Sound velocity as a function of depth in marine sediments

Abstract

Additional data from sonobuoys and the Deep Sea Drilling Project (DSDP) justify separating sound-velocity-depth functions and velocity gradients (in the first layer of soft marine sediments) into some geographic areas and sediment types. Based on sonobuoy and core measurements (where V is sound velocity in km/s, and h is depth in sediments in km), the following data are obtained: continental shelf basins off Sumatra and Java—V=1.484+0.710h−0.085h2; U. S. Atlantic continental rise—V=1.513+0.828h−0.138h2; deep-sea terrigenous sediments—V=1.519+1.227h−0.473h2; and siliceous sediments of the Bering Sea— V=1.509+0.869h−0.267h2. Selected DSDP data (through leg 74) in similar areas yield: continental terrace silt–clays—V=1.505+0.712h; deep-sea terrigenous sediments—V=1.510+1.019h; and deep-sea siliceous sediments—V=1.533+0.761h. Computed velocity gradients from sonobuoy measurements are generally supported by the DSDP gradients. Only DSDP data give the following: hemipelagic sediments—V=1.501+1.151h; deep-sea calcareous sediments—V=1.541+0.928h; and deep-sea pelagic clay—V=1.526+1.046h. Where fast sediment accumulation occurs, there has not been enough time to reduce sediment pore spaces under overburden pressure; areas of slow accumulation may have relatively high sediment structural strength. Both cases have lower velocity gradients because higher porosities and consequent lower velocities persist to deeper depths.