Abstract
Seismic refraction and electrical resistivity geophysical techniques were used to reconstruct the
internal architecture of a drumlin in Co. Down, Northern Ireland. Geophysical results were both
validated and complemented by borehole drilling, ground water flow modelling, and geologic
mapping. The geophysical anatomy of the drumlin consists of five successive layers with depth
including; topsoil, partially saturated and saturated glacial tills, and weathered and more competent
greywacke bedrock. There are numerous, often extensive inclusions of clay, sand, gravel, cobbles,
and boulders within the topsoil and the till units. Together geophysical and geotechnical findings
imply that the drumlin is part of the subglacial lodgement, melt-out, debris flow, sheet flow facies
described by previous authors, and formed by re-sedimentation and streamlining of pre-existing
sediments during deglaciation of the Late Devensian ice sheet. Seismic refraction imaging is
particularly well suited to 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 and of 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 the moisture content of the till units, as well as mapping sedimentary
inclusions within the till. The moisture content appeared to be elevated near the margins of the
drumlin, which may infer a weakening of the drumlin slopes. Our findings advocate the use of
seismic refraction and electrical resistivity methods in future sedimentological and geotechnical
studies of internal drumlin architecture and drumlin formation, owing particularly to the superior, 3-
D spatial coverage of these methods.
Original language | English |
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Pages (from-to) | 165-176 |
Number of pages | 12 |
Journal | Special Publication of the International Association of Sedimentologists |
Volume | 1 |
Publication status | Published - Nov 2007 |