Bottom Interaction in Long Range Acoustic Propagation

Analysis of data from the NPAL04 experiments has shown that there are three principal types of arrival in long-range ocean acoustic propagation: 1) arrivals predicted by PE model calculations (RR, RSR, RBR, SRBR ray paths and the mode-like finale region), 2) deep shadow zone arrivals arising from the spread of energy below PE predicted turning points (caustics) and attributed to internal waves (Van Uffelen et al., 2009), and 3) deep seafloor arrivals which can be the largest arrivals observed on seafloor receivers and do not correspond to turning points or any other features in the PE predicted path (Stephen et al., 2009; Stephen et al., 2008). Coda and "bottom junk" have been ubiquitously observed on acoustic receptions on seafloor receivers and are traditionally attributed to incoherent reverberation and scattering. The OBS data on NPAL04 has shown that there are robust, coherent, discrete arrivals that contribute to the coda and bottom junk. The long-term goal of this project is to understand the role of bottom interaction in long-range ocean acoustic propagation. At the moment we do no understand the physical mechanisms responsible for the deep seafloor arrivals and we do not understand the implications for seafloor receptions in shallower water.