Near‐surface seismic reflection profiling of the Matanuska Glacier, Alaska

Abstract

Several common‐midpoint seismic reflection profiles collected on the Matanuska Glacier, Alaska, clearly demonstrate the feasibility of collecting high‐quality, high‐resolution near‐surface reflection data on a temperate glacier. The results indicate that high‐resolution seismic reflection can be used to accurately determine the thickness and horizontal distribution of debris‐rich ice at the base of the glacier. The basal ice thickens about 30% over a 300‐m distance as the glacier flows out of an overdeepening. The reflection events ranged from 80‐ to 140‐m depth along the longitudinal axis of the glacier. The dominant reflection is from the contact between clean, englacial ice and the underlying debris‐rich basal ice, but a strong characteristic reflection is also observed from the base of the debris‐rich ice (bottom of the glacier). The P‐wave propagation velocity at the surface and throughout the englacial ice is 3600 m/s, and the frequency content of the reflections is in excess of 800 Hz. Supporting drilling data indicate that depth estimates are correct to within ± 1 m.

Macro‐ and micro‐sedimentology of a modern melt‐out till – Matanuska Glacier, Alaska, USA

“…A radar survey also documented the thickening of the basal ice with distance up the adverse slope of an overdeepening behind the glacier terminus (Baker et al . ).…”

“…A systematic ground‐penetrating radar survey near the cut bank has shown that the basal ice extends in the subsurface towards the glacier and into the moraine complex, and that the diamicton can be traced as a distinct horizon on the buried ice surface (Baker et al . ,b; Larson et al . : fig.…”

Glaciohydraulic supercooling in former ice sheets?

“…They have an average relief of 1-3 m and are commonly flanked by shallow closed depressions b 1 m deep. Recent geophysical studies have shown some of the hummocks to be ice cored (Baker et al, 2003a;Larson et al, 2003) and that some of the irregular terrain can be attributed to glaciotectonic deformation of stacked remnants of buried debris-laden ice (Baker et al, 2003b).…”

“…They are separated by linear depressions N1 m deep and generally have steep proximal and distal slopes. At least some of the ridges are ice cored (Baker et al, 2003a;. The debris fans occur mainly on the flanks of the ridges but some extend through ridge gaps and into linear depressions between ridges.…”

Active seismoelectric exploration of glaciers

“…Comparison of the isolated EIRs with cross‐borehole seismic and GPR data reveals that strong electrokinetic conversions occur within the surface snow pack, near the interface between dry and wet ice, and at the ice‐bed interface. Baker et al [2003] observed high‐frequency seismic reflections from the ice‐bed interface at depths of >100 m at Matanuska glacier, Alaska, generated by a hammer‐and‐plate source. Observation of EIRs from the ice‐bed interface (Figure 5), albeit weak compared to snow and dry‐wet ice interface EIRs, therefore does not appear unrealistic.…”