Abstract
AbstractGround‐penetrating radar (GPR) is widely used on polythermal glaciers to image bed topography and detect internal scatter due to water inclusions in temperate ice. The glaciological importance of this is twofold: bed topography is a primary component for modeling the long‐term evolution of glaciers and ice sheets, and the presence of temperate ice and associated englacial water significantly reduces overall ice viscosity. Englacial water has a direct influence on radar velocity, which can result in incorrect observations of bed topography due to errors in depth conversion. Assessment of radar velocities often requires multi‐offset surveys, yet these are logistically challenging and time consuming to acquire, hence techniques to extract velocity from common‐offset data are required. We calculate englacial radar velocity from common offset GPR data collected on Von Postbreen, a polythermal glacier in Svalbard. We first separate and enhance the diffracted wavefield by systematically assessing data coherence. We then use the focusing metric of negative entropy to deduce a migration velocity field and produce a velocity model which varies spatially across the glacier. We show that this velocity field successfully differentiates between areas of cold and temperate ice and can detect lateral variations in radar velocity close to the glacier bed. This velocity field results in consistently lower ice depths relative to those derived from a commonly assumed constant velocity, with an average difference of 4.9 ± 2.5% of local ice depth. This indicates that diffraction focusing and velocity estimation are crucial in retrieving correct bed topography in the presence of temperate ice.
Highlights
In this paper, we apply common-offset velocity analysis to conventional glaciological ground-penetrating radar (GPR) data acquired across an Arctic polythermal glacier
We show that this velocity field successfully differentiates between areas of cold and temperate ice and can detect lateral variations in radar velocity close to the glacier bed. This velocity field results in consistently lower ice depths relative to those derived from a commonly assumed constant velocity, with an average difference of 4.9 ± 2.5% of local ice depth. This indicates that diffraction focusing and velocity estimation are crucial in retrieving correct bed topography in the presence of temperate ice
We find that the velocities derived using this approach typically result in ice depths which are lower than those obtained using a commonly assumed constant velocity, with an average difference of 4.9 ± 2.5% of local ice depth, or 12.0 ± 8.4 m in absolute terms, for data collected over Von Postbreen
Summary
We apply common-offset velocity analysis to conventional glaciological ground-penetrating radar (GPR) data acquired across an Arctic polythermal glacier. Our motivation is to improve constraints on the englacial temperate-cold ice distribution, and to improve the accuracy of survey-derived basal topography, which is often degraded beneath polythermal ice masses. The importance of this approach is twofold. The application of GPR processing strategies to retrieve correct bed depth and topography is dependent on an accurate radar velocity field, yet this is often overlooked or poorly understood in studies quantifying ice volume or mapping bed topography of polythermal and temperate glaciers (Lapazaran et al, 2016).
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