Ocean Bottom Seismometer Augmentation of the NPAL 2010-2011 Philippine Sea Experiment.

摘要

This project addresses the coherence and depth dependence of deep-water ambient noise and signals. Seafloor signals are studied in the band from 50-400Hz and seafloor ambient noise is studied in the band from 0.03 - 80Hz. On NPAL04 we observed a new class of arrivals in long-range ocean acoustic propagation that we call Deep Seafloor Arrivals (DSFAs) because they are the dominant arrivals on ocean bottom seismometers. They either were undetected or very weak on the deepest DVLA hydrophone located near the conjugate depth about 750m above the seafloor. It appears that at least part of the path for DSFAs is through or on the seafloor perhaps as an interface wave. One goal is to see if DSFAs will be observed in the Philippine Sea at relatively short ranges of 50km or less and, if so, to study their characteristics and physical mechanism. The objective here is to understand the relationship between seafloor pressure and seafloor particle motion for both ambient noise and short-and long-range signals. What is the relationship between the seismic (ground motion) noise on the seafloor and the acoustic noise in the water column. What governs the trade-offs in contributions from local and distant storms and in contributions from local and distant shipping. How effective is seafloor bathymetry at stripping distant shipping noise from the ambient noise field. Clearly the ability of Navy systems to detect and identify ships and submarines by acoustic techniques will depend on at least the following factors: (i) the system noise of sensors used to detect the acoustic field, (ii) the true field noise for a given sensor type and location, and (iii) accurate knowledge of how sound travels in the ocean including bottom interaction if necessary. The observation of deep seafloor arrivals on NPAL04 showed that there is a significant bottom path for coherent sound propagation that was previously unrecognized and is still poorly understood.