Use Of Ground-Penetrating Radar To Determine The Depositional Environment Of Glacial Deposits In Southern New Hampshire

Ground-penetrating-radar (GPR) surveys were made along more than 8 miles of land and<br>water within the Flints Pond and Lyle Reed Brook areas in southern New Hampshire. The GPR<br>data revealed thick sequences of continuous reflectors that represent stratified drift. Surficial<br>geologic mapping of the area indicates that the stratified-drift deposits are graded to the various<br>levels of glacial Lake Nashua. Four distinct signatures were apparent on the GPR profiles and<br>were correlated with various glacial depositional environments: (1) thin, relatively flat-lying,<br>continuous reflectors draped over the underlying till or bedrock topography, which represent<br>fine-grained lacustrine sediment; (2) medium-grained, subparallel to slightly chaotic, continuous<br>reflectors, steeply dipping in places, which represent predominantly medium- to coarse-grained<br>sands deposited as deltaic sediments in a lacustrine environment; (3) subparallel to hummocky,<br>chaotic, coarse-grained reflectors, steeply dipping in places, which represent coarse-grained icecontact<br>deposits, such as eskers or crevasse fillings; and (4) sharply diffracted, fine-grained,<br>chaotic reflectors, which represent till and (or) till over bedrock. The signatures were correlated<br>with geologic structures by comparing the radar records to analyses of undisturbed sediment<br>samples collected at six test-boring sites in the area. Correlation of radar signatures with<br>geologic structures can be used to indicate the depositional environment of the structure.<br>Land and water surveys using GPR detected glacially scoured valleys in bedrock that have<br>been subsequently filled with stratified drift. Geologic structures are visible in greater detail<br>from GPR surveys on land than on water. Penetration depths of greater than 80 feet (800<br>nanoseconds) were common on land surveys done over stratified drift. Penetration depths of<br>approximately 45 feet (400 nanoseconds) were common on water surveys as a result of the<br>attenuation of the radar signal by the water column.<br>GPR also was used to delineate the surface contact between stratified drift and the adjacent<br>till and (or) bedrock. Typically, the signal was attenuated as the survey passed from the<br>stratified-drift surface to the till surface because of the increased electrical conductivity of the till.