The thickness and internal structure of Fireweed rock glacier, Alaska, U.S.A., as determined by geophysical methods

Abstract

ABSTRACT. Geophysical investigations on rock glaciers are often difficult because rock glaciers are covered by an unconsolidated debris mantle a few meters thick, are typically <50 m thick and are composed of an ice^rock mixture of unknown composition. Transient electromagnetics (TEM) is a method that allows some of these difficulties to be minimized, and data collection is relatively efficient. TEM, with calibration from terminus exposure, was used to determine the thickness ($60 m) of Fireweed rock glacier, Alaska, U.S.A., under complex valley geometry. A conductive layer beneath the rock glacier was identified, and its distribution is consistent with a till-like layer. Seismic refraction, used to resolve the debris-mantle thickness (2^4 m), suggests the presence of a discontinuity at 18^28 m depth within the rock glacier. The discontinuity is also indicated in the radio-echo sounding and theTEM data, but to a lesser extent. This discontinuity is important because the motion of the rock glacier may occur across this as a''shear plane'' . INTRODUCTIONEstimates of the thickness and cross-sectional shape of a rock glacier are important for understanding its stress distribution and motion. On ice glaciers this geometry is routinely determined using seismic and ice-radar methods. These methods are difficult to apply to rock glaciers because rock glaciers are thinner, are composed of a mixture of ice and rock of unknown composition and are covered by a 23 m thick layer of unconsolidated rock (the ''debris mantle''). Radio-echo sounding (RES) is difficult because the absorption and scattering of radar waves are stronger in rockglacier ice^rock mixtures than in clean ice and the basal interface may not be distinct. However, Berthling and others (2000), Degenhardt and others (2000), Isaksen and others (2000), Vonder Mu« hll and others (2001) and were successful in using ground-penetrating radar (GPR) to discern the basal interface and/or internal structures of some rock glaciers. Seismic methods are complicated by the debris mantle, which limits the transfer energy from the source (usually an explosion) into the rock glacier and inhibits geophone coupling. Seismic field techniques have been devised to overcome these problems , but are difficult to apply on a routine basis and have shown limited success. Direct-current (d.c.) electrical resistivity has been widely used in rock-glacier soundings (Fisch and others,1977; Evin and others, 1997;), but the debris mantle again poses problems with electrical coupling and requires laborintensive field set-ups. We have used all of these methods, with the exception of GPR, to investigate the geometry of Fireweed rock glacier with limited success. We have found that transient electromagnetic (TEM) methods provide the best means of investigating the internal structure and thickness distribution of the rock glacier. TEM methods do not require a high degree of physical or electrical coupling with the surface substrate. Here we discuss the methods and results for each of the techniq...

Geophysical imaging of alpine rock glaciers

“…Since the electromagnetic coupling is inductive, the sources and receivers need not be in direct contact with the ground. Consequently, the techniques can be used for ground and airborne surveying (Hoekstra, 1978; Todd and Dallimore, 1998; Hauck and others, 2001, Bucki and others, 2004).…”

Fast deformation of perennially frozen debris in a warm rock glacier in the Swiss Alps: An effect of liquid water

“…In contrast to the strength of polycrystalline ice and frozen soils derived from laboratory tests, the huge Fireweed rock glacier in Alaska had surface velocity fields requiring A values at least twice those of clean temperate ice [ Bucki and Echelmeyer , 2004]. The high A values indicating soft frozen ground appear to be associated with the large amount of debris, which was in fact seen at the calved front [ Elconin and LaChapelle , 1997; Bucki et al , 2004]. Bucki and Echelmeyer [2004] considered that the large A values originated from interstitial water between debris particles and ice crystals, which possibly allowed slip at the interface by lowering the effective viscosity, as Echelmeyer and Wang [1987] and Cohen [2000] explained the softness of dirty basal ice.…”

Characterizing seasonal groundwater storage in alpine catchments using time‐lapse gravimetry, water stable isotopes and water balance methods

“…A wide variety of geophysical methods have been employed to image internal structures and characterize physical properties within talus and moraine deposits. These include ground‐penetrating radar (Ardelean, Onaca, Urdea, & Sarasan, 2017), electrical resistivity tomography (McClymont, Hayashi, Bentley, Muir, & Ernst, 2010; McClymont et al, 2011), electromagnetic induction methods (Bucki, Echelmeyer, & MacInnes, 2004), seismic refraction (Langston et al, 2011), and surface nuclear magnetic resonance (Lehmann‐Horn et al, 2011). These methods give essential information on hydrological and geophysical characteristics of alpine aquifers, such as the volume of the deposits, the location of wet and dry areas and the presence of internal structures that may control groundwater flow paths (Clow et al, 2003; McClymont et al, 2011), but do not directly quantify groundwater storage variations.…”