TIME-FREQUENCY ANALYSIS OF BROADBAND SOUND PULSE PROPAGATION IN 3-D OCEANIC WAVEGUIDES

International audience
When considering low-frequency sources, the oceanic environment can act as a very dispersive waveguide, especially in shallow-water long-range sound wave propagation. One efficient tool for studying dispersion effects is based on time-frequency (TF) analysis of the received signals using a single receiver [1,2] . However, theses studies only considered two-dimensional (2-D) waveguides. In the present paper, TF analysis is used to study the propagation of low-frequency broadband pulses in three-dimensional (3-D) shallow-water waveguides. In particular, the paper focuses on the 3-D ASA wedge benchmark for which the received signals present characteristic modal structures including multiple mode arrivals. Indeed, it is well known that, due to the presence of a tilted bottom, a single propagating mode can have two distinct arrivals on a single receiver located in the across-slope direction. One objective of the present study is to show that TF analysis can be a suitable tool to better understand and easily illustrate this phenomenon. The paper is organized as follows. The 3-D ASA wedge benchmark is first recalled in Sec. 2. The simulated time signals are computed using a fully 3-D parabolic equation based code. The TF analysis is detailed in Sec. 3. The 2-D signals are first analyzed. Then comes the TF analysis for the 3-D signals. In particular, the spectrograms reveal that the second arrival of a given mode looks like the first one but is reversed in time. This observation is explained using mode-ray analogies. The paper ends with some concluding remarks.