Deducing the thermal structure in the tongue of Gornergletscher, Switzerland, from radar surveys and borehole measurements

Abstract

We present the thermal distribution in the confluence area of Gorner-and Grenzgletscher, Valais, Switzerland. The area was mapped by ice-penetrating radar at 1-5 and 40 MHz. The higherfrequency data reveal a thick surface layer of low backscatter in the center of the Grenzgletscher branch. Based on datasets of borehole-temperature measurements and flow velocity, we interpret this as a thick layer of cold ice, advected from the accumulation region of Grenzgletscher. Along seven profiles the base of the low-backscatter zone can be found at a maximum depths between approximately 100 and 200 m. Laterally, the layer extends some 400 m, ∼1/3 of the width of the Grenzgletscher branch. The lower boundary of the low-backscatter zone is systematically higher than the cold-temperate transition surface found in the boreholes. This discrepancy is attributed to the direct sensitivity of radar backscatter to liquid-water inclusions, rather than to the temperature distributions as observed in boreholes. We present the current state of the cold layer and discuss its influence on other glacier characteristics.

Results of an experimental radar survey on the gornergletscher glacier system (Zwillingsgletscher), Valais, Switzerland

“…A one-to-one correspondence between the subsurface pattern and the glacier surface is not expected since ablation processes would presumably affect the surface and subsurface differently. Our observations are different from those made by [7] with a lower frequency antenna on the adjacent Grenzgletscher. There the onset of scattering is about one order of magnitude deeper using lower frequency antennas.…”

“…The Bas Glacier d'Arolla shows very well-formed examples of band ogives, which [2] associated with "alternating reflection-rich and reflection-poor zones" on their radar profiles. Unlike the study of the Grenzgletscher [7], no association was drawn between zones of radar scattering and temperature or water inclusion effects. parameters would affect resolution, the single traverse was surveyed twice with different field frequency filters.…”

“…Previous geophysical investigations in the Valais area that involved GPR include a deep, low-frequency (1-5 and 40 MHz) survey on the Grenzgletscher, immediately northeast of our study area [7], and a higher frequency (100 MHz) survey on the Bas Glacier d'Arolla located roughly 25 km to the westnorthwest of our study area [2]. The former study detected a several-hundred meter wide layer of ice with a maximum thickness of 381 m (measured from a borehole).…”

“…Both migration and depth conversion were applied with a constant velocity of 0.16 m/ns, typical for solid freshwater ice [e.g., 9,10]. For the Grenzgletscher, located immediately to the northeast of our study area, [7] used a solid ice velocity of 0.168 m/ns, which differs by 5%. The GPR profiles were corrected for topographic variation by surveying locations (averaging 1 reading every 20 m) along the line.…”

Unlocking annual firn layer water equivalents from ground-penetrating radar data on an Alpine glacier

“…Therefore, studies have focused on the mapping of facies zones and other general glaciological characteristics (Pälli et al, 2003;Langley et al, 2008;Eisen et al, 2009;Dunse et al, 2009). On the other hand, GPR not only could accompany ongoing monitoring programmes to complement the sparse accumulation measurements with high spatial resolution but also could be used to retrospectively extend newly initiated time series into the past.…”