Temporal variability of narrow‐band tones in very shallow coastal waveguides

During an experiment near San Clemente, CA, a source moored on the bottom just outside the surf zone (500 m offshore, approximately 9‐m depth) transmitted eight tones between 70 and 700 Hz to two nearly perpendicular, 64‐element bottom‐lying hydrophone arrays located approximately 1.2 km downcoast and 1 km further offshore (approximately 12‐m depth). The acoustic tones were transmitted for 5 min every half hour over a 22‐h period. Surface wave data obtained from a depth sensor array located approximately 40 km upcoast, as well as data from a PUV sensor data deployed during the experiment, are used to compute concurrent directional surface wave spectra. Long fast Fourier transforms of the acoustic recordings reveal temporal and spatial variability of the width and shape of the received tones in temporal frequency space. Surface‐wave‐related mechanisms of acoustic frequency distortion are proposed and their relative significance is investigated through the comparison of the data with results from physics‐ba...