Continuous seismic-reflection profiling (CSP) and ground-penetrating radar (GPR) surveys were conducted by the U.S. Geological Survey in April 1998 over the northern part of John’s Pond, a glacial kettle pond southeast of Otis Air National Guard Base, Cape Cod, Massachusetts. The surveys were conducted to delineate the types and thickness of sedimentary units that may control the infiltration of contaminated groundwater into John’s Pond.
Sand-and-gravel deposits, collapse features and recent organic sediments were imaged with the CSP and GPR methods. Hummocky to chaotic reflections were interpreted as sand- and-gravel deposits. Slightly wavy, parallel reflections located in depressions in the sand-and- gravel deposits were interpreted as filled collapse features. Lower amplitude, horizontal, laminar reflections were interpreted as organic sediments. Entrapped methane gas within some of the organic sediments created a reflection zone that obscured deeper reflections in the CSP records.
The CSP and GPR methods provide complementary information over most of the surveyed part of the pond. The methods detect similar interfaces, but a particular interface may produce a stronger reflection in one record than in the other. For example, regions of the pond containing organic sediments with entrapped methane gas, which prevent penetration of the acoustic signal, were penetrated and imaged by GPR. Conversely, regions of the pond containing electrically conductive sediments or deep water, which attenuate the GPR signal, were imaged using CSP. The CSP and GPR data were interpreted to generate a bathymetric map and a map of sediment type and thickness beneath John’s Pond.
