Deep seafloor arrivals: An unexplained set of arrivals in long-range ocean acoustic propagation

Receptions, from a ship-suspended source (in the band $50–100Hz$⁠) to an ocean bottom seismometer (about $5000m$ depth) and the deepest element on a vertical hydrophone array (about $750m$ above the seafloor) that were acquired on the 2004 Long-Range Ocean Acoustic Propagation Experiment in the North Pacific Ocean, are described. The ranges varied from $50to3200km$⁠. In addition to predicted ocean acoustic arrivals and deep shadow zone arrivals (leaking below turning points), “deep seafloor arrivals,” that are dominant on the seafloor geophone but are absent or very weak on the hydrophone array, are observed. These deep seafloor arrivals are an unexplained set of arrivals in ocean acoustics possibly associated with seafloor interface waves.