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P. Borejko:
"Modeling three-dimensional propagation on the continental shelf";
Talk: The 12th International Conference on Theoretical and Computational Acoustics, Hangzhou, China (invited); 2015-10-11 - 2015-10-16.

During acoustic propagation experiments, conducted on the southeast Florida shelf in 2007, three-dimensional (3-D) effects were observed when the measured signal was produced by a low frequency source that was towed north parallel to the shelf from a bottom-mounted horizontal line array (HLA) [Kevin D. Heaney and James J. Murray, J. Acoust. Soc. Am. 125(3), 1394-1402 (2009)].

The beamformed data recorded on the HLA show the direct path arrival at the bearing of the tow ship and a second refracted path arrival coming from an angle inshore of the direct arrival. A significantly higher received level (RL) of the refracted arrival than that of the direct arrival is also observed in the data.

A modeling approach based on a 3-D adiabatic mode technique was applied to predict the measurements [Megan S. Ballard, J. Acoust. Soc. Am. 131(3), 1969-1977 (2012)]. It was shown that the arrival observed along the refracted path was due to horizontal refraction on the sloped shelf shoreward of the ship track. The higher RL of this arrival was explained by the local sediment properties. The direct arrival, which propagated over the limestone sediment (being treated as elastic in the model), was attenuated due to shear conversion, whereas the RL of the refracted arrival, which propagated over the sandy slope (being modeled as a fluid), was preserved.

Inspired by the 3-D adiabatic mode model predictions, in this paper, a 3-D generalized ray model is applied to explain the measurements. In the model the received signal is a superposition of arrivals along various ray paths that are represented by a series of integrals, each of which is evaluated exactly by applying the Cagniard method. By following the ray paths, the model enables one to analyze in detail the received signal. The arrival observed along the direct path is modeled by a group of arrivals along the ray paths of a few bounces, labeled the "direct group;" and the arrival observed along the refracted path is modeled by a group of arrivals along the (horizontally refracted) ray paths of multiple bounces, labeled the "refracted group." The model shows that the RL of the refracted group propagating over the sloped fluid bottom (the sandy slope on the shelf) is significantly higher than that of the direct group propagating over the elastic bottom (the limestone sediment on the shelf).