Gregory James Church

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Church

First Name

Gregory James

ORCID

0000-0003-0114-9950

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Publications 1 - 10 of 12

High-resolution helicopter-borne ground penetrating radar survey to determine glacier base topography and the outlook of a proglacial lake
Church, Gregory James; Bauder, Andreas; Grab, Melchior; et al. (2018)
2018 17th International Conference on Ground Penetrating Radar (GPR)
Ice volume estimates of Swiss glaciers using helicopter-borne GPR - An example from the Glacier de la Plaine Morte
Grab, Melchior; Bauder, Andreas; Ammann, Florian; et al. (2018)
2018 17th International Conference on Ground Penetrating Radar (GPR)
Seasonal evolution of an englacial conduit network on a temperate alpine glacier using ground-penetrating radar
Church, Gregory James; Grab, Melchior; Schmelzbach, Cédric; et al. (2019)
AGU Fall Meeting Abstracts
Monitoring the seasonal changes of an englacial conduit network using repeated ground-penetrating radar measurements
Church, Gregory James; Grab, Melchior; Schmelzbach, Cédric; et al. (2020)
The Cryosphere
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.
Using active geophysical methods to characterise a temperate glacier’s hydrological system
Church, Gregory James (2021)
Worldwide, glaciers are receding as a consequence of climate change. Due to the global glacier recession, glaciers are experiencing enhanced melting, which results in an increase in meltwater availability. This increased meltwater has the ability to alter the glacier's hydraulic conditions and ultimately affects the dynamics of glaciers. Glaciers move through a combination of basal motion and internal ice deformation. The motion at the ice-bed interface comprises of both ice sliding along the bed and the deformation of subglacial sediments. Sliding at the bed is partially controlled by the water pressure at the ice-bed interface and therefore, knowledge of the subglacial hydraulic system is paramount in order to predict glacier movement and fundamentally the future evolution of glaciers. To date, our understanding of the glacier’s hydrological system is limited due insufficient field observations. As part of my thesis, I used surface-based geophysical methods to detect and characterise an alpine glacier's hydrological system. The aim is to improve our understanding of the drainage network of temperate alpine glaciers by using active seismic and ground-penetrating radar (GPR) to determine the properties in space and time of the hydrological system. To detect and characterise englacial flow paths, I performed active seismic and GPR on Rhonegletscher (Switzerland). Both geophysical methods detected a hydrological flow path within the glacier. With the use of amplitude-versus-offset analysis from the 2017 active seismic data, I confirmed that the englacial conduit was water-filled, thereby indicating that this technique is able to characterise hydrological features of glaciers. From both geophysical datasets, I estimated the conduit thickness to be between 0.5 and 4 m. With the acquisition of a 2D GPR grid in 2018, the spatial extent of the englacial conduit was estimated to be approximately 14,000 m$^2$. The water within the englacial conduit was sourced by surrounding surface meltwater and morainal streams. These streams enter the glacier subglacially before flowing into an efficient conduit system indicating that surrounding melt water streams directly impact the glacier's hydrological system. Upon successfully detecting an englacial conduit in 2017 on Rhonegletscher, I performed repeated GPR surveys in 2018 and 2019 to detect seasonal and annual changes to the englacial conduit. I extracted the GPR reflectivity using an impedance inversion workflow. The workflow allowed an interpretation of the englacial conduit's infill material. During the summer melt season, a flowing, water-filled sediment-transporting englacial conduit was detected having a sinusoidal shape with shallow inclination and being 0.4 $\pm{}$ 0.35 m thick. The repeated GPR measurements revealed that the shape of the conduit did not dramatically alter throughout the melt season. During the winter periods, the englacial conduit transported neither water nor sediment. It was either physically closed, or it became very thin (< 0.1 m). Such detailed interpretations were only possible with the use of the impedance inversion workflow and synthetic GPR waveform modelling. Even though temperate glaciers have a dynamic hydrological system, I was able confirm that the englacial conduit reactivated in an identical position from 2018 to 2019. The observations into Rhonegletscher's englacial conduit were conducted using 2D geophysical data. Such 2D geophysical reflection data represents the current state-of-the-art in glaciological applications; however, 2D reflection data falsely assume that all reflections originate from the vertical plane of acquisition. Complex englacial structures or basal geometries cause this assumption to be violated. Therefore, for accurate subsurface imaging, 3D GPR