MULTI-FREQUENCY GROUND-PENETRATING RADAR (GPR) STUDY OF GLACIOFLUVIAL OUTWASH DEPOSITS, LIMEHOUSE, ONTARIO

Abstract

<p>In this study, the spatial trends in heterogeneity and sedimentary architecture of a coarse-grained glaciofluvial deposit were investigated in a gravel pit near Limehouse, Ontario using multi-frequency ground-penetrating radar (GPR). The well-exposed outcrops in the pit (> 10 m) allow direct comparison of radar and outcrop lithofacies and provide ‘outcrop analogues’ for regional glaciofluvial aquifers present in the shallow subsurface. GPR surveys were acquired using four different frequencies (25, 50, 100 and 250 MHz) over a 5800 m² grid on the pit floor and along an adjacent outcrop and roadway. Profiles were acquired at 2 m line spacing using D-GPS positioning and processed to a quasi-3D volume in GPR-SLICE software.</p> <p>Radar reflection patterns were grouped into radar facies and interpreted using architectural element analysis (AEA) with reference to outcrops and a nearby geophysically logged borehole (MW-22). Using this approach, five distinctive lithosomes and two depositional phases were identified. The lithosomes consist of gravel clinoform (GC) elements, gravel bar (GB) and gravel foreset (GF) elements, vertical aggradation (VA) elements and incised channel elements (CH). RP-1 (elements GC, CH, GB) records the progradation of a sub-aqueous fan delta into a shallow proglacial lake. RP-2 records a subsequent phase of incision of deltaic deposits by glaciofluvial spillways and infilling of channels by aggradation of braided river sand and gravel deposits.</p> <p>Comparison of the radar profiles with borehole and outcrop data indicates that lithosome bounding surfaces are associated with major changes in sediment texture. Lithosomes defined using radar reflection patterns can therefore be used to predict changes in sediment grain size and associated hydrogeologic parameters (e.g. hydraulic conductivity). These results demonstrate that multi-frequency GPR is a viable approach for evaluating spatial trends in sedimentary heterogeneity and can be applied at other sites under consideration for artificial recharge projects.</p>