Bottom-diffracted surface-reflected arrivals in the North Pacific

Abstract

Bottom-diffracted surface-reflected (BDSR) arrivals were first identified in the 2004 Long-range Ocean Acoustic Propagation Experiment (LOAPEX) in the North Pacific (Stephen et al., 2013, JASA, 134, 3307-3317). In deep water, ambient noise and PE predicted arrivals are sufficiently quiet that BDSR arrivals, scattered from small seamounts, can be the largest amplitude arrivals observed. The Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) Experiment in June-July 2013 studied BDSRs at the same site in detail. They are most readily identified by their move-out on lines of transmissions and are clearest on the vertical component channel. There appear to be two classes of diffractors. The diffraction point for Type 1 BDSRs occurs on the side of small seamounts and is substantially out of the source-receiver sagittal plane (for example, the BDSR observed on NPAL04). The diffraction point for Type 2 BDSRs is essentially in the source-receiver sagittal plane and occurs on the relatively featureless abyssal plain. In at least one case the same BDSR is observed for 77.5, 155 and 310 Hz M-sequence transmissions. Other characteristics of BDSRs will be discussed. [Work supported by ONR.]