedimenta ry basin, of a prominent basement horst and of a normal fault penetrat ing to the basement.