Sediment sound velocity measurements made in situ from BathyscaphTrieste

Abstract

The first deep-water, in situ measurements of sound velocity in sea-floor sediments (other than seismic) have been made from the bathyscaph Trieste. These measurements were made at three stations off San Diego during August–October 1962 at depths of 338 to 1235 meters. Specially designed probes measured sound travel time over a 1-m path, 46 cm below the water-sediment interface at frequencies of 25 kc/s; probe accuracy was about ±0.5 m/sec. Laboratory measurements of sound speed in sediment can be corrected to in situ conditions in the sea floor by applying full corrections for temperature and pressure, using tables for sound speed in sea water. A significant portion of the vertical sound velocity gradient within the sea floor must be due to thermal gradients. At the Mohole (Guadalupe) site, for example, the thermal gradient (plus a small increment due to pressure) accounts for an increase in sound velocity of 53 m/sec in the upper 100 meters of the sediment. In situ sound speeds of less than 1 per cent error can be predicted for fine-grained, high-porosity sediments off San Diego by means of present sound-speed versus porosity curves; these curves need revision for sands.