GPR characterization of a lacustrine UXO site

We used 135-MHz ground-penetrating-radar (GPR) reflection profiles to determine if high-explosive (HE) depth bombs were possibly detonated within a small New England lake formerly used as a bombing range, as well as the depths and distribution of inert and possibly unexploded ordnance (UXO) within the subbottom sediments. Maximum water depth was 8.6 m. The primary munitions were unarmed practice bombs, but terrestrial fragments from HE depth bombs led us to seek stratigraphic obliteration and distortion as evidence of their use. The low water conductivity and quartz-rich sediments provided more than 8 m of subbottom penetration beneath slope and shelf sediments. The high permittivities of the water and subbottom sediments provided good antenna directivity, excellent 2D migrations, multicentimeter vertical resolution, and a unique pulse waveform signature for metal objects. Our line profiles revealed hundreds of subbottom diffractions, which implied an areal distribution of thousands of suspicious objects. Most lay beneath the central basin, but many resided beneath the slopes and shelves as well. Supplementary profiles of a nearby pristine lake and of controlled munitions, a side-scan sonar bottom survey, and time-domain reflectometry dielectric measurements of sediment samples corroborated our findings of natural stratigraphy, a unique metallic diffraction phase signature, the existence of many nonmetallic objects including logs, and subbottom relative permittivities ranging from 28 to 76. The intact natural lacustrine strata, including deltaic foresets and bottom sets, faults, and conformable and unconformable horizons, suggest that live bombs likely did not detonate in the lake. The only stratigraphic disturbances we found were apparent perforations beneath about 1 m of water, within which diffractions of metallic origin Originated. The dielectric results and the prominent slope strata over the deepest horizons led us to conclude that the exceptional subbottom penetration existed under predominantly sandy sediments.