| 02 Jul 2026
Ice-core break-off as an opportunistic seismic source on the Northeast Greenland Ice Stream
Abstract. Ice-core drilling generates repeated mechanical disturbances within the ice column, raising the possibility that operational drilling signals could be used as opportunistic seismic sources. We test whether impulsive ice-core break-off events during the 2022 EastGRIP (Greenland) field season can be detected on surface-based, near-offset three-component geophones and used to estimate apparent P-wave velocity. A catalogue of 67 candidate core-break times was identified from drill-log load peaks, of which 37 had sufficient simultaneous near-offset seismic station coverage for assessment. Only three of these assessable events produced plausible impulsive arrivals. Automatic STA/LTA triggering was useful for identifying candidate windows, but did not reliably pick first arrivals. Where clear arrivals were identifiable above the background noise, manually picked arrivals gave apparent P-wave velocities consistent with firn/ice propagation, and showed a small across-flow faster-than-along-flow tendency, though this difference remains below the level of uncertainty required for robust anisotropy interpretation. These results show that ice-core break-off may provide a useful opportunistic seismic source, but only if future deployments include direct source-time measurement, quiet acquisition conditions, and receiver geometries designed for this purpose.
This manuscript explores an interesting idea: using ice-core break-off during deep drilling as an opportunistic downhole seismic source. The concept could be useful for future glaciological seismic-while-drilling experiments, and the authors appropriately acknowledge the low detection rate and substantial operational limitations. However, the present data do not yet demonstrate that the three selected impulsive signals originated from core break-off at the borehole bottom. More importantly, the acquisition geometry and uncertain source time do not support an independent P-wave velocity estimate or a meaningful directional comparison. I therefore recommend major revision.
Major comments
1. I am afraid that the receiver geometry cannot resolve travel-time moveout. The source depth is approximately 2.2–2.4 km, whereas receiver offsets are only 10–100 m. For a source at 2300 m depth, the travel-path difference between 10 and 100 m offset is only about 2 m, corresponding to approximately 0.5 ms at 3800 m/s. This is much smaller than the 2.5 ms sampling interval of the 400 Hz data.
Therefore, the array cannot independently constrain velocity from differential arrival times. The reported apparent velocities are controlled primarily by the assumed source time rather than by measurable moveout. This limitation should be explicitly quantified.
2. The tower-load maximum is only an indirect proxy for the actual fracture time, and the authors acknowledge that the true break may occur before, during, or after the load peak. The approximately 50 m/s difference between along-flow and across-flow groups corresponds to only several milliseconds of travel-time difference, which is likely smaller than the source-time and picking uncertainty. The directional comparison should therefore be removed or presented only as evidence that the current experiment is insufficient for anisotropy measurement. The calculated values should also be described as equivalent velocities conditional on the assumed source time, rather than independently measured P-wave velocities.
3. The identified impulses may also originate from the drill tower, winch, drill cable, core barrel, camp activity, or other near-surface mechanical sources. Because the array provides almost no resolvable moveout, a deep source cannot currently be distinguished from a surface operational source.
The authors should use the three-component data to provide additional evidence, such as: polarization and incidence angle;
waveform coherence and polarity across stations; amplitude variation with offset; comparison with surface-source and downhole-source hypotheses. Until this is demonstrated, the events should be called “candidate signals associated with core-break operations,” rather than confirmed core-break arrivals.
4. The expected arrival window was defined using velocities between 2000 and 6000 m/s, and signals were then manually selected within that window. Any random impulse inside the window will necessarily produce a physically plausible apparent velocity. The authors should apply the same detection procedure to randomly shifted times or control windows without core breaks and estimate the false-positive rate. Ideally, manual classifications should be performed blindly, without displaying the predicted arrival window.
5. Only three candidate detections and one non-detection are shown. For a Lessons Report, the unsuccessful cases are equally important. Please provide a table or summary figure for all assessable events, including: depth and load peak;
available stations; background-noise level; detection confidence; signal-to-noise ratio; number of coherent stations. The authors should test whether detection depends on load, source depth, station coverage, or operational noise.
6. I am also thinking that the strongest contribution of this dataset is not a velocity or anisotropy measurement, but the identification of experimental requirements for future seismic-while-drilling studies. The manuscript would be more convincing if it focused on: detection probability; or source-time uncertainty; or geometric resolution; or required drill instrumentation. A known firn–ice VP(z) model should also be used to predict theoretical travel times. Observations could then be reported as timing residuals relative to the load maximum, rather than converted directly into apparent velocities.
Minor comments:
1. Figure 3 should state which component is displayed. Three-component or rotated waveforms should be included for the three candidate detections.
2. Picks from the N, E, and Z components of the same station are not independent observations. Figure 4 should preferably show one arrival estimate per station with an uncertainty.
3. The 20–200 Hz filter requires clarification because 200 Hz is the Nyquist frequency for data sampled at 400 Hz. Please report the actual filter corners, order, and implementation.
4. Raw or less strongly filtered records should be provided to show that the identified impulses are not filter-induced ringing.
5. Correct “propogate” to “propagate” and “ambiguious” to “ambiguous.”