Characteristics of bottom-diffracted surface-reflected arrivals in ocean acoustics

Bottom-diffracted surface-reflected (BDSR) arrivals are a ubiquitous feature in long-range ocean acoustic propagation. BDSRs are distinct from bottom-reflected surface-reflected (BRSR) arrivals because the angle of emergence is not equal to the angle of incidence. They are not predicted by existing forward models based on available bathymetric and bottom properties data. Research cruises in the Philippine Sea and North Pacific, in 2011 and 2013, respectively, were carried out to understand BDSRs in more detail for transmissions out to 50 km range. Transmissions from a controlled source at about 60m depth were received on ocean bottom seismometers and a deep vertical line array of hydrophones. In the North Pacific experiment alone over 40 distinct bottom diffractor locations were identified. Based on these data sets BDSRs can be characterized in terms of: (a) in-plane or out-of-plane diffractors, (b) diffractor location relative to bathymetric features, c) grazing angle of the incident field, d) transmission frequency (from 75 to 310 Hz), (e) receiver type (vertical or horizontal seismometer, hydrophone, etc.), (f) receiver location, and (g) signal strength relative to direct and BRSR paths. [Work supported by ONR.]