Acoustic measurements on mid-shelf sediments with cobble: Implications for reverberation

The vast majority of sediment acoustics research has focused on rather homogeneous sandy sediments. Measurements for sediments containing cobbles (grain size greater than 6 cm) are rare. Here, measurements are presented for mid-shelf sediments containing pebbles/cobbles mixed with other grain sizes spanning 7 orders of magnitude, including silty clay, sand, and shell hash. The 2 kHz sediment sound speed in two distinct layers with cobble is 1531±5 m/s and 1800±20 m/s at the 95% credibility interval. The dispersion over the 400–2000 Hz band was relatively weak, 2 and 7 m/s respectively. The objective is to (1) present results for a sediment type for which little is known, (2) motivate development of theoretical wave propagation models for wide grain size distributions, and (3) speculate on the possibility of cobble as a scattering mechanism for mid shelf reverberation. The presence of cobbles from 1 to 3 m (possibly extending to 6 m) sub-bottom suggest they are the dominant scattering mechanism at this site. Though sediments with cobbles might be considered unusual, especially on the mid-shelf, they may be more common than the paucity of measurements would suggest since typical direct sampling techniques (e.g., cores and grab samples) have fundamental sampling limitations. [Research supported by ONR Ocean Acoustics.]