acquisition is required. Consequently, I acquired a 3D GPR survey that provided unprecedented high-resolution and unaliased 3D images of the Rhonegletscher's drainage network situated in the lower ablation zone. With this dataset, I was able to confirm a long-standing theory regarding melt water flow pathways around overdeepenings. With the development of unmanned aerial vehicles, future 3D GPR surveys are looking to be carried out in a fast and efficient manner. The research carried out and presented in this thesis demonstrates that both active seismic and GPR can be used to successfully identify, characterise and monitor a temperate glacier's hydrological network. Such studies will have a substantial impact on future investigations of the glacier hydrological system, and I conclude that future 3D GPR surveys conducted by unmanned aerial vehicles have the potential to revolutionise the way GPR data are acquired and processed in cryosphere applications.
Modeling the Re-appearance of a Crashed Airplane on Gauligletscher, Switzerland
Compagno, Loris; Jouvet, Guillaume; Bauder, Andreas; et al. (2019)
Frontiers in Earth Science
In this study we used a modeling approach to reconstruct the space-time trajectory of the Dakota airplane which crashed on the Gauligletscher in 1946 and was subsequently buried by snow accumulation. Our aim was to localize its present position and predict when and where it would re-appear at the surface. As a first step we modeled the ice flow field and the evolution of Gauligletscher from 1946 using a combined Stokes ice flow and surface mass balance model, which was calibrated with surface elevation and velocity observations. In a second step the modeled ice velocity fields were integrated forward-in-time, starting from the crash location. Our results suggest that the main body of the damaged aircraft will be released approximately between 2027 and 2035, 1 km upstream of the parts that emerged between 2012 and 2018. Our modeling results indicate that the recently found pieces of the Dakota might have been removed from the original aircraft location and moved down-glacier before being abandoned in the late 40s.
Ice thickness distribution of all Swiss glaciers based on extended ground-penetrating radar data and glaciological modeling
Grab, Melchior; Mattea, Enrico; Bauder, Andreas; et al. (2021)
Journal of Glaciology
Accurate knowledge of the ice thickness distribution and glacier bed topography is essential for predicting dynamic glacier changes and the future developments of downstream hydrology, which are impacting the energy sector, tourism industry and natural hazard management. Using AIR-ETH, a new helicopter-borne ground-penetrating radar (GPR) platform, we measured the ice thickness of all large and most medium-sized glaciers in the Swiss Alps during the years 2016–20. Most of these had either never or only partially been surveyed before. With this new dataset, 251 glaciers – making up 81% of the glacierized area – are now covered by GPR surveys. For obtaining a comprehensive estimate of the overall glacier ice volume, ice thickness distribution and glacier bed topography, we combined this large amount of data with two independent modeling algorithms. This resulted in new maps of the glacier bed topography with unprecedented accuracy. The total glacier volume in the Swiss Alps was determined to be 58.7 ± 2.5 km3 in the year 2016. By projecting these results based on mass-balance data, we estimated a total ice volume of 52.9 ± 2.7 km3 for the year 2020. Data and modeling results are accessible in the form of the SwissGlacierThickness-R2020 data package.
Ground-penetrating radar imaging reveals glacier's drainage network in 3D
Church, Gregory James; Bauder, Andreas; Grab, Melchior; et al. (2021)
The Cryosphere
Hydrological systems of glaciers have a direct impact on the glacier dynamics. Since the 1950s, geophysical studies have provided insights into these hydrological systems. Unfortunately, such studies were predominantly conducted using 2D acquisitions along a few profiles, thus failing to provide spatially unaliased 3D images of englacial and subglacial water pathways. The latter has likely resulted in flawed constraints for the hydrological modelling of glacier drainage networks. Here, we present 3D ground-penetrating radar (GPR) results that provide high-resolution 3D images of an alpine glacier's drainage network. Our results confirm a long-standing englacial hydrology theory stating that englacial conduits flow around glacial overdeepenings rather than directly over the overdeepening. Furthermore, these results also show exciting new opportunities for highresolution 3D GPR studies of glaciers.
Repeated ground-penetrating radar measurements to detect seasonal and annual variations of an englacial conduit network
Church, Gregory James; Bauder, Andreas; Grab, Melchior; et al. (2020)
EGUsphere
Using active geophysical methods to characterise a temperate glacier's hydrological system
Church, Gregory James (2021)
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Gregory James Church

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