Intellectual Merit: In recent years it has been shown theoretically that, under very weak assumptions, a good approximation to the acoustic Green's function G(A, B, t) can be extracted by computing the cross-correlation of simultaneous measurements of ambient acoustic noise at locations A and B. G(A, B, t) is the response at location A to an impulsive point source at location B; by reciprocity G(A, B, t) = G(B, A, t) in a quiescent environment. This procedure is sometimes referred to as noise interferometry. The practical utility of noise interferometry has been demonstrated in a variety of settings, including seismic applications. Because this technique does not involve the use of active acoustic sources, it requires only very modest experimental resources. This project seeks to demonstrate the feasibility of noise interferometry in the ocean, and explore several issues relevant to understanding the limitations of noise interferometry for ocean remote sensing. The work involves a combination of experiment, data analysis, theory and numerical simulations. It is anticipated to provide a foundation for using noise interferometry as a practical means of acoustic remote sensing of the ocean's interior, and to that end the focus will be on using real ocean data to demonstrate the utility of the method under practical ocean conditions, rather than such things as idealized simulations.
Broader Impacts: The proposed work has applications to several fields with obvious societal benefits. These include noninvasive medical ultrasound for diagnostic and monitoring purposes, and nondestructive evaluation of structures, e.g., crack detection in bridges using only traffic-generated vibrations. In addition to developing the tools to understand and address these problems, the project will support and train two graduate students. For ocean remote sensing, the ability to use ambient sound for remote sensing of the ocean volume eliminates the possibility of harming marine life with active sound sources. The work will be done in loose, unfunded collaboration with a small group of Russian scientists, thereby contributing to the strengthening of international ties.
Observing the interior of the ocean is importnat for a variety of reasons. Like the atmosphere, ocean circulation patterns are constantly changing. These changing circulation patterns are important for a variety of reasons. They control the distribution of nutrients, biota, pollutants, heat, etc. Surface currents are important for shipping, pollutant dispersal, etc. The oceans role in redistributing heat is of critical importance in understanding the ocean's role in moderating the Earth's climate. Observing the interior is a daunting task due to its size and the remoteness of much of the ocean volume. It is impractical to make direct in situ measurements of the ocean interior with the resolution required to resolve the most energetic scales. Furthermore, remote sensing of the interior of the ocean cannot be done using electromagnetic waves (light, microwaves, radar, etc). This is because electromagnetic waves are strongly attenuated in seawater. On the other hand, sound can be used for underwater remote sensing as sound can travel long distances in the ocean. Traditionally, underwater remote sensing using sound has been performed using active (mechanical) sources. But there are drawbacks to using mechanical sources. Low frequency (under a few hundred Hz) sound sources are large, heavy, expensive and difficult to deploy. Also, there are some concerns about potential impacts of man-made sounds on marine life, which has lead to some regulatory restrictions. The work performed as part of this NSF grant focussed on using a new technique to perform remote sensing of the ocean interior. The technique is based on using the ambient underwater sound field, rather than using an active sound source. Ambient underwater sound is generated by shipping, weather-related phenomena (wind, breaking waves, rain, etc) and biological sources. The remote sensing technique that we used has been used successfully in other fields, most notably seismology, and is referred to as noise interferometry. In an oceanographic context, the utility of noise interferometry had previously been demonstrated at ranges up to 3 km in very simple geometries. Our effort focussed on exploring the utility of noise interferometry in more typical and challenging ocean environments, and at longer ranges. To explore the utility of noise interferometry in the ocean, we fabricated 3 ocean acoustic sound receiver and recorder systems; we deployed the three systems three times in the Florida Straits in different acoustic environments, and we analyzed the data. The results were positive and encouraging. We were able to demonstrate the utility of noise interferometry in a very challenging shallow water environment at ranges up to 10 km. We were able to extract estimates of both sound speed structure and horizontal currents from the measurements collected. Theoretical arguments lead us to expect that in typical deep ocean environments (with depths of approximately 5 km), noise interferometry should be feasible at ranges of 50 km, and possibly longer.