Integrated GPR and ERT data interpretation for bedrock identification at Clericy, Quebec, Canada

Ground-penetrating-radar (GPR) and electrical resistivity tomography (ERT) are two geophysical methods widely used for shallow subsurface structure investigations. Although GPR can provide high precision information, it is very limited by the detection depth, especially when there is a medium with high conductivity. This situation is common in the Abitibi greenstone belt of Canada, where fine-grained glaciolacustrine sediments are widespread. Therefore, GPR and electrical methods were combined to investigate the undulating topography of bedrock beneath the clay, to help an infrastructure installation within an urban area. GPR measurements were carried out with four frequencies (450 MHz, 160 MHz, 100 MHz and 80 MHz) and ERT survey were accomplished using a Syscal Pro Switch from Iris Instruments, with 72 electrodes. Shallow boreholes helped in the recognition of the lithology (up to 4 m deep) and in the calibration of geophysical data interpretation. Known infrastructures (underground pipes) were used to estimate the velocity of the radar wave in the uppermost layers, which is estimated at 0.14 m/ns. The data show an unconsolidated material cover, composed of clay, sand and backfill materials, overlying a rugged bedrock surface. The present study highlighted places where the burial of new infrastructures will require rock excavation, and the observations made during the excavation carried out as part of the underground infrastructure installation work made it possible to test the validity of the geophysical measurements.