SBIR-STTR Award

We propose to use the finite element technique to model propagation of acoustics waves in a realistic shallow water (10-100 m) environment in the presence of man-made or natural targets. Such modeling accuracy is paramount in detection and classification of underwater targets. Currently available models make simplifying assumptions in modeling both scattering and propagation. Targets are usually modeled as non-penetrable (rigid) or having simple shapes. In most cases, simple scattering models such as the Kirchhoff approximation is used to compute the scattered field. The existing propagation models are also unable to accurately model a realistic ocean environment. For instance, there is no propagation model that can accurately compute propagation in a range-dependent ocean overlying an elastic bottom. These models cannot provide the accuracy that is needed for use in detection and classification of underwater targets. To properly account for the full physics of the problem, propagation and scattering should be solved as a single boundary value problem. Our approach will consist of three stages: in stage 1, we will use the finite element technique in 2-D to propagate the acoustic field to the target, in stage 2, we will use the finite element technique in 3-D to compute the scattered field, and in stage 3, we will use the finite element technique in 2-D to propagate the scattered field to the receiver. For shorter ranges and/or lower frequencies it may be possible to solve the whole problem in 3-D as a single boundary value problem. In Phase 1 of this effort, we will demonstrate the capability of our proposed approach to model scattering from a simple elastic target in a shallow water environment overlying an elastic bottom. In this phase, we will solve this problem for a source to target separation of approximately 1000 wavelengths. In Phase 2, we will extend the capability of our approach to model the problem for a source to target separation of approximately 10,000 wavelengths, and for arbitrary man-made or natural targets. The novelty of our approach is the use of the finite element method in propagation modeling, which has only become possible with advances in computer speed along with progress in modern finite element techniques.

Benefit:
The products resulting from this work will be used in areas such as seismic and electromagnetic modeling.

Keywords:
Propagation, Propagation, Target Scattering, Finite element modeling

The US Navy needs the capability to model acoustic propagation in complex ocean environments containing natural or man-made objects. Such accurate modeling requires the solution of the wave equation in the ocean containing scatterers. In the absence of the scatterer, the oceanic waveguide can be assumed to be axially symmetric. This allows certain types of physically valid approximations to be made, which are 2D in nature, but can accurately compute the acoustic field in a 3D ocean. The presence of the scatterer breaks this symmetry and forces one to model the ocean as fully 3D. Due the large size of the computational domain, the solution of the wave equation in an oceanic waveguide in the presence of scatterers is a daunting numerical task. In Phase I of this STTR we developed finite element models in the frequency and time domains to compute the acoustic field in a 2D, elastic, range-dependent waveguide in the presence of an elastic object. The objective of this Phase II effort is to develop the numerical capability to accurately solve the wave equation in a range-dependent elastic waveguide in the presence of a general, 3D elastic target to ranges of at least 10,000 acoustic wavelengths. To cope with the numerical complexities in 3D, we plan to accomplish this objective by developing numerical tools that implement a hybrid model composed of finite element and propagation models.

Benefit:
The anticipated benefits of this work is the development of computer software that can be used to accurately model sound propagation in the ocean in the presence of natural or man-made objects. This capability can be used to simulate various naval operational scenarios, from mine countermeasure to active ASW. It will also provide the capability to study how the bodies of marine mammals respond to exposure to sound.

Keywords:
Propagation, wave equation, Scattering, finite element technique