Experimental detection of a slow acoustic wave in sediment at shallow grazing angles

Following recent experimental results at sea (N. P. Chotiros, Proc. Oceans '89) that suggest the existence of a previously undetected type of acoustic wave in sandy sediments, an experiment was designed to detect and measure the speed of acoustic waves in an isolated environment. The experiment was conducted in a laboratory tank containing 1 m of unwashed river sediment under a 3‐m water column. Observations were made of the travel time and attenuation of a pulse from an acoustic source located above the water‐sediment interface to a set of probes below the interface. It was observed that, at normal incidence the pulse traveled at about 1750 m/s, while at shallow grazing angles, the pulse traveled through the sediment at close to 1200 m/s. An interesting possible explanation exists in the Biot model [M. A. Biot, J. Acoust. Soc. Am. 28, 168–191 (1956)], which predicts a slow acoustic wave in porous materials. [Work funded by ONR under NOARL management.]