Abstract
Abstract. Englacial conduits act as water pathways to feed surface meltwater into the subglacial drainage system. A change of meltwater into the subglacial drainage system can alter the glacier's dynamics. 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 2012, 2016, and 2017 GPR measurements were carried out only once a year, and an englacial conduit was detected in 2017. In 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations in the detected englacial conduit. 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 Rhonegletscher's englacial conduit network. During the summer melt seasons, we observed an active, water-filled, sediment-transporting englacial conduit network that yielded large negative GPR reflection coefficients (<-0.2). The GPR surveys conducted during the summer provided evidence that the englacial conduit was 15–20 m±6 m wide, ∼0.4m±0.35m thick, ∼250m±6m long with a shallow inclination (2∘), and having a sinusoidal shape from the GPR data. We speculate that extensional hydraulic fracturing is responsible for the formation of the conduit as a result of the conduit network geometry observed and from borehole observations. Synthetic GPR waveform modelling using a thin water-filled conduit showed that a conduit thickness larger than 0.4 m (0.3× minimum wavelength) thick can be correctly identified using 25 MHz GPR data. During the winter periods, the englacial conduit no longer transports water and either physically closed or became 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.
Highlights
Surface meltwater is routed through the glacier’s interior by englacial drainage systems, before it reaches subglacial drainage systems (Fountain and Walder, 1998; Cuffey and Paterson, 2010)
The strength of the reflected ground-penetrating radar (GPR) signal is a function of media’s electrical properties that form an interface and can be used to determine subglacial environments. Such studies have been conducted with an impulse ice-penetrating radar system within a cold-ice environment (Macgregor et al, 2011; Christianson et al, 2016); no such analysis has been performed using a commercial GPR within a temperate ice environment or to characterise an englacial conduit network
These common midpoint (CMP) measurements were performed in April, May, September, and October 2018 over the englacial conduit in order to detect any seasonal changes to the EM-wave velocities
Summary
Surface meltwater is routed through the glacier’s interior by englacial drainage systems, before it reaches subglacial drainage systems (Fountain and Walder, 1998; Cuffey and Paterson, 2010). Englacial drainage systems often provide the meltwater pathways that can facilitate changes in subglacial water pressure, and as a result they can impact the glacier’s dynamics. The strength of the reflected GPR signal is a function of media’s electrical properties that form an interface and can be used to determine subglacial environments Such studies have been conducted with an impulse ice-penetrating radar system within a cold-ice environment (Macgregor et al, 2011; Christianson et al, 2016); no such analysis has been performed using a commercial GPR within a temperate ice environment or to characterise an englacial conduit network. GPR imaging and reflectivity analysis facilitates studying the temporal and spatial changes of an englacial conduit network on a temperate glacier. We used a GPR data simulation algorithm using a variety of 3D englacial conduit models in order to quantify the spatial dimensions of an active englacial conduit network
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