Through-the-sensor simulated sonar signals can be used to assess the design of a sonar platform or a possible modification to an existing platform. Simulated signals can also augment available measured data in the training and testing of classification algorithms. The Target-in-the-Environment Response (TIER) model is a fast acoustic model that can predict stave level sonar signals under various environmental and geometrical conditions. It is an Applied Physics Laboratory, University of Washington inhouse model that has undergone continuous development, testing, and validation with partial support under SERDP MR-1665, MR-2231, MR-2505, and MR19-1234. The current implementation of the TIER model is a high-quality research code, but it requires some familiarity with its internal implementation details and acoustic scattering in an underwater environment. The objective of the project is to package the TIER model into a tool that can be used by other munition response researchers and contractors.
Key features of the TIER model include acoustic ray tracing and free-field scattering amplitudes. Ray tracing accounts for sound propagation within an environment. The free-field scattering amplitude for an object allows one to synthesize sonar signals for various scattering geometries and environments. Geometries are defined by the spatial location of sources, receivers, and objects. Environments include free-field (i.e., far from any boundary), a half-space with water above a sea floor, a layer above a sediment basement, and a waveguide (i.e., air-water boundary cannot be ignored). An object can be placed anywhere within a half-space environment (i.e., lying proud on a sea floor, partially buried, or completely buried). In a layer or waveguide environment, an object is contained completely within the layer or waveguide. While the TIER model originally addressed isolated objects, it has been updated to consider an object in close proximity to neighboring objects, where multiple scattering may become important. Multiple scattering can be simulated in free field and half-space environments. In TIER, the sediment and layer are assumed to be homogeneous, dissipative, fluid media.
Successful completion of the project has two benefits. First, the TIER model has been shown to be a useful tool for sonar design. Informed decisions for both the Multi-Sensor Tow Body (MR-2501) and Sediment Volume Search Sonar (MR-2545) were made based on simulations of possible configurations of sources and receiving array elements. Availability of the TIER tool can benefit an organization when designing new sonar platforms or modifying current systems. Second, training and testing classification algorithms requires access to a large set of sonar data. Portions of data collected during Pond Experiment 2010 and Target and Reverberation Experiment 2013 have received public-release status through the Office of Naval Research, and have been made available to others (MR-2416). However, the measured field data are limited to a fairly small set of objects and environmental conditions. An earlier version of the TIER model was made available to the performers of MR-2416 to augment the measured data, and it was found to improve the performance of their classifier. The TIER tool would allow researchers to produce synthetic data for research on classification schemes and deep learning, and it would provide vendors of sonar platforms and remediation practitioners a means to update classification schemes of fielded systems while on site.