CHARACTERIZING THE SUB-BOTTOM GEOLOGY AT ANDOVER LAKE, CT USING GROUND-PENETRATING RADAR

Ground-Penetrating Radar (GPR) was used to investigate the structure of sub-bottom geologic materials beneath Andover Lake, an artificial water body dammed in 1927. Radar surveys used a 50 MHz pulseEkko Pro system mounted on an inflatable raft with an integrated Novatel GPS. A total of 10.5 km of data were collected along 27 transects with radar penetration to depths of ~13.5 m. These data were processed using Ekko View software applying de-wow filters and spreading exponential gains. Radar velocities were estimated by hyperbola fitting for lake bottom targets and CMP analysis at an adjacent beach. Radar interpretations are constrained by comparison with known transitions in bottom conditions (e.g. sand lapping onto bedrock) and with 6 vibracores (to 2 m) and 11 shorter percussion cores (to ~1-2 m).

Five distinct radar facies occur. F1, interpreted as bedrock, consists of high amplitude reflectors, sub-parallel to the lake bottom. F1 merges with exposed outcrops at some locations but extends to greater depths elsewhere. Transparent soft sediments (F2) overlying bedrock as pockets of low amplitude parallel reflectors that are horizontal to gently dipping are common at flat bottom and submerged stream channel locations. F3 is comprised of long (to ~225 m) parallel high amplitude reflectors often occurring near a sandy island and beach. F4 is characterized by parabolic point returns, hummocky to chaotic and discontinuous reflectors with low to high amplitudes thought to be boulder-rich till. Finally, the deepest facies (F5) consists of low to moderate amplitude reflectors that are variably chaotic to cross-cutting and occasionally channelized.

Boulder-rich till (F4) and bedrock (F1) dominate the northern half of the lake where bedrock forms a confined outlet valley. Soft sediment (F2) is widespread in low areas on the lake. High amplitude sandy reflectors (F3) are prevalent in the southern third of the lake where deeper channel-like reflectors (F5) and thick trenched deposits at a beach suggest a glacial fluvial origin. Although speculative at present, topographic constraints near the lake outlet may have facilitated ice-dammed deposition and the accumulation of thick sandy deposits at the south end of the lake.