Preliminary Seismic Refraction Results Using a Borehole Seismometer in Deep Sea Drilling Project Hole 395A

Three seismic refraction lines shot to the Marine Seismic System, a vertical-component borehole seismometer in DSDP Hole 395A at 609 m sub-bottom depth, have been analyzed.Despite inconsistencies between various velocity determinations in the upper crust, the velocity of the uppermost 600 m appears to be high, about 4.5 km/s.Between 600 m and 1.8 km sub-bottom, the crust yields an apparent velocity of 4.6 km/s.This value is somewhat lower than that determined near the base of the hole using acoustic logging tools (5.0-5.5 km/s), and is also lower than the average velocity of core samples from the same depth (5.7 km/s).The lower crust is unusually thin, only 1.8 to 2.4 km thick, and has compressional-wave velocities of 6.8 to 7.3 km/s.Mantle velocities range from 7.8 to 8.2 km/s.There is little or no velocity gradient in the uppermost kilometers of the crust, but our results indicate widespread lateral inhomogeneities, supporting the observations based on refraction profiles shot for the original site survey.