ANALYSIS OF THE INTERNAL STRUCTURE OF ANDOVER LAKE BEACH (CT) USING 3D GROUND PENETRATING RADAR (GPR)

Andover Lake was dammed in 1927 for recreation that includes swimming at a small beach. The origin of this beach and it’s suitability for a clubhouse with a septic system are in question. Accordingly, GPR surveys were undertaken to determine the sedimentary architecture and physical characteristics of the beach. Surveys consisted of 27 transects ≈ 46m long using 50 and 100 mHz antenna. Topography was measured with a total station; a 4.02m vibracore and 2.5m deep trench provide ground truth. Radar data were dewowed, gained, and topographically shifted to prepare 3D plots with various software.

Radar penetration to ≈16m reveals 5 radar facies described from deepest to shallowest. Facies 1 (F1) consists of 2 high amplitude continuous reflectors that are subparallel to topography and overlie numerous hyperbolas. F1 is similar to radar collected over bedrock elsewhere on the lake. Remaining facies are composed of sand, affirmed by direct observation and transparency to radar waves. F2 (2m thick) contains high amplitude wavy lenticular reflectors that span the grid. F3 (1-3m thick) is characterized by chaotic, wavy reflectors with low amplitude that downlap toward the water. F4 (1-3m thick) contains continuous parallel high amplitude reflectors that become oblique sigmoidal near the shore. F5 (1-2m thick), only visible on the NE quadrant of the grid, consists of chaotic low amplitude reflectors that onlap other facia.

Vibracore and trenching enable direct observation of F2-4 and provide insight as to the suitability of the site for a septic system. The beach is underlain by 6-8m of fine to coarse sand (F2-5) in a general coarsening up sequence that rests on bedrock (F1) which slopes towards the water. The lowermost 5m of beach sands (F2, 3) are cross- bedded, contain few pebbles and are interpreted as gloaciofluvial, perhaps deltaic. An upper lag surface (4 cm thick) of coarse sand and subrounded gneissic pebbles marks the transition to F3. Overlying sands (F4) exhibit pedogenic overprintings, structureless and mottled sands that contain small wood fragments. The radar air/ground wave obscures 35 cm of coarse sands that comprise modern beach deposits. The thick and permeable nature of sand overlying gneissic bedrock suggests septic seepage would drain rapidly to the lake.