Analysis of acoustic transmissions from ONR Inner Shelf Dynamics Direct Research Initiative
Abstract
The inner shelf is defined as the transition zone between the surface wave dominated surf zone and themid-shelf, beyond which ocean dynamics are governed by Ekman transport. The relatively little-studied innershelf is characterized by processes such as non-linear internal waves, Langmuir mixing, rip currents, and wind driven upwelling and downwelling. A consequence of the lack of knowledge into processes that govern dynamics on the inner shelf is that their impact on acoustic propagation is also under-studied. A thorough understanding of acoustic fluctuations in this region is of considerable interest to naval applications such as mine-hunting sonar, search and rescue operations, and underwater communications. The recently completed Inner Shelf DRI (IS-DRI) experiment off of Vandenberg Air Force base in Central California included detailed measurements of physical oceanographic features from the surf zone to the inner shelf. The goal of the experiment was to understand the role of bottom and surface boundary processes, and, wind- and wave-driven circulation on inner-shelf dynamics. Measurements included bottom-mounted current meters (ADCPs), thermistor chain arrays, wave buoys, surface radars and aerial imagery. This comprehensive and diverse set of measurements provides an exceptionally high-resolution dataset of near-shore circulation and shelf exchange processes. In an effort to characterize sound propagation on the inner shelf, a modest acoustic experimental effort was conducted alongside the IS-DRI to support an initial understanding of acoustic propagation in the dynamical nearshore environment. A 27 kHz acoustic source was deployed in 10 m water depth offshore of Point Sal. Sounds transmitted by this source were simultaneously received on two arrays consisting of three acoustic receivers each, located at was at a distance of 500 m and 1 km respectively from the source along the 10 m isobath. Gathered active and passive acoustic data, when analyzed together with vast amounts of physical oceanographic data will help answer key questions about acoustic propagation in this dynamic region. Scientific examples include transmission loss characteristics through rip currents and nearshore eddies, background noise levels from energetic physical oceanographic features such as breaking surface waves, and a description of the anisotropy of ambient noise on the inner shelf. While the original award funded a single deployment lasting two weeks, a request for additional deployments was made by ONR to obtain data to cover as much of the experiment duration as possible. Consequently, budget that was originally allocated for analysis was spent to cover the extended field effort. This proposal requests additional funding to cover analysis and reporting.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 26, 2018
- Source ID
- N000141812433
Entities
People
- Kaustubha Raghukumar
Organizations
- Integral Consulting (United States)
- Office of Naval Research
- United States Navy