Blue Water Acoustics Research

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
	Study the spatial and temporal coherence of long-range, low-frequency resolved rays and modes Explore the source-to-receiver distance and frequency dependence of the fluctuation statistics of resolved acoustic (ray and mode) arrivals and of the highly scattered (unresolved) signal finale observed in previous experiments Understand the surprisingly large amount of acoustic energy scattered into the geometric shadow zone beneath deep caustics as measured with the NPAL network of bottom-mounted SOSUS receivers (shadow-zone arrivals) Define the characteristics, determine the causes, and estimate the trends of ambient acoustic noise on ocean basin scales Elucidate the relative roles of internal waves, ocean spice, and internal tides in causing acoustic signal fluctuations Improve basin-scale ocean sound speed predictions via assimilation of acoustic travel time and other data into numerical ocean-dynamic models

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 mixed-layer spice. All have a randomizing influence on propagating acoustic signals, just as atmospheric interference has on light from distant stars--the 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 well-understood, and there are numerous implementations of well-accepted approximations to the wave equation. These facts enable us to conduct "virtual" experiments that repeat famous at-sea 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 at-sea experiments, which probe imprecisely known ocean processes and their sometimes startling acoustic signatures.