Laboratory measurements of compressional sound speed, and absorption, have been made on 111 unconsolidated marine sediment samples, ranging from shallow water sands to deep‐sea clays. In addition, determinations were made of porosity, wet density, and grain size distributions. Frequencies between 20 kc/sec and 37 kc/sec were used for the acoustic studies. Sound speed values at room temperature range from 1.474 km/sec for a red medium clay to 1.785 km/sec for a medium sand. More than one‐third of the values are lower than the value for sea water alone. Variations in the speed of sound in unconsolidated sediments as found in nature are caused by the following factors, in order of importance: (1) porosity, because of the great difference in compressibility of water and mineral grains; (2) the factor which produces rigidity, which appears to be related to the abundance of coarse grains; (3) pressure; (4) temperature; (5) compressibility of the grain aggregate, determined from compressibilities of individual minerals. Sound absorption measurements ranged from 0.5 db/m for a medium clay (28.4 kc/sec) to about 20 db/m for silts and fine sands (between 30 and 37 kc/sec). An absorption maximum occurs for sediments of intermediate porosity (0.45–0.6) and intermediate grain size (0.031 mm–0.25 mm). The expression [Formula: see text], where α is the linear absorption coefficient, M is a frequency‐dependent factor related to the sediment volume fraction of grains in mutual contact, and [Formula: see text] is a computable total acoustically effective grain surface area, predicts the absorption values and the absorption maximum. Absorption measurements at more than one frequency between 20 kc/sec and 37 kc/sec were obtained for 65 samples. Assuming that absorption is directly proportional to frequency raised to a power n, the data yield an average value of n equal to 1.79, with a standard deviation of 0.98.

Compressional and shear wave properties of marine sediments: Comparisons between theory and data

This paper reports on a continuing study of the mass physical properties of surficial, shallow-water marine sediments off San Diego, California. Sound velocity measurements in situ at 100 kc were made by pulsing between small transducers inserted into the bottom by diver. Density, porosity, and size analyses were determined on relatively undisturbed samples taken by diver. The following averaged representative values have been obtained: Velocity in sediment at Impedance Density Porosity Med. diam 60°F (105 g/ Sediment type (g/cc) (%) (mm) (ft/sec) cm2 sec) Fine sand 1.93 46.2 0.19 5520 3.25 Very fine sand 1.92 47.7 0.12 5435 3.18 Silty very fine sand 1.68 61.3 0.05 5075 2.60 Medium silt 1.69 60.9 0.03 4825 2.49 Clayey fine silt 1.60 65.6 0.02 4800 2.34 Laboratory measurements of velocity and attenuation 25 to 35 kc were made by a resonant-chamber method using diver-taken samples; average attenuation values of about 0.5 db/ft (silt) to about 5 db/ft (fine sand) were obtained. At three stations the sediment sound velocity was less than the velocity in the bottom water.

No abstract

The relationships between sound speed and the more important physical properties of unconsolidated sediments have become known in recent years largely as a result of the studies of Hamilton, 1956; Hamilton et al., 1956; Shumway, 1956, 1958; Nafe and Drake, 1957; Sutton, Berckhemer and Nafe, 1957; and Laughton, 1954, 1957. The suite of samples used in this study is larger and more varied than the suites used in any of the earlier studies, and the measurements probably were more carefully taken. A large number of shelf and harbor sediments were used in addition to sediments from deeper water in the Pacific and Arctic Oceans.