The City of Fredericton, New Brunswick, Canada operates a water well field adjacent to the Saint John River. Water is extracted from a semi-confined aquifer that consists of a buried sand and gravel esker ridge discontinuously covered by a clay∕silt aquitard. Geological, hydrogeochemical, and hydraulic data have long pointed to infiltration of river water as the main source of aquifer recharge but the shape and size of the recharge zone has, until recently, remained speculative. A combination of three geophysical surveys, including resistivity imaging along the shoreline, and seismic and electromagnetic profiling on the river itself, have now revealed the complex outline of the area beneath the river where the infiltration likely occurs. Broadband seismic reflections, with dominant frequencies of [Formula: see text] were recorded from depths as great as [Formula: see text] using a boomer source and 1KB Seistec line-in-cone receiver well suited to operation in the shallow water environment. EM31 and EM34 terrain conductivity meters, deployed in an outboard-motor-powered canoe and raft, revealed the subsurface conductivity structure in the same depth range. While the seismic method imaged the clay aquitard stratigraphy including its termination against the esker ridge, the electromagnetic method provided a more complete plan view of the extent of the clay window recharge zone. The two riverine data sets are highly complementary; as a start towards joint interpretation, we introduce the concept of bathymetry-corrected apparent conductivity and show how it helps to sharpen the boundary of the inferred recharge area. Confirmation of the geophysical interpretation is provided by the presence or absence of aquitard material in boreholes that have recently been drilled from a barge on the river.

I. ABSTRACTA geophysical survey conducted off Liverpool, Nova Scotia, Canada in November 1998 collected data using multibeam bathymetric, sidescan sonar, high-resolution subbottom profiler, and acoustic seafloor classification systems. During the survey, multibeam bathymetric and sidescan sonar data were processed on-site and imported into a Geographical Information System for further analysis and display. Shaded relief images derived from the multibeam bathymetric data and sidescan sonar mosaics were combined with data from maps and aerial photographs of the area. These maps and images formed the basis for a preliminary interpretation of geological processes and features on the seabed. Post-processing of the multibeam bathymetric data using newly-developed algorithms improved the resolution of seafloor features and provided acoustic backscatter intensity measurements. These data were used to defme the distribution of coarse and fine-grained sediments and seabed features and to compare and contrast various system resolutions. Seafloor samples and photographs were also taken to provide information for the interpretation of the various acoustic data sets. A variety of seafloor features are presented, using both sidescan sonar and multibeam bathymetric data for illustration.The seafloor is dominated by glacial features deposited during the recession of the last glaciers across the area (ca. 14,000-12,000 years BP). These include a large curvilinear moraine, smaller ribbed moraines and glaciomarine sediment. Bedrock crops out in many areas forming shoals. The glacial features were slightly modified, largely by shore-face processes, during a subsequent rise of post-glacial sea level from a low stand of approximately 65 m below present-day sea level to its present position. During the marine transgression, several coastal sand bodies were deposited at the northern flank of bedrock outcrops. Ribbed or lift-off moraines, which are normally thought to be destroyed in transgressed zones on the inner shelf, dominate the topography of the study area. Their distribution, in depths as shallow as 15 m, suggests that they survived the effects of the marine transgression largely intact. Bottom photographs confirm that the moraines are erosional remnants, armoured by boulders and cobbles. Existing models of marine transgression and sea level history must therefore consider the idea that some glacial features can survive shoreface erosion during marine transgression.

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