Bottom Interaction in Ocean Acoustic Propagation

Abstract

LONG-TERM GOALSThis project addresses "the effects of environmental variability induced by ocean internal waves, internal tides and mesoscale processes, and by bathymetric features including seamounts and ridges, on the stability, statistics, spatial distribution and predictability of broadband acoustic signals..." (quote from the Ocean Acoustics web page). The long-term objective here is to understand the dominant physical mechanisms responsible for propagation and scattering in the deep ocean where the sound channel is not bottom limited. Understanding long range acoustic propagation in the ocean is essential for a broad range of Navy applications such as i) the acoustic detection of ships and submarines at long ranges, ii) avoiding detection of ships and submarines, iii) long range command and communications to submerged assets, and iv) improved understanding of the environment through which the Navy operates.Bottom interaction plays a significant role in the physics of both short and long-range ocean acoustic propagation and impacts physical models of both signals and ambient noise. As discovered on NPAL04 for controlled sources at long range (500-3200km) in deep water and frequencies around 75Hz, the sound field at the bottom is much more complex than the sound field in the ocean. The deep seafloor arrival pattern is not entirely random, but has some components that are regular, coherent and repeatable. OBJECTIVESThis project focuses on five tasks: 1) Participation on the PhilSea10 Mobile Operations (MOPS) cruise in July 2010 (three weeks, Kaohsiung-Kaohsiung). 2) Study the physical mechanisms controlling the strength of BR and SRBR paths as a function of frequency in the band 50-400Hz and ranges out to 300km or more. 3) Study the ambient noise as a function of depth and time on the NPAL04 and PhilSea10 DVLAs in the band 2-100Hz. 4) Analyze the controlled source DVLA data acquired on PhilSea09 and PhilSea10 to look for and explain "deep seafloor arrivals". 5) Further analysis of the mysterious "deep seafloor arrivals" on the NPAL04 OBSs. For convenience, we have split these five tasks into two parts, A and B. Part A is the ongoing analysis of the new "deep seafloor arrivals" observed on NPAL04 (Task 5). The first year of Part B includes the cruise participation and analysis of existing DVLA data (Tasks 1 and 3). The second year of Part B focuses on Tasks 2, 3 and 4 using all three DVLA data sets (NPAL04, PhilSea09 and PhilSea10).