Among the sensor technologies expected to play a significant role in underwater unexploded ordnance (UXO) remediation, acoustics (sonar) is especially promising based on extensive assessments of other sensor technologies carried out at the Naval Surface Warfare Center Panama City Division (NSWC PCD) for Navy littoral operations. Acoustics can be used to probe for targets over a significant range and, being a wave phenomenon, can be used to discriminate buried targets from clutter by extraction of spectral, temporal, and/or spatial (imaging) features from received signals. However, environmental factors can make detection and discrimination problematic.
The objective of this project was to identify and understand factors that affect sonar performance and use this knowledge to develop a simulation tool to optimize sonar design for use against UXO under various conditions.
In 2003, SERDP initiated an effort to expand NSWC PCD’s Personal Computer compatible Shallow Water Acoustic Toolset (PC SWAT) software to allow sonar performance simulations with underwater UXO targets in proud and buried configurations. This development included formulation, validation, and incorporation of new models to account for target burial and surface roughness effects in underwater environments. The resulting software tool is meant to be a numerical testbed to help mitigate expensive test and evaluation of sonar-based UXO sonar concepts.
The project was the completion of a 3-year follow-on effort that continued to leverage on-going Navy sponsored sonar tests to collect data to both continue the model validation, refinement, and maintenance needed to keep PC SWAT up to date for UXO applications and also to assess sonar effectiveness against UXO. Since validation requires high quality data taken over a wide range of bottom target configurations, much of the data collection was performed in NSWC PCD’s freshwater test pond under near laboratory conditions. This facility enabled investigation of the acoustic response of canonical and UXO bottom targets as a function of frequency, target orientation, monostatic vs. bistatic scattering geometry, bottom surface profile, and burial depth on a well-characterized sand bottom.
As part of the model validation process, this project continued to refine the scattering solution benchmarks to improve fidelity in comparisons with PC SWAT and investigated better ways to measure environmental parameters required as model inputs. Enhancements to existing transition (T-) matrix solutions extended validation of the physics models to simple cylindrically shaped targets buried in sand and consideration of biharmonic ripple profiles. In addition, a finite element modeling (FEM) capability was initiated to make possible simulations with more realistic targets. While initially useful for verifying current simulations and analyzing target responses, this capability was envisioned to be the ultimate follow-on to PC SWAT, enabling sonar simulations encompassing the full 3D physics of a complex target coupled to its environment.
An improvement to benchmark models used to verify simulations and understand physical scattering mechanisms was made through formulation and development of a spheroidal-basis transition matrix for elongated buried targets. Despite a possible glitch in PC SWAT’s implementation of evanescent wave effects at low frequencies, good agreement between modeled and measured signal levels were seen as a function of frequency when sediment parameters were chosen within measurement error bounds. This provides an important check on the fidelity of the algorithms in PC SWAT as well on the basic physics built into models to explain observations for very nonspherical targets. As a result, updates to PC SWAT to correctly account for low frequency phase errors when evanescent wave scattering is important will be implemented and tested in coming years.