Next Generation Buried Object Scanning Sonar (BOSS) for Detecting Buried UXO in Shallow Water
Dr. Jason Sara | Edge Tech
The objective of this work will be to improve the ability to remediate underwater sites by developing a system for the detection and classification of proud, partially buried, and buried ordnance in shallow water (<5m). The focus of this research is to develop algorithms and to design an acoustic array that will image buried UXO with sufficient detail to measure their length, diameter, and shape. Those metrics can be used as features for a standalone UXO classifier or as an input constraining the solution of a classifier using acoustic color map features. This enhanced Buried Object Scanning Sonar (BOSS) technology will increase the probability of detecting and classifying objects that have settled or have been buried in shallow water sites.
The main objective of this work will be to perform a limited-scope paper-study that develops an acoustic array and a signal and image processing design for a sonar optimized for imaging, detecting, and classifying buried UXO in shallow water including water depths less than 5 meters. A supporting objective is determining the optimal sonar parameters for buried UXO imaging such as source spacing, bandwidth, image range and depth, sonar altitude, number of parallel hydrophone arrays, and transmission sequence for sources.
The Buried Object Scanning Sonar (BOSS) will be redesigned to generate UXO imagery that will measure length, diameter, and shape of buried objects. BOSS was originally designed to detect and to generate images of mines buried 1 meter in the seabed. This operational requirement and the constraints of installing BOSS on a 12-inch diameter UUV drove the design of BOSS about 12 years ago. SERDP sponsored experiments demonstrated that BOSS could a) detect UXO and b) generate low resolution imagery and burial depth estimates of UXO over a limited range of target orientations. However, the BOSS imagery lacked the resolution and dynamic range for accurate measurement of UXO dimensions and for UXO classification.
This study will investigate an EdgeTech BOSS (eBOSS) design for imaging and classifying UXO. The technical approach is to generate synthetic datasets consisting of scattering from buried UXO-shaped rigid bodies including frequency-dependent sediment attenuation and sediment–water interface transmission losses, sediment scattering, and shallow water multipath. Multiple acoustic sources will be used to illuminate all aspects of the UXO targets. The synthetic data, acquired by parallel discrete arrays of hydrophones, will be processed by new eBOSS algorithms designed to produce imagery suitable for measuring UXO length, diameter, and shape.
One of the design considerations is to avoid strong specular scattering from the seabed that can obscure weaker returns of buried objects. This approach is to use signal and image processing algorithms that will construct UXO imagery using only echoes with grazing angles of approximately 45 degrees, which will spatially separate target images from the specular returns of the seafloor and subfloor. Angles of incidence of approximately 30 degrees will be avoided because the slopes of sand ripples and burrows of bottom dwelling marine organisms can be as high as 30 degrees, which is the angle of repose for sand. Additionally, high two-way transmission loss at the sediment-water interface prevents sub bottom imaging near and beyond the critical angle. This image processing design is one of many novel features in the eBOSS design for imaging UXO and pipelines. The eBOSS performance will be quantified by comparing UXO dimensions and shape with those measured from synthetic imagery for various UXO orientations and burial depths.
The eBOSS will be capable of detecting and classifying buried ordnance using multiple, spatially separated acoustic sources and a mixture of real and synthetic aperture sonar. By providing multiple aspects of buried objects, the system delivers two benefits: 1) map target strength as a function of frequency and target aspect and 2) identify objects by their size and shape. eBOSS will have improved bandwidth and a wider aperture than previous BOSS systems. Benefits include improved area coverage rates and improved probability to detect and classify targets. The scientific community will have a sensor that will allow them to a) generate acoustic color maps and b) estimate the location, size, and shape of buried objects.
In addition to high quality imagery, classification algorithms being developed by other programs will benefit from the multipath datasets produced by the eBOSS sources, with each acoustic source capable of producing an independent monostatic acoustic color map of a given UXO target.