Geophysical Characterisation and Evolutionary Model of the Quaternary sediments in the North Offaly region of Central Ireland

Data from field mapping of 133 exposures, 57 boreholes and 190 petrographic and particle size analysis samples were used to produce Quaternary geological and depth to bedrock maps for the 600 Km² research area located in the North Offaly region of central Ireland. Based on the mapping, the area was subdivided into four physiographic units with specific landscape characteristics, namely: Shannon Basin esker-dominated landscape; Shannon - East Basin watershed area; East Basin and the Brosna River Basin. The capabilities of geophysical techniques were tested as Quaternary sediments mapping tools and the geophysical results obtained, combined with field mapping, were used in the production of an evolutionary model of deglaciation. The geophysical methods employed were Electrical Resistivity Tomography (ERT), time-lapse resistivity imaging, azimuthal resistivity, Ground Penetrating Radar (GPR) and Very Low Frequency (VLF). Forward modelling with GPRSIM and RES2DMOD software and geophysical ground truth evaluation in three test sites aided in the selection of the most efficient data collection, processing and interpretation methods, evaluation of the potential and limitations of the techniques and inversion models and yielded a better interpretation of the geophysical data. Seventeen sites were geophysically surveyed in association with borehole and exposure data where available. GPR proved to be an excellent technique for the classification and characterisation of sedimentological and deformational structures within low conductivity soft sediments, expressed as esker and morainic ridges, glaciodeltas, sub-aqueous fans or raised bog. A centre antenna frequency of 200MHz gave a better spatial resolution that 100MHz but a lower penetration and also took longer to collect the data. Amplitude analysis of 2D GPR data assisted in the lithological classification of these sediments. ERT data permitted accurate depth to bedrock detection and lithological classification of soft sediments. ERT and GPR data combined yielded the best results. Monthly time-lapse resistivity showed that effective recharge, lithology and depth influenced the observed variation in resistivity. Geophysical data and field mapping suggest that a glacial lake expanded westwards, confined to the east by the Shannon Basin watershed and to the west by two ice-sheets, one retreating northwest and the other south-southwest