the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: open (until 05 Sep 2026)
- RC1: 'Comment on egusphere-2026-3762', Anonymous Referee #1, 21 Jul 2026 reply
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RC2: 'Comment on egusphere-2026-3762', Anonymous Referee #2, 02 Sep 2026
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This manuscript tests whether ice-core break-off events can serve as an opportunistic seismic source during deep ice-core drilling, using drill-log load peaks as source-time proxies and near-offset geophones from the 2022 EastGRIP field season. Of 67 candidate events, only three yielded interpretable P-wave arrivals, giving velocity estimates broadly consistent with expected firn/ice values. The authors frame this honestly as a feasibility/lessons-learned study rather than a full success, and use it to outline what future deployments would need to make the method robust.
I found this a valuable and refreshingly transparent contribution. My main concerns are around confidence in the three detected events given the low detection rate and unconstrained source location (and time), the presentation of per-component picks in Figure 4, and the weight given to the anisotropy result relative to its stated uncertainty, detailed below, along with some minor comments.
Major comments:
- The detection rate of the 67 candidate break-off events is very low, which makes it hard to judge how confident the authors can be that the arrivals identified in the three usable core breaks are indeed genuine core-break signals rather than something else. This is especially true since the velocity estimates rely on an assumed source location, and that location is not independently constrained. Would it be possible to independently constrain the source location using these arrivals, to give more confidence in the origin of the signal? I suspect this would only be feasible where coverage is sufficient across several detections, especially including the larger-offset stations — so I'm not sure it's practical here, but it seems worth considering.
- I don't fully understand why Figure 4 presents the arrivals from different components as separate measurements. The fact that they vary so much mainly seems to convey that the picks are highly uncertain at each station, rather than adding independent information. I would suggest showing one pick per station instead, colour-coded as needed.
- I think the anisotropy aspect is worth mentioning, but I would be more cautious about presenting it as a result, including in the abstract As it is mentioned more than once that the difference between across and along flow apparent velocities lies well below the actual threshold of certainty. More generally, I would shift the paper's emphasis toward the lessons learned for improving the experimental setup, so that future work could produce a more extensive and usable dataset from core-break events.
Minor comments:
- Line 18: I would change "would" to "could," since you cannot be certain this approach would provide information on all of the subjects mentioned.
- Line 68: The reference to Figure 2 here is a bit misleading, as the figure only shows a limited time window of candidate core breaks.
- Figure 2: The caption could state more clearly that this is an example illustrating how candidates are selected.
- Figure 3: Please clarify in the caption whether the amplitude of each trace is normalized individually or shown on a common scale. Also, the pick lines are quite thick, which makes it somewhat hard to see the underlying waveform and first arrival — perhaps make them thinner and/or semi-transparent so the waveform remains visible.
- Figure 4: As noted above, I would avoid showing individual components separately and instead present one velocity estimate per station.
- Line 96: Space missing after (Figure 3)
Citation: https://doi.org/10.5194/egusphere-2026-3762-RC2
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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.”