Abstract
Abstract. Subglacial water modulates glacier-bed friction and therefore is of fundamental importance when characterising the dynamics of ice masses. The state of subglacial pore water, whether liquid or frozen, is associated with differences in electrical resistivity that span several orders of magnitude; hence, liquid water can be inferred from electrical resistivity depth profiles. Such profiles can be obtained from inversions of transient (time-domain) electromagnetic (TEM) soundings, but these are often non-unique. Here, we adapt an existing Bayesian transdimensional algorithm (Multimodal Layered Transdimensional Inversion – MuLTI) to the inversion of TEM data using independent depth constraints to provide statistical properties and uncertainty analysis of the resistivity profile with depth. The method was applied to ground-based TEM data acquired on the terminus of the Norwegian glacier, Midtdalsbreen, with depth constraints provided by co-located ground-penetrating radar data. Our inversion shows that the glacier bed is directly underlain by material of resistivity 102 Ωm ± 1000 %, with thickness 5–40 m, in turn underlain by a highly conductive basement (100 Ωm ± 15 %). High-resistivity material, 5×104 Ωm ± 25 %, exists at the front of the glacier. All uncertainties are defined by the interquartile range of the posterior resistivity distribution. Combining these resistivity profiles with those from co-located seismic shear-wave velocity inversions to further reduce ambiguity in the hydrogeological interpretation of the subsurface, we propose a new 3-D interpretation in which the Midtdalsbreen subglacial material is partitioned into partially frozen sediment, frozen sediment/permafrost and weathered/fractured bedrock with saline water.
Highlights
Subglacial structure and material properties are one of several important controls on ice flow, both through composition and ice–material interactions
We provide a mechanism for the constrained inversion of transient electromagnetics (TEM), with depth constraints derived from ground-penetrating radar (GPR), to provide geophysical insight into the structure and water characteristics of the subglacial environment
This paper focuses on the specific TEM survey design used in this study, a ground-based 10 × 10 m transmitter with receiver 15 m away, MuLTI-TEM can be used with most TEM datasets
Summary
Subglacial structure and material properties are one of several important controls on ice flow, both through composition and ice–material interactions. The ability to develop accurate ice-flow models is limited by poor understanding of processes acting at the ice–bed interface and the composition of subglacial material. Non-invasive geophysical imaging methods are widely and successfully applied to characterise the internal properties of glacier ice and its immediate basal environment. Such methods (including reflection seismology and groundpenetrating radar) can underperform when characterising material properties beyond the first few metres of the glacier bed (Booth et al, 2012), yet subglacial aquifers, sediment accumulations and permafrost can extend to much greater depths Glaciological surveys often involve the acquisition of multiple geophysical datasets: given the typical absence of ground-truth data, imaging the target with several methods
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