Ocean Bottom Seismometer Augmentation of the Philippine Sea Experiment (OBSAPS) cruise report

Abstract

The Ocean Bottom Seismometer Augmentation to the Philippine Sea Experiment (OBSAPS, April-May, 2011, R/V Revelle) addresses the coherence and depth dependence of deep-water ambient noise and signals. During the 2004 NPAL Experiment in the North Pacific Ocean, in addition to predicted ocean acoustic arrivals and deep shadow zone arrivals, we observed "deep seafloor arrivals" that were dominant on the seafloor Ocean Bottom Seismometer (OBS) (at about 5000m depth) but were absent or very weak on the Distributed Vertical Line Array (DVLA) (above 4250m depth). These "deep seafloor arrivals" (DSFA) are a new class of arrivals in ocean acoustics possibly associated with seafloor interface waves. The OBSAPS cruise had three major research goals: a) identification and analysis of DSFAs occurring at short (1/2CZ) ranges in the 50 to 400Hz band, b) analysis of deep sea ambient noise in the band 0.03 to 80Hz, and c) analysis of the frequency dependence of BR and SRBR paths as a function of frequency. On OBSAPS we deployed a fifteen element VLA from 12 to 852m above the seafloor, four short-period OBSs and two long-period OBSs and carried out an 11.5day transmission program using a J15-3 acoustic source.

Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) - cruise report

“…They either were undetected or very weak on the deepest DVLA hydrophone located near the conjugate depth about 750m above the seafloor (Stephen et al, 2011). It appears that at least some of the DSFAs corresponded to long-range guided wave energy that diffracted from a small seamount near the receivers and reflected back down from the sea surface (Stephen et al, 2013).…”

“…The long-period OBS have broadband, three-component Trillium 240 seismometers and a Differential Pressure Gauge (DPG). The long period OBSs provided seafloor ambient noise data for comparison with other deep-water, broadband data sets in the Pacific such as OBSAPS (Stephen et al, 2011), H2O (Duennebier et al, 2002;Stephen et al, 2006) and the OSNPE (Stephen et al, 2003). SAIC Ultra-Low-Noise hydrophones (ULN-SAIC) were integrated into two of the long-period OBSs (LPA and LPC).…”

“…See Section 5 of the OBSAPS cruise report (Stephen et al, 2011) for a discussion of some of the issues involved in determining the depth of sensors. Based on input from Peter Worcester, we assume that the "best" depths are those determined from an amalgamation of multi beam data (from previous cruises as well as our own) that has been edited for "glitches" by Tom Bolmer (see Section 8.6).…”

“…Seafloor signals were studied in the band from 50-350Hz and seafloor ambient noise was studied in the band from 0.03-700Hz. The first cruise was the OBSAPS cruise in the Philippine Sea (Stephen et al, 2011). Both cruises focused on elucidating the physics and characteristics of Deep Seafloor Arrivals (DSFA) and Bottom-Diffracted Surface-Reflected (BDSR) arrivals Stephen et al, 2013).…”

The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea

“…A near-seafloor Ocean Bottom Seismometer Augmentation of the 2010-2011 NPAL Philippine Sea Experimentdistributed vertical line array (O-DVLA) and array of ocean bottom seismometers (OBS) were deployed in the Philippine Sea during April-May 2011, immediately following recovery of the PhilSea10 moorings, to study the relationship between the acoustic field in the water column and the seismic field in the seafloor for both ambient noise and signals transmitted by a J15-3 source (Stephen et al, 2011). The experiment was motivated in part by unexpected arrivals observed on OBSs at 5000-m depth in the Northeast Pacific from broadband signals transmitted by a ship-suspended 75-Hz source during the 2004 Long-range Ocean Acoustic Propagation Experiment (LOAPEX) Stephen et al, 2009Stephen et al, , 2012.…”