Detecting supraglacial debris thickness with GPR under suboptimal conditions

Alexandra Giese,Steven Arcone,Robert Hawley,Gabriel Lewis,Patrick Wagnon

doi:10.1017/jog.2021.59

Abstract

The thickness of a supraglacial layer is critical to the magnitude and time frame of glacier melt. Field-based, short pulse, ground-penetrating radar (GPR) has successfully measured debris thickness during a glacier's melt season, when there is a strong return from the ice–debris interface, but profiling with GPR in the absence of a highly reflective ice interface has not been explored. We investigated the performance of 960 MHz signals over 2 km of transects on Changri Nup Glacier, Nepal, during the post-monsoon. We also performed laboratory experiments to interpret the field data and investigate electromagnetic wave propagation into dry rocky debris. Laboratory tests confirmed wave penetration into the glacier ice and suggest that the ice–debris interface return was missing in field data because of a weak dielectric contrast between solid ice and porous dry debris. We developed a new method to estimate debris thicknesses by applying a statistical approach to volumetric backscatter, and our backscatter-based calculated thickness retrievals gave reasonable agreement with debris depths measured manually in the field (10–40 cm). We conclude that, when melt season profiling is not an option, a remote system near 1 GHz could allow dry debris thickness to be estimated based on volumetric backscatter.

Highlights

19% of glacier area across Eastern South Asia is debris-covered compared to a figure of only 7.3% on mountain glaciers excluding Antarctica and Greenland (Herreid and Pellicciotti, 2020)
By measuring the relative permittivity of mineralogically similar, dry porous debris in the rock box, we found that a lack of dielectric contrast between glacier ice (ε = 3.18) and debris likely accounted for the absence of interface detection on Changri Nup
This study presents a method to calculate debris thicknesses along five transects of Changri Nup Glacier based upon the depth decay of the volumetric backscatter that dominates our recorded profiles

Summary

Introduction

19% of glacier area across Eastern South Asia is debris-covered compared to a figure of only 7.3% on mountain glaciers excluding Antarctica and Greenland (Herreid and Pellicciotti, 2020). The share of glacier area covered in debris is greater than average in the eastern Himalaya, at 25% (Kraaijenbrink and others, 2017) in the East Nepal region defined by Kääb and others (2015) (Brun and others, 2017). Understanding the impact of climate change on all glaciers in High Mountain Asia is critical to predicting the future water supply of a highly populated region (Lutz and others, 2014; Brun and others, 2017; Rowan and others, 2017). As shown through experiments (Östrem, 1959; Reznichenko and others, 2010) and in situ field measurements (Mattson and others, 1993; Nicholson and Benn, 2006), a debriscovered glacier’s response to climate depends on debris thickness.

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