Observing the subglacial hydrology network and its dynamics with a dense seismic array

Abstract

Subglacial water flow strongly modulates glacier basal motion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismic monitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming.

Dynamic Imaging of Glacier Structures at High‐Resolution Using Source Localization With a Dense Seismic Array

“…This evolution was identified by Nanni, Gimbert, et al. (2021) as a transition from a distributed to a more localized drainage system in response to an increase in surface melt, consistent with expectations based on numerous observational and numerical studies in different contexts (Irarrazaval et al., 2021; Lewington et al., 2020; Tranter et al., 1996; Vincent & Moreau, 2016).…”

“…At high frequencies ((10–20) Hz), short and repeated pulses mainly composed of surface waves (phase velocity ∼1,590 m. sec −1 ) cross the entire array and dominate the seismic signal with a few seconds recurrence time (Gimbert et al., 2021). At low frequency ((3–7) Hz), subglacial water flow has been observed to generate continuous seismic noise (Nanni, Gimbert, Vincent et al., 2020) and a dynamic mapping of subglacial hydrological flows was carried out via the study of spatially dispersed seismic sources generating low phase coherence (Nanni, Gimbert, et al., 2021).…”

“…We select three frequency bands of (5, 13, 17) ±2 Hz over which we coherently apply the MFP for each bin of 0.1 Hz within this range. To preserve the quality of the source localizations, we only keep sources detected within the glacier at distance less than 400 m from the seismic array center and with realistic phase velocities ((1,000–3,500) m. sec −1 , Nanni, Gimbert, et al., 2021; Sergeant et al., 2020). We also only keep sources with MFP output greater than a threshold of 0.01, which is one order of magnitude above the level of phase coherence associated with a random phase field.…”

“…The complementary data associated with the dense array experiment, including the actives crevasses identification, are available via https://doi.org/10.5281/zenodo.3971815 under a creative commons attribution 4.0 international license (Nanni, Gimbert, Helmstetter, et al., 2020; Gimbert et al., 2021). The complementary data associated with the subglacial hydrology investigation (Section 3.3) are available via https://doi.org/10.5281/zenodo.4024660 under a creative commons attribution 4.0 international license (Nanni, Gimbert, Roux et al., 2020, Nanni, Gimbert et al., 2021).…”

Seismic Mapping of Subglacial Hydrology Reveals Previously Undetected Pressurization Event

“…Expanding observational scales of subglacial hydrology is vital to predicting how Measurement of glaciohydraulic tremor, the continuous seismic signal generated by turbulent water flow in a glacier, can improve the spatial and temporal scales of subglacial hydrological observations. By adapting theoretical descriptions of tremor generation from fluvial seismology (Burtin et al 2008, Burtin et al 2011, Tsai et al 2012, Schmandt et al 2013, Gimbert et al 2014, glaciohydraulic tremor has already been shown to reveal properties of subglacial hydrologic systems such as water flux (Bartholomaus et al 2015, Winberry et al 2009, sediment transport (Gimbert et al 2016), water pressurization (Gimbert et al 2016, Lindner et al 2020, Nanni et al 2020, water flow source location (Vore et al 2019, Nanni et al 2021, and flood front propagation (Eibl et al 2020). In addition, analysis of tremor signals at glaciers can also be interpreted to locate and describe other glaciohydraulic sources such as moulin activity (Aso et al 2017, Röösli et al 2014, Röösli et al 2016, Lindner et al 2020) and crack waves in the basal water layer (Gräff et al 2019).…”