The seismic velocity structure of some NE Atlantic continental rise sediments; a lithification index?

Abstract

SUMMARY Two separate sets of experiments with digital ocean-bottom seismographs (DOBS) and airguns, on continental rise areas off Madeira and west of Portugal, produced en echelon second arrivals from the sediment layer on record sections. Traveltime and synthetic seismogram modelling indicate that the arrivals represent multiplyreflected refracted phases which have undergone reflection within the sediment layer itself. Further, although the P-wave contrast at the intrasediment reflecting horizon is relatively small, the modelling indicates a large downward increase in S-wave velocity from 100-250ms-' (Poisson's ratio of at least 0.42) to about 1200ms-' (Poisson's ratio of about 0.25). A reflection event can usually be found on reflection profiles along the refraction lines at almost exactly the same 'depth' as the intrasediment reflector. In one case such an event can be traced to a nearby Deep Sea Drilling Project (DSDP) borehole where it is associated with the transition from ooze to chalk. This, and other circumstantial evidence, suggests that the intrasediment reflector marks an important increase in lithification within the sediment layer. If so it means that, in future, straightforward OBS experiments may be used to measure the depth of this increase without resorting to the drill.