Low-frequency sound propagation in short ranges: A case where leaky modes can be observed

Abstract

In an ocean waveguide, sound propagation can be modeled by the normal-modes theory. In a lossless ocean floor, the acoustic field is a finite sum of the normal modes that are trapped in the water column. These modes have real acoustic wave numbers and their equivalent rays are at angles less than the critical grazing angle. When attenuation is present in the ocean floor, the number of normal modes increases. Those additional modes, called “leaky modes,” have complex acoustic wave numbers, and their equivalent rays are steeper than the critical grazing angle. Since leaky modes attenuate in short ranges, they are not typically observable in measurements. In this presentation, I show that leaky modes can be observed on the data collected during marine seismic reflection surveys. Marine seismic reflection surveys are used to image the structure of the seafloor. They generate low-frequency signals from airguns and record the reflections off the seafloor up to a few kilometers from the source. I use simulations and experiments to characterize the short-range propagation of airgun pulses in shallow and deep water. The experimental data utilized in this study are from the COAST experiment conducted in the Northeast Pacific continental shelf.