Monitoring the seasonal changes of an englacial conduit network using repeated ground penetrating radar measurements

Abstract

Between 2012 and 2019, repeated 25 MHz ground penetrating radar (GPR) surveys were carried out over an active englacial conduit network within the ablation area of the temperate Rhonegletscher, Switzerland. In 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations. The resulting GPR data were processed using an impedance inversion workflow to compute GPR reflection coefficients and layer impedances, which are indicative of the conduit's infill material. The spatial and temporal evolution of the reflection coefficients also provided insights into the morphology of the 5 Rhonegletscher's englacial conduit network. During the summer melt seasons, we observed an active, water-filled, sedimenttransporting englacial conduit network that yielded large negative GPR reflection coefficients (<-0.2). For all the GPR surveys conducted during the summer, the englacial conduit was 15-20 m wide,~0.4 m thick,~250 m long with a shallow inclination (2°) and having a sinusoidal shape. We speculate that such a geometry is likely the result of extensional hydraulic fracturing.Synthetic GPR waveform modelling using a thin water-filled conduit showed that a conduit thickness larger than 0.4 m (0.3 10 × minimum wavelength) thick can be correctly identified using 25 MHz GPR data. During the winter periods, the englacial conduit shuts down and either physically closed or becomes very thin (<0.1 m), thereby producing small negative reflection coefficients that are caused by either sediments lying within the closed conduit or water within the very thin conduit. Furthermore, the englacial conduit reactivated during the following melt season at an identical position as in the previous year.