Exact modeling of targets in littoral environments

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.