Distributed Acoustic Sensing of Seismic Properties in a Borehole Drilled on a Fast‐Flowing Greenlandic Outlet Glacier

Abstract

Distributed Acoustic Sensing (DAS) is a new technology in which seismic energy is detected, at high spatial and temporal resolution, using the propagation of laser pulses in a fiber‐optic cable. We show analyses from the first glaciological borehole DAS deployment to measure the englacial and subglacial seismic properties of Store Glacier, a fast‐flowing outlet of the Greenland Ice Sheet. We record compressional and shear waves in 1,043 m‐deep vertical seismic profiles, sampled at 10 m vertical resolution, and detect a transition from isotropic to anisotropic ice at 84% of ice thickness, consistent with the Holocene‐Wisconsin transition. We identify subglacial reflections originating from the base of a 20 m‐thick layer of consolidated sediment and, from attenuation measurements, interpret temperate ice in the lowermost 100 m of the glacier. Our findings highlight the promising potential of DAS technology to constrain the seismic properties of glaciers and ice sheets.

Thermodynamics of a fast-moving Greenlandic outlet glacier revealed by fiber-optic distributed temperature sensing

“…Consequently, deformation rates are typically 2.5 to 5 times greater in pre-Holocene ice than in Holocene ice under similar temperature and stress conditions ( 30 , 38 , 39 ). The inferred transition from Holocene to pre-Holocene ice (i.e., the Last Glacial-Interglacial Transition) at 889 m depth (85% of the ice thickness) matches the transition from isotropic to anisotropic ice recorded at ~880 m depth in the same borehole using distributed acoustic sensing with 10-m spatial resolution ( 40 ). The inferred Last Glacial-Interglacial Transition also agrees well with the depth inferred previously from seismic studies of this transition at site S30 on Store Glacier [84%, ( 36 )], and it falls within the estimated range for the Last Glacial-Interglacial Transition in a wider regional study [82 to 85%, ( 35 )].…”

“…Booth et al . ( 40 ) show that borehole R30 at Store Glacier is likely to be underlain by consolidated sediment, the shear strength of which may vary depending on porosity and water supplied in subglacial drainage pathways ( 51 ), and Hofstede et al . ( 36 ) report notable differences in the availability of water at the ice-sediment interface beneath Store Glacier across distances of only a few kilometers.…”

“…The fiber-optic cable was attached to a thermistor-piezometer cable weighted with a ~2-kg chain 280 mm above the piezometer diaphragm; this assembly was lowered to 1043 m below the ice surface, whereupon pressure readings stopped increasing, and then raised 50 mm above the inferred bed location. We used a Solifos BRUsens fiber-optic sensing cable containing two single-mode fibers [OS2, for separate distributed acoustic sensing covered by Booth et al (40)] alongside four multimode (OM3) bend-insensitive fibers for DTS in a duplex arrangement. A stainless steel basal turnaround with an outside diameter of 15 mm and internal fiber loop of 5 mm diameter was used.…”

Breaking the Ice: Identifying Hydraulically Forced Crevassing

“…Obviously, this would be costly and logistically challenging. However, novel instrumentation such as distributed acoustic sensing (DAS) (Booth et al, 2020;Walter et al, 2020) or seismic nodes would make this feasible. We suggest that one should use a spiral or similar network geometry, so as to have sufficient radial and azimuthal coverage of the study site to constrain source mechanisms.…”

Distributed Acoustic Sensing (DAS) of Seismic Properties in a Borehole drilled on a Fast-Flowing Greenlandic Outlet Glacier

“…Poisson's ratio describes how a material deforms when under stress, specifically as the ratio of transverse extension to axial compression. In ice, Poisson's ratio is typically 0.3-0.34 (Köhler et al, 2019) but is sensitive to the strength of anisotropy (Nanthikesan & Shyam Sunder, 1994) and increases weakly with temperature (Weeks & Assur, 1967).…”