Anatomy and facies association of a drumlin in Co. Down,Northern Ireland, from seismic and electrical resistivity surveys

摘要

Seismic refraction and electrical resistivity geophysical techniques were used to reconstruct theinternal architecture of a drumlin in Co. Down, Northern Ireland. Geophysical results were bothvalidated and complemented by borehole drilling, flow modelling of ground water, and geologicalmapping. The geophysical anatomy of the drumlin consists of five successive layers with depth includ-ing; topsoil, partially saturated and saturated glacial tills, and weathered and more competent greywackebedrock. There are numerous, often extensive inclusions of clay, sand, gravel, cobbles, and boulderswithin the topsoil and the till units. Together geophysical and geotechnical findings imply that thedrumlin is part of the subglacial lodgement, melt-out, debris flow, sheet flow facies described by pre-vious authors, and formed by re-sedimentation and streamlining of pre-existing sediments duringdeglaciation of the Late Devensian ice sheet. Seismic refraction imaging is particularly well suitedto delineating layering within the drumlin, and is able to reconstruct depths to interfaces to within±0.5 m accuracy. Refraction imaging ascertained that the weathered bedrock layer is continuous andof substantial thickness, so that it acts as a basal aquifer which underdrains the bulk of the drumlin.Electrical resistivity imaging was found to be capable of delineating relative spatial changes in themoisture content of the till units, as well as mapping sedimentary inclusions within the till. The mois-ture content appeared to be elevated near the margins of the drumlin, which may infer a weakeningof the drumlin slopes. Our findings advocate the use of seismic refraction and electrical resistivitymethods in future sedimentological and geotechnical studies of internal drumlin architecture anddrumlin formation, owing particularly to the superior, 3-D spatial coverage of these methods.