SOUND-VELOCITY GRADIENTS IN MARINE-SEDIMENTS

The presence of positive sound velocity gradients in the sea floor is of importance in underwater acoustics; especially in refracting energy back into the water mass. This report summarizes velocity-depth relations and values of velocity gradients in four principal marine sediment types as computed from measurements of velocity at the sea floor from cores and at depths from sonobuoys. Compressional wave velocity (Vp, km/s) versus depth (D, km) in three major sediment types are, for 20 turbidite areas: VP= 1.511 + 1.304D—0.741 D2+0.257D3; siliceous sediments (Bering Sea): Vp= 1.509 + 0.869D—0.261 D2; and calcareous sediments (Ontong-Java Plateau): Vp= 1.559 + 1.713D— 0.374D2. Vpgradients, surface, and 0–1000 m, average 1.3-0.8 s-1(turbidites); 0.9-0.6 s-1(siliceous); and 1.9-1.3 s-1(calcareous). In sands Vpgradients vary from 18 s-1(1–2 m) to about 4 s-1between 1 and 20 m. Major factors in Vpgradients in silt clays and turbidites to 500m are pressure-induced porosity reductions and effects on the sediment mineral frame (about 66% of the gradient), temperature increases due to heat flow (17%), pore-water pressure increases (2%), and increases in rigidity caused by lithification (15%). These percentages vary with sediment depth. Velocity ratios and gradients may indicate a small sediment sound channel. © 1979, Acoustical Society of America. All rights reserved.