Lakes, rivers, and wetlands are common on many military bases, especially within artillery and bombing ranges, some of which have been mandated to close and convert to public use. Many of these ranges have been in use since the 1930s, but environmental mapping of stray ordnance has been implemented on some bases only within the last 20 years. Of particular concern are dangers associated with shallow lakes because unexploded ordnance (UXO) are then close to waders, fishermen, motors, and paddles. Lakes and ponds may dry up and leave buried UXO dangerously close to the surface.
This project’s objectives were to prove and quantify the penetration of 100−400 MHz ground-penetrating radar (GPR) signals within freshwater subbottom sedimentation and to define the electromagnetic and geologic matrix profile characteristics of buried, single, typical UXO targets.
GPR is a competing method against SONAR (sound navigation and ranging). As does SONAR, GPR can delineate subbottom strata and detect localized objects within it to decimeter resolution or better. Unlike SONAR, GPR is able to operate from either an ice or a water platform, can propagate within a heavy suspended load, is not affected by vegetation or gaseous sediments, is not subject to masking reverberation in shallow water, and is less expensive.
The technical approach consisted of field observations and experimentation with GPR in addition to simplified field exercises of controlled UXO detection, numerical modeling of target responses, and laboratory investigations of sediment complex permittivity to aid interpretation of field data.
In the field, researchers applied well-developed commercial 16-bit GPR technology. The advantage of GPR is the shortening of its wavelengths by the high refractive index of water and saturated sediments. This shortening affords centimeter resolution of targets and therefore the possibility of imaging larger targets such as bombs, or their effects upon the geologic matrix.
In the laboratory, researchers implemented the time-domain spectroscopy technique and applied it to measure the frequency-dependent properties of wet sediments. In this method, a pulse is reflected from a sediment sample and from a metal reflector, and the two reflections are then analyzed for their comparative Fourier components. The resulting ratios are used to calculate the complex permittivity at each frequency. There is no commercially available system for this; the researchers developed their own system and were able to vary frequency over 6 orders of magnitude.
Numerically, researchers used pseudospectral finite element modeling to find the response of cylindrical targets and flat plates to GPR pulse excitation.
This project determined that likely UXO provide unique pulse waveform phase signatures that distinguish UXO from false targets; UXO create their own stratigraphic disturbances such as small craters, new layers, and draping of overburden sedimentation; resonance signatures are likely false metal targets; and subbottom penetration of as much as 10 meters is likely to be caused by the lack of phyllosilicate clay minerals.
3D reconstructions provide a valuable, compact format for storing, sharing, and summarizing ROV video, providing an information-dense, interactive synopsis of Munitions and Explosives of Concern (MEC) as it sits in the local environment. These reconstructions provide direct benefits to the DoD by simplifying and expediting the identification of MEC, while also reducing risks to divers in the water. Looking forward, active robotic situational awareness and perception is an essential component of semi- and fully-automatic behaviors, including automated mapping, classification, and computer-assisted remediation.