Statistics 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 and are not predicted by existing forward models based on available bathymetric and bottom properties data. In a research cruise in the North Pacific in 2013 over 40 distinct bottom diffractor locations were identified within a 25 km radius survey (at least one diffractor location every 50 square kilometers). The BDSRs can be characterized in terms of: (a) grazing angle of the incident field, (b) in-plane or out-of-plane diffractors, (c) frequency (transmissions were made from 77.5 to 310 Hz), (d) receiver type (vertical or horizontal seismometer, hydrophone, etc.), (e) receiver location, (f) signal strength relative to direct and water multiple paths, and (g) location relative to bathymetric features. In this talk, we present the statistics of the BDSRs in terms of these criteria. This information will inform efforts to predict and exploit BDSRs and will be valuable in planning future cruises to identify the geologic features responsible for them. [Work supported by ONR.]