![]() ![]() Stuart and others ( Reference Stuart, Murray, Brisbourne, Styles and Toon2005) describe resonances from water-filled fractures close to the glacier base during a surge at Bakaninbreen, Svalbard, and West and others ( Reference West, Larsen, Truffer, O'Neel and LeBlanc2010) emphasize the similarity between glacial fracture resonances within Bering Glacier, Alaska, and fluid chambers during an eruption of Augustine Volcano, Alaska. Passive seismic observations of such resonances in the cryosphere have often been attributed to resonant water-filled fractures: Anandakrishnan and Alley ( Reference Anandakrishnan and Alley1997) and Winberry and others ( Reference Winberry, Anandakrishnan and Alley2009) found narrow-banded seismic tremor signals from the bed of MacAyeal and Kamb ice streams, Antarctica, respectively, whereas Métaxian and others ( Reference Métaxian, Araujo, Mora and Lesage2003) detected low-frequency seismic signals from resonant water-filled ice cavities on the flanks of Cotopaxi Volcano, Ecuador. Some studies attribute similar resonance observations in other geological contexts to an intrinsic resonance of hydraulic fractures (Aki and others, Reference Aki, Fehler and Das1977) while other studies explain such observations as wave propagation effects (Bean and others, Reference Bean2014). These recordings often show resonances, whose interpretation is challenging (Clarke, Reference Clarke2005 Podolskiy and Walter, Reference Podolskiy and Walter2016). Therefore numerous studies either concentrate on point measurements directly at the glacier bed through boreholes, or on recordings of signals emitted from hydraulic events acquired with passive seismic techniques. ![]() Despite this importance, it is inherently difficult to observe processes at the glacier bed. Subglacial hydrology exerts a significant control on glacier flow. We demonstrate that high-frequency observations of subglacial hydraulic processes provide new insights into this evolving dynamic system. Using both the resonance frequencies and attenuation of recorded crack waves we estimate thickness, aperture and length of the resonating basal water layer patch into which we drilled. Our borehole observations confirm the occurrence of both sound and crack waves within the basal water layer. Long-wavelength resonances, in contrast, experience restoring forces due to elasticity and are composed of anomalously dispersed crack waves or Krauklis waves. Within such structures, short-wavelength waves experience restoring forces due to compressibility and are composed of sound waves. ![]() We apply a previously established theory of wave propagation along thin, water-filled structures such as water-filled voids, basal water layers, or hydraulic fractures. Here, we explore these mechanics using observations from a kHz-sampled pressure sensor installed in a borehole directly above the hard granite bedrock of a temperate mountain glacier in Switzerland. Within the subglacial system, rapid changes in these processes may excite resonances whose interpretation requires an understanding of the underlying wave mechanics. Hydraulic processes within and beneath glacial bodies exert a far-reaching control on ice flow through their influence on basal sliding. ![]()
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