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.