Integrated Use of Continuous Seismic-reflection Profiling and Ground-penetrating Radar Methods at John’s Pond, Cape Cod, Massachusetts

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 sandand-gravel deposits. Slightly wavy, parallel reflections located in depressions in the sand-andgravel 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. INTRODUCTION Recent studies involving land-based drilling have traced a contaminated groundwater plume to the western edge of the John’s Pond, but it is unknown where the plume is entering the pond (Air Force Center for Environmental Excellence (AFCEE, 1997a; AFCEE, 1997b). Using continuous seismic-reflection profiling and ground-penetrating radar methods, the U.S. Geological Survey, in cooperation with the Air Force Center for Environmental Excellence imaged the subbottom of the northern part of John’s Pond. CSP and GPR were used to identify layers and structures that could control groundwater flow and contaminant transport into John’s Pond in order to aid in focusing “on-pond” drilling and environmental sampling (AFCEE, 1997~). This paper presents the results of the geophysical surveys conducted in April 1998. The pond is located in the Town of Mashapee, southeast of Otis Air National Guard Base (Figure 1). John’s Pond is a glacial kettle pond that was formed during the Wisconsinan