THE SHALLOW-WATER 2006 EXPERIMENT (LEAR-NLIWI-AWACS)

Interdisciplinary Team Tackles Unanswered Questions in Acoustical Oceanography:

1. The Shallow-Water 2006 experiment promises to shed light on fundamental questions in acoustical wave propagation physics, physical oceanography, seabed properties, and to provide results of great practical significance.

2. Recent advances in technology and the concentration of research efforts on specific locations make this research particularly exciting and promising.

3. This project will bring together approximately 40 scientists from 12 research institutions, with funding support from the Office of Naval Research and Defence Research Development Canada.

An interdisciplinary team of acoustical oceanographers, geophysicists, and engineers has embarked on a series of experiments designed to answer some of the most fundamental questions in acoustical wave propagation physics, namely: (1) What is the influence of the seabed on sound transmission from a source to a receiver in the water column? (2) What is the influence of internal waves and fronts on sound transmission in the ocean? (3) Can the transmitted sound waves be used to determine the geological and sediment structure of the seabed? (4) Can measurements of the acoustic field radiated by natural and manmade sources be used to track and localize these sources? (5) Can the effects on sound transmission of the variable water column be distinguished from effects due to the complexities of the seabed? (6) What are the most relevant environmental parameters to measure for acoustic field prediction, and where and how often to measure them? The answers to these questions are not only of fundamental scientific interest, but also have great practical significance. Sound is the most effective remote sensing tool available to mankind underwater. Increasing our understanding of ocean acoustic propagation improves our ability to describe the earth’s structure, to monitor the ocean’s moving water masses, to communicate underwater, and to localize and track sources, such as vessels from hostile nations and marine mammals. The central problem in shallow-water acoustics is how to integrate and manage environmental uncertainties.

Although these basic research questions have been asked and studied for many years, there has never been a time when the scientific and engineering community has been better positioned to address these critical issues. This fortunate state of affairs is attributable to several factors. First, ocean instrumentation has advanced to a point where high-resolution, highly accurate measurements of the sound field as well as of ocean water and seabed properties can be made. The Global Positioning System has revolutionized our ability to pinpoint precisely where acoustic, oceanographic, and bottom measurements are made. Advances in microprocessor technology have led to the ability to acquire and record enormous amounts of data in instruments of manageable size. Second, the exponential growth in computing power has enabled the rapid processing, display, and interpretation of these data as well as comparisons with sophisticated theoretical models. Third, the ocean science community has adopted the strategy of selecting field sites where many investigators make measurements in the same areas, thereby enabling the intercomparison of results and the evaluation of different measurement methodologies.

These three factors provide the underpinnings for the Littoral Environmental Acoustics Research (LEAR) Project led by James Lynch from the Woods Hole Oceanographic Institution and Dajun Tang from the Applied Physics Laboratory of the University of Washington. They are coordinating with scientists from the Non-Linear Internal Waves Initiative (NLIWI) project and with scientists participating in the Autonomous Wide Aperture Cluster for Surveillance (AWACS) project. Altogether the joint experiment is known as Shallow-Water 2006 (SW06) and consists of a group of about forty scientists from twelve research institutions. They include physical oceanographers, geophysicists, and engineers specializing in ocean and seabed acoustics, geology, physical oceanography, and remote sensing. With support from the Office of Naval Research and Defence Research and Development Canada, they have assembled a unique set of state-of-the-art instrumentation designed specifically to address the research questions described above. These include low-power sound sources as well as vertical and horizontal arrays of receivers that can be used to discriminate signals arriving from different directions. Moored, mobile, and drifting measurement systems, including GPS-navigated buoys, sea gliders, and autonomous underwater vehicles will also be used to make high-resolution maps of the ocean environment and the ambient sound field. Using four vessels from the academic research fleet, experimental configurations will be implemented to provide quantitative information about how much the propagated acoustic field is attenuated due to scattering and refraction in the water column and the seabed. An extensive suite of oceanographic and geophysical instruments will be deployed to provide environmental data in support of the acoustic measurements. These include conductivity, temperature, and salinity probes, turbulence and current meters, and sediment sampling devices. A wide range of state-of-the-art microprocessors, laptop and desktop computers, and workstations will be used to acquire and process all of these data, in many cases in real time.

Finally, the East Coast STRATAFORM area has been chosen as the site of the Shallow-Water 2006 experiment. This area, which is shown in the figure, is about 100 miles east of Atlantic City, New Jersey and has been the site of extensive previous acoustic, geophysical, and physical oceanographic measurements. The specific locations where the activities will be concentrated are in water depths ranging from 50 m to 150 m. In addition, the time of year chosen for SW06 (late summer) when combined with the low power levels of the sound sources being used, virtually guarantees no impact on any marine life in the area.

The Shallow-Water 2006 experiment promises to shed light on fundamental questions in acoustical wave propagation physics, physical oceanography, seabed properties, and to provide results of great practical significance. Results will be published and available in the leading journals of peer-reviewed scientific literature.