James M. Sabatier
Christopher C. Lawrenson
James B. Shadwick
Craig J. Hickey
F. Douglas Shields
Natl. Ctr. for Phys. Acoust., Univ. of Mississippi, University, MS 38677
A sediment tank 11 ft deep and 10 ft in diameter has been installed at the National Center for Physical Acoustics. This tank has been instrumented with a vertical array of B&K 8103 hydrophones along the axis of the tank, and has been filled with washed ``pit run'' sand. In studying the sound transmission through the sediment, the hydrophones are used both as senders and receivers. The tank is being used to develop ``shaded'' array transducers for sub-bottom profiling for measuring sound speeds and absorptions in sediments. Difficulties have been experienced in getting a homogeneous sediment. However, two distinct waves have been observed. The first is a fast wave with a velocity equal to approximately 1600 m/s that propagates readily at frequencies up to 100 kHz. The second is a much slower wave that cuts off above 2 kHz and has a velocity of a few hundred meters per second. Experiments are under way to determine the frequency dependence off the speed and absorption of these two waves. Calculations are being made in an effort to explain theoretically the experimental results. [Work supported by ONR, MMRI.]