Comparison of modeling approaches to low-frequency noise propagation in the ocean

In this study, the performance of a ray model for the prediction of long-range acoustic propagation in shallow-water scenarii, such as those encountered in coastal areas, is tested and discussed. Increased human activities in the marine environment over the recent years (seismic exploration, wind farming, shipping, etc.) has caused a significant general increase in ocean noise level, raising concerns over its impact on marine life and fisheries. Anthropogenic noise in the oceans covers a wide spectral range, including very low frequencies down to 10 Hz, and can propagate over large distances up to several tens of kilometers. Modeling the acoustic propagation to predict the noise impact in the marine environment thus becomes important for acoustic-system design in various applications and in compliance with the regulations regarding the uses of sound in the ocean. The study compares transmission-loss-versus-range curves calculated by a ray-tracing model (PlaneRay) with solutions by two wave-theoretic models particularly suited ti the shallow-water environment, namely a wavenumber integration (WI) model and a parabolic equation (PE)-based in a number of canonical test cases of increasing complexity. In particular, the WI model at hand allows for a combination of several fluid and elastic bottom layers in range-independent environments. On the other hand, the PE model at hand allows for comparisons in both range-independent and range-dependent environments with the bottom being described as all-fluid or all-elastic. The test cases are accordingly chosen to consist of a water layer lying over a fluid or elastic bottom half-space. Each case is analyzed and interpreted at several frequencies in the spectral range from 15 to 100 Hz. The comparison results show a good and promising agreement of the ray-tracing model with the PE and WI models. In particular, although based on the high-frequency approximation, the ray-tracing model appears to be an acceptable and efficient choice providing reliable predictions even at low frequencies and/or shallow-water environments.