Measurements of three-dimensional propagation in a continental shelf environment

Although a significant amount of theoretical and numerical modeling effort has been put into the study of three-dimensional (3D) acoustic propagation on a coastal wedge, including the development of the ASA 3D benchmark problem set, there have been few observations of the predicted 3D propagation effects. Significant horizontal multipath arrivals were observed in a pair of acoustic transmission tests on the continental shelf off the east coast of Florida in September 2007 and February 2008. For many transmissions, arrivals were received coming from nearly the global positioning system (GPS) bearing of the ship, as well as up to 30 deg inshore of the true bearing. The inshore path was up to 25 dB stronger than the direct path in some cases. The experimental waveforms transmitted included continuous-wave transmissions ranging in frequency from 24 to 415 Hz as well as wideband linear frequency modulation pulses (20-420 Hz). Horizontal multipath arrivals were observed for source ranges from 10 to 80 km, source depths of 20 and 100 m, and along several different bearings (inshore and along the 250 m isobath). It is a conclusion of this paper that the bearing bias and multiple horizontal arrivals are the result of 3D propagation due to the local shoaling bathymetry.