Electrical resistivity and Ground Penetrating Radar for the characterisation of the internal architecture of Quaternary sediments in the Midlands of Ireland

Abstract

Abstract Geophysical techniques Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) supported by traditional field methods are used for the geological mapping, description and interpretation of Quaternary unconsolidated sediments in a site located in the Midlands of Ireland. The site comprises a broad range of glacial and postglacial sediments (diamicton, esker sand and gravel, glaciolacustrine sand, glaciolacustrine silt/clay and peat). Preliminary fieldwork comprising, geomorphological mapping, lithostratigraphic analysis of exposures and borehole drilling and laboratory testing encompassing particle size distribution analysis were carried out to broadly characterise the geology of the study area. These data aided locating the geophysical profiles and supported the geophysical data interpretation. Five GPR radargrams were collected and permitted depicting the subsurface internal architecture within low conductivity unconsolidated sediments and aided to the classification and characterisation of sedimentological and deformational structures. Four ERT profiles allowed the depth to bedrock to be determined and lithological classification of the sediments. The use of these geophysical techniques in combination with geotechnical and geological data allowed (i) the determination of the lithological composition and detailed internal architecture of the subsurface, (ii) the characterisation and description of the geology of the site and (iii) understanding the depositional processes acting in the area during ice withdrawal. Diamicton and esker gravels were deposited subglacially by an ice sheet withdrawing westwards; glaciolacustrine sediments located along the south margin of the esker ridge were laid down in an ice marginal environment as a subaqueous fan composed of silt, sand and gravel, and as distal deposits composed of silt and clay in the lower ground area between the fan and the esker ridge. Peat developed during postglacial times and was partially cut away by anthropogenic action at a later stage.