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.
Delimiting a saline water zone in Quaternary fluvial–alluvial deposits using transient electromagnetic: a case study in Punata, Bolivia
