Mode coupling and the environmental sensitivity of shallow-water propagation loss predictions

Abstract

Variations of seabed parameters, particularly the acoustic compressional velocity relative to that of the water column, have a very strong effect on the predicted magnitude of the propagated acoustic field. In many cases, the sensitivity of the predicted losses to the assumed seabed parameters is sufficiently severe to render the predictions meaningless. In this article, coupled-mode theory is employed to study how the presence of lateral seabed inhomogeneities affects this sensitivity. The dependence of predicted propagation losses on sediment sound speed is first examined for horizontally stratified sediments; then a rough layered structure of clay-silt interbedded with sand is assumed. With the introduction of rough sub-bottom layering, coupling occurs between modes, so that the conversion of energy into progressively higher-order modes, which attenuate rapidly, becomes an important loss mechanism. The extent to which acoustic energy penetrates the seabed and interacts with the sub-bottom inhomogeneities is governed by the sediment sound speed. Hence, the mode coupling that is induced is simply another loss mechanism dependent on sediment sound speed, leading to an increase in sensitivity to this parameter. Studies to determine the effect of inhomogeneous sediment layering on the transverse horizontal spatial coherence of the propagated field reveal that, even in the presence of mode coupling, the coherence remains high over many acoustic wavelengths, in agreement with measured data trends.

Analysis and modeling of broadband airgun data influenced by nonlinear internal waves

“…Resonant coupling between acoustic modes is caused by range-dependent anomalies in the bottom 38 or water column 7,11 when the following condition is satisfied:…”

Mid Frequency Bottom-Interacting Sound Propagation and Reverberation in the Baltic

“…In the USC, a plausible explanation is channel leakage due to variations of the sound speed profile with range. For a lossy bottom, the process is partly irreversible and it leads to a larger TL as compared with rangeindependent conditions [22][23][24]. Fig.…”

Spatial coherence and rough bottom scattering in shallow water

“…She also derived the mode coupling due to lateral seabed inhomogeneities on propagation loss and transverse horizontal spatial coherence [71]. Bellis [77] and McDaniel [75] have studied coupled mode approach on rough surface scattering.…”