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The first Ocean Drilling Program (ODP) vertical seismic profile (VSP) experiment in a thick sedimentary section was conducted at Site 765 to measure compressional wave velocities and to correlate drilling results with seismic data. Measurements were performed through the cased portion of the hole between 186 and 915 mbsf using both air-gun and water-gun sources. Average velocities for the air-gun source were 1.967 km/s (284-915 mbsf), 1.815 km/s (186-284 mbsf), and 1.633 km/s (seafloor to 186 mbsf, extrapolated). Separation of the wavefield into upgoing and downgoing components by f-k filtering plus deconvolution of the upgoing wavefield produced good VSP records and revealed reflectors that correlate well with seismic profiles. These data were used to correlate the drilling results (lithologic units) with the seismic sequences at the drill site, which will be used to extrapolate the drilling results regionally. A weak sub-basement reflection at approximately 600 m into basement may correspond to the contact between pillow basalts and sheet flows. A comparison between VSP velocities, laboratory measurements, and sonic logs shows generally good agreement below about 420 mbsf.

Vertical Seismic Profile at Site 765 and Seismic Reflectors in the Argo Abyssal Plain