The Blue Water Acoustics Research group is a multidisciplinary team of investigators committed to solving the fundamental physical problems of oceanic acoustic propagation across ocean basins. Our inquiry is focused to maximize application to tactical and environmental monitoring systems. 
Inquiry 


Science Problems The forward problem The ocean is an acoustic waveguide with strong deterministic refractive properties and weak random refractive perturbations. These are due to small and large features in the ocean including internal waves, mesoscale eddies, and mixedlayer spice. All have a randomizing influence on propagating acoustic signals, just as atmospheric interference has on light from distant starsthe perturbations cause the acoustic signals to "twinkle." Solving this problem means understanding the statistics of acoustic twinkling. The inverse problem Acoustic signals propagating through a water mass essentially interrogate that mass. Inferring the internal structure of the water mass from the measured statistics of the interrogatin signal is the inverse problem. Success in solving this problem depends critically on an adequate understanding of the corresponding forward problem. Inverse problems include velocimetry (using the signal Doppler profile to estimate water velocity); tomography (using the signal arrival times to estimate the water volume's interior thermal structure); and internal wave tomography (using signal fluctuations to estimate parameters of the internal wave spectrum). Oceanic ambient noise Undersea noise includes the vocalization of its inhabitants, from shrimp and toadfish to baleen whales. Winds on the surface, too, excite breaking waves that generate sound through bubble injection. Ambient sound is anthropogenic, too: ships radiate engine noise through their hulls and cavitation noise from their props. These sounds made be people generate a distinctive signal throughout the world's oceans.We study ambient noise to mitigate it negative influence on signal detection and extraction. Listening to the undersea environment also provides insight to the ocean's inhabitants and our influence on the environment. 
Methods Experimental Measurements in the World's Deep Ocean Basins (Information to come ... still under construction) Computational Physics The wave propagation physics of acoustic fields is considered to be wellunderstood, and there are numerous implementations of wellaccepted approximations to the wave equation. These facts enable us to conduct "virtual" experiments that repeat famous atsea experiments such as MATE, AFAR and ATOC, as well as new experiments that use transmitter and receiver configurations too complicated or prohibitively expensive to actually attempt. These experiments explore the propagation of statistical properties of randomized acoustic fields and also serve as testbeds for studying the fundamental limits of signal processing algorithms. Theoretical Development The evolution (in space and/or time) of the statistical properties of a scalar field propagating in a random medium waveguide remains an unsolved problem. It is an example of a stochastic differential equation. Ray theory — which underpins the first successful WPRM theories from the 1960s — appears to break down in refractive media at turning points, where the Markov approximation may no longer hold. Mode based solutions for even the field second moment appear to require a prohibitive number of modes for accurate solutions. Contemporary efforts synergize with numerical experiments, which simulate physics in a controlled and understood domain, and with atsea experiments, which probe imprecisely known ocean processes and their sometimes startling acoustic signatures. 
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experiments © 2006 Applied Physics LaboratoryUniversity of Washington 