Seismograms from vertical seismic profiles (VSP) on Ocean Drilling Program Legs 111 and 148 were merged. First arrival traveltimes picked on the best quality traces are equivocal in the sediment but suggest an average velocity of about 1.8 km/s for the lower 110 m of section. In basement, VSP velocities follow the trend of the sonic log velocities but are generally equal to the highest sonic velocities measured in an interval. The general trend of VSP velocity follows that of refraction velocities, but the decrease in gradient marking the seismic Layer 2/3 boundary occurs ~500 m shallower at Hole 504B than in the surrounding 1–2 km region and appears to correlate with the top of the sheeted dikes. A downgoing arrival with velocities of 3–4 km/s is interpreted as a shear wave converted either by rough basement surface near the borehole or by shallow-buried lateral velocity variations. Several other arrivals can be most easily explained as coherent energy scattered from basement roughness or velocity inhomogeneities within the upper 500 m of basement. Downhole increases in VSP velocities correlate with events in the upgoing wavefield suggesting internal crustal reflections, but the amplitudes are no greater than side-scattered energy and coherency is generally poor over borehole depths exceeding 100–200 m.

Date of initial receipt: 17 August 1994
Date of acceptance: 24 February 1995

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