High-Resolution Glacier Bed Imaging with Passive Seismology Using a Novel Dense Seismic Array at Isunnguata Sermia, West Greenland

Accelerating mass loss from the Greenland Ice Sheet is strongly modulated by meltwater-driven changes of bed conditions in ice dynamics, but these processes remain poorly constrained due to limited high-resolution observations of glacier hydrological systems. Constraints on basal properties can be obtained from borehole measurements and active reflection seismic surveys; however, these methods are inherently limited by their localized spatial coverage and are time intensive. As a result, they are poorly suited to monitoring the subglacial hydrological system, which evolves over seasonal timescales and across spatial scales larger than can be practically sampled. Thus, dense seismic node arrays offer a powerful alternative, enabling passive, high-resolution imaging and continuous monitoring of englacial and subglacial processes over broad areas and extended time periods.Here, we first present a dense seismic array experiment covering approximately 2.5 km² we conducted in the ablation zone of Isunnguata Sermia, West Greenland. The experiment comprised 82–117 autonomous seismic nodes deployed during one-month long monitoring periods in spring 2023 and in summer and fall 2024, complemented by multi-week surface Distributed Acoustic Sensing (DAS) measurements conducted in 2024. We assess data quality using power spectral densities and ambient noise cross-correlations to detect sensor tilt and GPS timing desynchronization, which are critical issues for data quality in remote and highly dynamical glacial environments such as the Greenland Ice Sheet.Then, we demonstrate we can use natural icequakes to perform seismic reflection analysis for bed mapping and interface property evaluation. We locate numerous natural seismic events with meter-scale resolution using Matched Field Processing (MFP) beamforming applied to array data in the 4–6 Hz frequency band. These events reveal a large population of near-surface sources associated with crevassing that generate both surface and body waves. We extract body waves and enable event stacking following a two-step synchronization procedure first synchronizing signals in the 4–6 Hz band dominated by surface waves, and subsequently refining timing in the 40–80 Hz band where P waves dominate. By sorting and stacking waveforms from these synchronized natural sources into common midpoint gathers, we identify high signal-to-noise ratio reflected P waves from the ice–bedrock interface at offsets of up to 1 km, significantly greater than those typically achieved in traditional active reflection seismic surveys on glaciers. This extended offset range makes the passive approach better suited for estimating basal conditions using classical amplitude-versus-offset (AVO) analyses due to the stronger dependence of reflectivity to bed properties at large incidence angles.Finally, following a workflow analogous to active reflection seismology, we derive a two-dimensional map of ice thickness beneath the array with a vertical resolution of approximately 15 m, comparable to that of conventional active surveys. This ice-thickness model will serve as a critical constraint for future high-resolution surface-wave tomography, enabling improved imaging of glacier structure and basal conditions at depth.