## Ocean Environmental Acoustics

The ocean acoustics research in LAMSS falls largely in two main areas, environmental acoustic modeling and simulation, and autonomous, adaptive and collaborative acoustic sensing by distributed ocean sensing networks.

## Modeling and Simulation

The environmental acoustic modeling in LAMSS has traditionally focused on hybrid and spectral methods for wave theory modeling of seismo-acoustic propagation and scattering in ocean waveguides. The laboratory maintains and distributes a wide suite of acoustic models, including OASES, Csnap, and OASES-Scatt.

## Autonomous, Adaptive and Collaborative Acoustic Sensing

The development of autonomous behaviors for adaptive environmental sensing by undersea networks andautonomous, multistatic acoustic sensing by undersea distributed networks is the principal objective of the field experiments LAMSS is involved in. Thus, under SWAMSI and GOATS we are investigating the benefits of multistatics and collaborative, autonomous adaptation for Detection, Classification, Localization and Tracking of undersea acoustic targets.

## Recent Thesis Projects

### 3-D Mode Coupling around Seamounts

The propagation over and around a seamount is inherently a three-dimensional problem, except for the trivial case of the source located at the top of a cylindrically symmetric seamount. With the interest in exploring global propagation for climate monitoring etc., a significant effort has been invested in the development of adequate numerical models capable of addressing the effect of mid-ocean seamounts on long-range propagation.

To allow the modeling of 3-D propagation and scattering around realistic seamounts at relevant frequencies, a spectral coupled-mode model for the field generated by an offset acoustic source in an ocean with axisymmetric bathymetry was developed by LAMSS under a PhD thesis project by Wenyu Luo. This approach combines a spectral decomposition in azimuth with a coupled-mode theory for two-way, range-dependent propagation. Numerical stability and high efficiency is achieved by making a number of significant modifications to the theoretical and numerical formulation, leading to orders-of-magnitude improvement in numerical efficiency for realistic problems compared to earlier implementations. Further, by using a standard normal-mode model for determining the fundamental modal solutions and coupling matrices, and by applying a simple superposition principle the computational requirements are made independent of the distance between the seamount and the source and receivers, and dependent only on the geometry of the seamount and the frequency of the source. As a result, realistic propagation and scattering scenarios can then be modeled, including effects of seamount roughness and realistic sedimentary structure.

The 3-D, coupled-mode model has been used to analyze the mode coupling occurring at the edge of a conical seamount, demonstrating that the out-of-plane scattering leads to significantly stronger shadowing behind the seamount than predicted by traditional two-dimensional models, as evident in the figure above.

- W. Luo, “Three-Dimensional Propagation and Scattering around a Conical Seamount,” Ph.D. thesis, MIT, 2007
- W. Luo and H. Schmidt, “Three-Dimensional Propagation and Scattering around a Conical Seamount,” J. Acoust. Soc. of Am., 125(1), 52-65, 2009

### Understanding and utilizing the waveguide invariant

The range-frequency waveguide invariant describes striations that often appear in plots of acoustic intensity versus range and frequency. An example of these striations is shown in the figure below, which was collected during the Glint08 cruise.

Signal processing techniques based on the range-frequency waveguide invariant are able to exploit the effects of the ocean acoustic waveguide without requiring detailed knowledge of the sound speed profile or of the seafloor. A method for understanding and calculating the waveguide invariant was developed by LAMSS under a PhD thesis project by Kevin Cockrell. This project achieved four main results:

- A method for passively estimating the range from an acoustic source to a receiver was developed, and tested on the experimental data shown above, which was from Glint08. Heuristics were developed to estimate the minimum source bandwidth and minimum horizontal aperture required for range estimation.
- A semi-analytic formula for the waveguide invariant was derived using WKB approximation along with a normal mode description of the acoustic field in a range-independent waveguide. This formula is applicable to waveguides with arbitrary SSPs, and reveals precisely how the SSP and the seafloor reflection coefficient affect the value of the waveguide invariant.
- Array processing techniques designed specifically for the purpose of observing range-frequency striations were developed and demonstrated.
- A relationship between the waveguide invariant and wavenumber integration was derived, which may be useful for studying range-frequency striations in elastic environments such as ice-covered waveguides.

Associated Publications:

- K. L. Cockrell, "Understanding and utilizing waveguide invariant range-frequency striations in ocean acoustic waveguides," Ph.D. thesis, MIT-WHOI, 2011
- K. L. Cockrell, H. Schmidt "A relationship between the waveguide invariant and wavenumber integration" (Submitted, JASA)
- K. L. Cockrell and H. Schmidt, "Robust passive range estimation using the waveguide invariant," J. Acoust. Soc. Am. 127 2780 (2010)