Shallow Water Sound Propagation Studies in an Intermediate Scale Model

Abstract

A one-half square mile of constant water depth of about 10 ft is situated close to Pacific Naval Laboratory in the Esquimalt Lagoon and forms a natural intermediate scale model tank for shallow water propagation studies. Bottom velocities and layering depths have been obtained from a seismic survey. The dependence of sound level of a cw signal on transmitter depth at range to depth ratios of 30, 45, 60, 75, 90, 105 has been obtained for the first three modes of propagation. The received sound level from a transducer moved at constant depth towards the receiver has been recorded with acoustic wavelength and transducer depth as parameters. Instrumentation to (a) permit rapid measurement of vertical temperature profiles from a remote position, and (b) to record surface wave amplitude at the Tx and Rx position has been installed. Fluctuations in level of received cw signals appear well-correlated in frequency with surface waves. The dependence of the percentage of fluctuations on (a) surface wave amplitude, and (b) acoustical wavelength has been determined. A wave packet experiment, using the bow wave from a boat to produce the sound fluctuations, throws some light on the mechanism involved.