The problem is to detect, classify, and remediate military munitions found in aquatic environments such as ponds, lakes, rivers, estuaries, and coastal and open ocean areas. A specific need is for technology to solve this problem with munitions residing in depths less than five meters. This shallow-water domain includes an assortment of unexploded ordnance that are the most likely to be encountered by the public and are expected to experience the most mobility. Many sensor technologies designed to detect, classify, and remediate munitions are challenged by this unique environment and suffer in performance, access, navigation, deployment, viewing, sensor standoff distance, and damage by changing bottom topography or obstructions.
The project's objective was to investigate how water conditions (i.e. wavy surfaces and turbid water columns) might impact the feasibility of using a new above-water lidar technology for the classification of the aquatic environment and the identification of munitions in waters less than five meters deep with vertical and horizontal resolutions at centimeter levels. The project studied the interactions of pulsed laser light with wavy water surfaces and turbid water columns and their effect on the lidar’s three dimensional mapping capability.
The technical approach used a variety of materials and methods involving numerical simulation, controlled lab experimentation, instrument prototyping, and outdoor experiments. Simulations included a Monte Carlo scheme that followed photons as they propagated through turbid media and a ray trace scheme to map the effects of surface reflection, transmission, and refraction due to wavy surfaces. Indoor experiments were carried out using the lidar technique with a graduated cylinder and a water tank to determine turbidity and surface wave effects on lidar measurements. A prototype drone-based, scanning topographic/bathymetric lidar system was also used to demonstrate outdoor capabilities.
Wave properties were simulated and experimentally generated to evaluate the lidar detection of underwater objects. Variable refraction due to changing wave slope and time delay due to changing wave amplitude were the dominant factors in producing variability in lidar range and cross-range measurements. In turbid waters, both particle size and particle concentration matter when considering the lidar depth performance. Larger particles, relative to the excitation wavelength, proved less detrimental to lidar detections than smaller particles due to their preferential forward scatter. Outdoor drone-flight experiments demonstrated the potential capability of this technology to detect underwater munitions in shallow waters (< 5 m) with a total empirical precision estimate of < 10 cm (lidar error < 1 cm, platform error < 5 cm, water conditions error < 5 cm). These lidar measurements experienced both wavy and turbid conditions and provided empirical evidence for the technique’s ability to operate in environments relevant to munitions response.
This research is of great interest to the Department of Defense (DoD) and scientific communities due to its ability to observe at high resolution a wide variety of shallow-water aquatic environments that have been, up to now, were inaccessible due to limited technologies. The results further address DoD needs by establishing a new technology with the ability to detect, range, and classify underwater objects with high vertical and horizontal resolution without making contact with the water. This lidar capability can be highly complementary to other SERDP techniques.