Within the domain of munitions response, the problem of conducting detailed acoustic surveys in very shallow water is particularly difficult, owing to sensor deployment challenges and multipath interference in these environments. The Sediment Volume Search Sonar (SVSS) improves buried unexploded ordnance (UXO) detection and classification performance through design, fabrication, and demonstration of a sonar system that is deployed from a shallow-draft surface vessel. The SVSS sensor produces a novel form of three-dimensional synthetic aperture sonar imagery of both surficial and buried UXO across a range of environments. The platform and sensor are designed to operate in less than five meter water depth. These very shallow water areas are particularly important to address because of the increased likelihood of human interaction with ordnance.
The sensor’s hardware design was based on an acoustic modeling and simulation effort. The simulation effort was accomplished by combining The Applied Research Laboratory at the Pennsylvania State University’s (ARL/PSU) point based sonar scattering model with the Applied Physics Laboratory at the University of Washington’s target in the environment response model. Using this integrated toolset, parameters of the sensor/environment/target space were modified to explore the expected operating conditions, and to adapt existing back projection image reconstruction algorithms used to create three-dimensional acoustic imagery.
These model and simulation efforts informed both the sensor and signal processing design and resulted in the development of a prototype SVSS system. The system was deployed from an ARL/PSU shallow-draft research platform and tested at two trial sites: the Foster Joseph Sayers Reservoir near Howard, PA and the Littoral Warfare Environment at the Aberdeen Test Center (ATC) in Maryland. Both sites were prepared with a wide range of surficial and buried man-made objects. In total, over 150 objects were emplaced, which varied in size from small shell casings to one meter long cylinders. The geolocation of object positions was recorded with high precision, such that acoustically-detected objects could be referenced to ground truth demonstrating a localization errors less than 10 centimeters. Field testing conducted in the Bush River, adjacent to the ATC, demonstrated sediment penetration depths up to three meters and imagery contained indications of likely man-made clutter from historical activity in the area.
One of the main remaining challenges to reliable detection is understanding the environmental effects that influence object detection capability. For example, a strong, seasonally-dependent acoustic response from the upper layer of sediment at the Foster Joseph Sayers Reservoir test site was shown to mask the initial acoustic response from objects within the same sediment region. Note, however, that the late-time acoustic response was still visible in imagery. Model-data comparison suggests that the strong, seasonal acoustic response is likely due to entrapped gas. During testing in the Bush River, localized regions of high-reflective seafloor were observed that occluded the sediment below, again indicating the presence of entrapped gas.
The SVSS system developed under this effort has demonstrated a prototype sensor deployed from a surface craft. This provides a means for the detection and localization of ordnance at SERDP-designated test sites. The system will fill the need for a very shallow water detection system that is currently unmet.