Sound Speed and Absorption Studies of Marine Sediments by a Resonance Method

Abstract

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.

Vitesse du son dans les sédiments marins durant les premiers stades du tassement

“…LAUGHTON (1957) à partir de l'étude de la compaction de divers sédiments naturels introduit pour sa part le concept d'incompressibilité de structure k = k w . n + k s (1 -n) + k c , et SHUMWAY (1960) suppose un coefficient de rigidité µ.…”

Sound Velocity-Density Parameters of Sediment and Rock from DSDP Drill Sites 315–318 on the Line Islands Chain, Manihiki Plateau, and Tuamotu Ridge in the Pacific Ocean

“…For the Leg 33 data a 5% porosity rebound on removal of the uncemented sample from the hole to the laboratory is probably closer to reality than not; therefore, as a very rough estimate, a porosity rebound of 5% will be used for sediment between 30% and 60% porosity, and for lithologies with porosities between 20% and 30% only a 2.5% porosity rebound will be assumed, and below 20% porosity rebound will be considered very small. Of course, near the sediment surface the overburden rebound becomes smaller until it is zero at the sea floor, but fortunately, velocity variations with porosity in these high porosity sediments are also very small (Hamilton, 1959;Nafe and Drake, 1963;Sutton et al, 1957;Shumway, 1960); therefore, it is not a major problem for Leg 33 data (Figure 44).…”

“…Naval Oceanographic Office, Special Publication 58; Press, 1966). Velocities of the high porosity sediments (with velocities near that of sea water) have been empirically shown to have a similar percentage velocity change as that of sea water with temperature and pressure variations (Laughton, 1957;Shumway, 1960). Wet and dry limestone, and sandstone (5% porosity) velocity variations with temperature and pressure have been measured by Hughes and Cross (1951), which suggests that a pure mineral end member such as calcite may not have a velocity variance of more than 1% for the hydrostatic pressure and temperature estimated at Leg 33 sites.…”

Scattering and Propagation of Acoustic Waves in the Presence of Rough, Penetrable Boundaries

“…The absorption used in Table I is based on the data in Shixway, 23 Wood and weston,27 Cole,28 and Hampton.26 The absorption is based on an assumed first-power frequency dependence, which allows the wave number, k, to be wr!LLen 16…”