Assessments conducted at many former and active Department of Defense (DoD) ranges and installations have discovered unexploded ordnance (UXO) in the underwater environment, posing a potential current or future hazard. Relatively little is known about these underwater sites, which cover a broad spectrum of environments including shallow near-coastal and deep offshore marine sites, as well as estuaries and freshwater sites. This project demonstrates that a remote sensor survey utilizing laser imaging technology can provide an effective means of locating potential UXO in an underwater setting where these items lie exposed on the seafloor. The laser imagery also allows for the determination of size and condition (intact or clutter), which would greatly facilitate the ground-truthing effort or any potential intervention (removal or in-place detonation).
The objective of this project was to demonstrate the utility of the SM-2000 Laser Line Scan System (LLSS) for improved wide-area detection, identification, and assessment of UXO and ordnance-related materials in the marine environment. Under ideal conditions (calm, clear waters; flat topography), the optical images produced by LLSS can georeference and resolve centimeter-sized objects at two to five times the range of conventional video and photographic systems and therefore offer enhanced potential for positive identification and discrimination of UXO items. As a result, the performance of the LLSS was evaluated within a matrix of mission requirements and environmental conditions typical of a range of UXO disposal sites. Three main parameters were selected on which to base the performance evaluation: (1) image quality, (2) target morphometry, and (3) target positioning. Water clarity requirements, in terms of beam attenuation coefficient, for optimal system performance were also evaluated during the demonstration.
The LLSS is composed of an underwater optical sensor consisting of a solid-state, 200 milliwatt (mW), neodymium-doped yttrium aluminum garnet (Nd:YAG), 532 nanometer (nm) (blue-green) laser with synchronized rotating mirrors and an optical receiver. The light energy reflected by bottom features is projected on a photo-multiplier tube (PMT) and converted into an analog signal. Each individual scan line is passed to the topside electronics where it is digitized and written to file on a sample by sample basis. The LLSS is currently part of an integrated seafloor mapping system residing on Science Applications International Corporation’s (SAIC) FOCUS 1500 remotely operated towed vehicle (ROTV). The FOCUS ROTV provides a stable platform that can be maneuvered along discrete survey track lines and provides a coordinated uplink of sensor and instrumentation data over a fiber-optic tow cable manipulated by a LEBUS hydraulic drum winch. High resolution, precision positioning of each seafloor LLSS scan and resulting images are based on data provided by the combined use of the IXSEA Global Acoustic Positioning System (GAPS). The GAPS combines ultra-short baseline (USBL), inertial navigation system (INS) and Global Positioning System (GPS) technologies in one integrated package designed to determine real-time geodetic positioning of an aquatic sensor (i.e., the FOCUS ROTV) as it moves through the water. Ocean Imaging Corporation (OIC) GeoDAS seafloor imaging and data management software is the primary platform used to support acquisition, real-time processing and geocoding of digital LLSS data and to simultaneously interface navigation, attitude, altitude and environmental sensors providing real-time feedback regarding system operation. Following data acquisition and initial processing with GeoDAS, OIC CleanSweep2 software is also used to provide complete post-survey data processing and quality control (QC) to facilitate the enhancement of LLSS imagery.
Regarding image quality, LLSS images were found to be sufficient for the purpose of target identification in terms of clarity, contrast, aspect ratio, and visible details. Three LLSS image software platforms (GeoDAS, CleanSweep2, SAIC) were evaluated and showed various differences in final image quality. The images produced by the SAIC software were considered the best overall representation of the targets to be used for final identification.
In terms of target morphometry, the overall length and width of LLSS measurements showed good correlation to actual measurements (slope=1.05; r2=0.97), with the regression slope indicating that image measurements were on average 5% larger than actual.
With regard to target positioning precision, data indicated a 50% probability that repeated LLSS-detected target positions would fall within 1.65 meters (m) of the originally measured position and would be no greater than 4 m from the original position at a 95% confidence level. The results also indicated accuracy of 2 to 3 m (depending on which software application was used for measurement) between LLSS determined positions for select targets and “true” positions for the same targets as determined by collocated mobile transponder beacons attached to the targets.
Transmissometer data collected during the Year 2 survey indicated that the LLSS will produce high quality images in bottom water with a beam attenuation coefficient <1.115 m-1 and will not produce successful images in bottom water with a beam attenuation coefficient >2.453 m-1. The lowest observed beam attenuation coefficients were correlated to bottom types consisting primarily of rock or sand while the highest observed beam attenuation coefficients were correlated to sandy silt. Divers confirmed the presence of a high turbidity layer concentrated within approximately 1 m of the bottom during the survey of soft bottom areas.
For the blind testing element, Tier I classification success (target correctly identified as UXO simulator or dummy object) was 89% for targets surveyed. Tier II classification success (specific target identification [ID] correctly identified) was 78%. The probability of false alarm (Type I error; dummy objects falsely identified as UXO simulators) and the probability of false negative (Type II error; UXO simulators falsely identified as dummy objects) were both 0%. These results suggest that the technology is capable of providing accurate target discrimination for objects proud of the seafloor.
Other statistics from the demonstration surveys complied with established success criteria for depth station keeping (±0.8 m of target altitude), line station keeping (<3 m of the target survey line), data preprocessing (45 minutes per 500 m swath of LLSS imagery with georectification), target analysis (2.5 hours per 500 m swath of LLSS imagery), survey coverage (<1.7% survey area missed) and survey production rate (27,114±3390 m2/hour for low altitude [3-4 m] pass; 52,705 m2/hour for high altitude [7-8 m] pass). It should be noted that the probability of surrogate target detection and target location accuracy quantitative objectives were not fully qualified as desired given limitations of the available data resulting from lack of access to the intended survey area at the San Clemente Island Range Complex (SCIRC). The ease of use, system reliability and maintenance qualitative objectives were met within the scope of the available data.
There were no environmental or regulatory issues associated with the LLSS demonstration. All operations were non-intrusive and thus did not require permits and were not subject to any other local environmental regulations. All activities were overseen by Space and Naval Warfare Systems Command (SPAWAR) and conducted within the constraints of the project specific Demonstration Plan and Health and Safety Plan (HASP), both of which were reviewed and approved by the Environmental Security Technology Certification Program (ESTCP) and Navy prior to initiating work.
The most likely end users of this technology are commercial UXO service provider firms under contract to the DoD. Direct DoD users include Army, Navy, and Marine Corps installation managers who are responsible for training ranges with marine munitions and explosives of concern (MEC) contamination problems. The complete LLSS (including FOCUS ROTV) is the property of SAIC and is housed and maintained at the SAIC Marine Operations facility on Ponderosa Avenue, San Diego, California. It can be made available to support other demonstrations or marine UXO survey operations. Optional software platforms to perform LLSS image analysis (i.e., CleanSweep2) can be purchased separately or leased prior to any survey activities. No specialized training is required to operate the LLSS aside from company-specific or project-specific health and safety training for both marine offshore and laser operation. However, specialized skills in maintaining and troubleshooting the LLSS, piloting the FOCUS vehicle and operating the LLSS software are necessary to ensure optimal performance, and these skills can only be acquired through prior work with the LLSS technology.