Abstract
We report a series of two-ship multichannel seismic experiments carried out on the continental shelf off northwest Scotland in February 1981. Both expanding spread and constant offset profiles were shot using explosives and a 16.4-1 airgun. The expanding spread shot-receiver configurations yield detailed information on the variation of seismic velocity with depth, whilst constant offset profiles allow us to monitor lateral variations in the upper crustal velocity structure. We show an example of marked variations in the efficiency of compressional to shear wave conversion along a constant offset profile over shallow Lewisian basement west of the Outer Hebrides which we interpret as due to changes in the uppermost basement velocity structure. From profiles north of Cape Wrath we show that where the Precambrian Lewisian basement is buried beneath Mesozoic sediments of variable thickness we can use the constant offset profile of the basement refractor to strip off the effect of the sediments and so correct deeper crustal returns on the coincident expanding spread profiles.
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