3D Acoustic Model for Geometrically Constrained Environments
Navy STTR 2016.A - Topic N16A-T018
ONR - Ms. Dusty Lang - dusty.lang@navy.mil
Opens: January 11, 2016 - Closes: February 17, 2016

N16A-T018 TITLE: 3D Acoustic Model for Geometrically Constrained Environments

TECHNOLOGY AREA(S): Battlespace, Human Systems

ACQUISITION PROGRAM: Advanced Underseas Weapons System (AUWS)

OBJECTIVE: Produce a 3D Acoustic model for predicting three-dimensional acoustic field parameters in environments characterized by complex geometries with variable boundary and propagation conditions. Assess the new model for use in existing, or newly developed, sonar performance estimation tools to address the optimal placement of sensors in constrained environments.

DESCRIPTION: Sonar performance models are used to assess the use of a particular sonar system for specific tasks including submarine detection, mine hunting, or swimmer detection. Feeding the performance models are acoustic models which are numerical solutions to a wave equation based on knowledge of the underlying physics and physical conditions in the prevailing environment. For active sonar systems, Transmission Loss (TL) and Reverberation Level (RL) are key sonar equation parameters derived from the acoustic fields predicted by models and used in the sonar equation to assess performance. For the majority of the existing propagation codes used in the Navy, whether based on ray theory, normal modes, wavenumber integration, or the parabolic equation, the starting point is the assumption of a horizontally stratified waveguide. Likewise, excepting volumetric contributions which are primarily biologic scattering, reverberation models are predominantly based on acoustic scattering from rough horizontal surfaces such as the seabed or sea surface. Consequently, most existing acoustic propagation models are concerned with predicting forward propagating and scattered acoustic energy. While three-dimensional acoustic models exist, or are being developed, they are based on refraction of acoustic energy owing to bathymetric changes and or internal waves or fronts that do not scatter energy strongly in the back-propagating direction. The existing models are adequate for applications in the deep ocean or open littorals, but sonar operators are increasingly being asked to perform tasks including navigation or detection in more confined waterways such as rivers or ports. However, models are generally not available for predicting the acoustic field in such highly geometrically constrained and dynamic environments. These environments can be characterized by vertical or near vertical boundaries such as piers and breakwaters and have large tidally driven depth variations over short time periods. They also may be populated with large scattering objects such as deep draft vessels and mooring dolphins that impact the acoustic field.

We seek a capability to model the three-dimensional acoustic field, including propagation, scattering, and reverberation in complex environments. Approaches should include, but are not limited to, predicting complex pressure from a point source, with a minimum frequency of 1 kHz, placed arbitrarily within a representative harbor environment, e.g. Mayport Basin, Florida. Solutions should provide wavelength resolution for area dimensions greater than six million square feet for a typical depth of 50 feet. The environment may be open to the sea, but must include at least one vertical boundary representative of a quay wall, a breakwater, and a blockage representing a deep draft vessel with draft of 60%-90% of the channel depth. A broadband model is preferred, but a narrowband solution is acceptable if accompanied by a conceptual plan for development into a full broadband solution. Computational efficiency and speed is not a priority, but will be given consideration. Amongst other things, it is expected this capability will form the basis for existing or new sonar performance estimation tools. In particular, the model combined with an appropriate decision aid could address the optimal placement of sensors in complicated environments for tasks including establishing underwater communication links or harbor surveillance.

PHASE I: Define and develop a concept to predict acoustic field parameters in highly geometrically constrained underwater environments. Concepts should include approaches to predict the three-dimensional complex acoustic pressure field for a point source in representative environments such as described above. Develop concepts for incorporating the new acoustic model into sonar performance estimation models, existing or proposed, to address optimal placement of acoustic sensors to achieve basin wide communications coverage or object detection.

PHASE II: Produce an acoustic model capable of generating a 3D complex acoustic pressure field in a geometrically constrained environment described as for Phase I. Perform initial validation and verification testing of the new model and document changes in the acoustic field for changes in source position and the presence or absence of quay walls, breakwaters, deep draft vessels, etc. Document the associated mathematical development and implementation in technical reports and user manuals. Provide details on software and hardware requirements for the new code. Provide a plan for integrating the new acoustic model into existing sonar performance estimation models or a development plan for a new integrated sonar performance model.

PHASE III DUAL USE APPLICATIONS: Complete the integration of the acoustic model into an existing sonar performance estimation model, or complete the development of a new integrated model for optimal placement of acoustic assets in confined environments. Design and deliver a prototype TDA to the AUWS program to guide acoustic communications sensor placement. The baseline 3D acoustic model should be submitted for consideration in the Oceanographic and Atmospheric Master Library (OAML) suite of Navy applied acoustics codes. There is potential to spin off the technology for private security clients in the protection of marinas and private or commercial vessels.

REFERENCES:

1. P.C. Etter, Underwater Acoustic Modeling and Simulation, 4th edition (CRC Press, Boca Raton, FL, 2013)

2. I.F. Akyildiz, D.Pompili, and T. Melodia, "Underwater acoustic sensor networks: research challenges", Ad Hoc Networks, vol. 3, no. 3, pp. 257 279 (2005)

3. F.B. Jensen, W.A. Kuperman, M.B. Porter, and H. Schmidt, Computational Ocean Acoustics, 2nd edition (Springer, 2011)

4. R.P. Goddard, "The Sonar Simulation Toolset, Release 4.6: Science, Mathematics, and Algorithms," APL-UW TR 0702 (October 2008)

KEYWORDS: underwater acoustics, propagation, scattering, communications, sonar, 3D acoustic modeling, sonar simulation, performance estimation

TPOC-1: Kyle Becker

Email: kyle.becker1@navy.mil

TPOC-2: Robert Headrick

Email: bob.headrick@navy.mil

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