the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Dec 2025
The role of structural heterogeneity in glacier ice deformation: Insights from Planpincieux Glacier
Abstract. Natural glacier ice is not a monophasic, isotropic material as commonly assumed in models based on Glen-Nye's flow law. It can contain crevasses, develop crystallographic preferred orientations, and include mixtures of debris and interstitial water in temperate glaciers. Understanding the influence of such structural heterogeneities on deformation is therefore essential for accurately modeling glacier dynamics. In this study, we investigate the multi-scale evolution of structural heterogeneities with depth in the Planpincieux Glacier (Italian Mont Blanc massif) and evaluate their respective influence on ice deformation using a borehole instrumented with an optical televiewer, a full-waveform sonic logger, a piezometer, and an inclinometer chain. Complementary GNSS and seismic data provide additional constraints on hydrological activity and surface motion. Optical and sonic logging reveal two main families of heterogeneities: open and closed crevasses in the upper 60 m, and debris-rich layers near the bedrock interface. Acoustic data show continuous but opposite trends in both P- and Stoneley-wave velocities with depth, interpreted as reflecting an increase in water content but a decrease in permeability. Tiltmeter measurements indicate that roughly one-third of the surface velocity is accommodated by internal deformation, with pronounced strain localization near the bedrock, particularly within debris-rich layers. These layers exhibit enhanced strain following hydrological drainage events, suggesting a coupling between mechanical heterogeneity, basal hydrology, and strain localization. The results highlight that glacier friction laws may be significantly influenced by such heterogeneities, including the effects of interstitial water and debris on local mechanical behavior.
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Status: open (until 19 Feb 2026)
- RC1: 'Comment on egusphere-2025-5714', Anonymous Referee #1, 03 Feb 2026 reply
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- 1
Review of “The role of structural heterogeneity in glacier ice deformation: Insights from Planpincieux Glacier”
This paper reports on an interesting dataset where a seismometer and a GPS station placed on the glacier surface (and two additional seismometers placed in the forefield of a glacier) were compared with observations from a borehole. The borehole observations include sonic measurements, caliper measurements of the borehole diameter, water pressure, and borehole inclinometer measurements to estimate the strain and strain rate of the glacier. In addition, there was also optical televiewer data that was used to assess the structure of the ice near the borehole and whether or not debris was included in the ice. They report that approximately 30% of the motion of the glacier results from internal deformation, and the remainder comes from slip at the ice-bed-interface. They use two different methods to estimate stress within the glacier near the borehole and the resulting strain rates they’ve measured from the inclinometer to estimate flow law parameters (A and n) for the ice. The measurements show a large degree of temporal and spatial heterogeneity in strain rates, especially near the ice-bed-interface. Also, the entire glacier motion balance varies in response to hydrologic forcing at the ice-bed interface. They advocate for the role that structural heterogeneity within the ice can play in altering glacier flow in ways that varies from just utilizing the bulk ice properties, which is likely the most important point of the paper.
I found the paper easy to follow and well written. The methods were clearly described, and the data set is very interesting. However, there are some issues I have with the results and how they were interpreted. Overall, I think the data is interesting and should be published, and that the main speculation in the discussion is largely accurate, but some of the main findings of the data do not make physical sense (e.g., water contents are too high, flow exponent is too high, the ice is flowing backwards at times) that need additional attention.
Major points ---
Error estimates and bounds need to be reported throughout the paper. In particular, average velocity estimates are given, but we have no sense of, for example, the one sigma error associated with them to know if their deviations from the predicted values and, in some ways, the way they are used to interpret things like relative changes and permeability are meaningful or not.
The estimates of the A and n parameters in table 1 are abnormal when they’re both allowed to be utilized as fitting parameters. For example, the best fit gives a value of n= 7.76, and the A parameter is raised to the -41st power. It is unclear what deformation mechanism within the ice could give rise to an exponent of n this high. For example, dislocation creep commonly results an exponent of 4, while grain boundary sliding produces 1.8, and diffusion mechanisms results in an exponent of 1. None of these mechanisms must be active within glacier or something completely different is occurring within the ice to result in an exponent of n equals 7.76. If n is truly this high, it is a significant deviation from the findings in most other studies. It requires more attention to address what is causing these values to deviate so much from the known deformation mechanisms.
In the situation where the n is fixed at 3, somewhat more realistic values for A are resolved; however, what is the justification for fixing n equals 3, especially when modern results have shown that for tempered ice, which I assume this entire glacier is temperate, the value of n=1 is more appropriate, especially when water contents are in the range of 1 to 2%, and here they are reported to be much higher than that value.
The use of equation 9, which is extrapolated from Duval (1977) to estimate the water content, is problematic. Duval never found water contents greater than 1%, and so it’s unclear if his findings for the relationship between the fluidity parameter and water content should be extended outside of his bounds with the same functional relationship that occurs inside the bounds. Furthermore, Duval assumed n was equal to three in his relationship, and so it’s unclear if his relationship between fluidity and water content is even accurate in the first place if n does not equal 3. Furthermore, work by Schohn et al., 2025 showed that the fluidity parameter does not vary with water content in temperate ice above 0.5%.
A water content within the ice of 8% would be extremely high. For example, Vallon et al 1976 never measured anything in excess of 2% and the work by Haseloff et al., et al, 2019, in which they looked at water flow through tempered ice; the only mechanism to produce water contents above 2% would be to have unrealistically low permeability within the ice. Now that there are better estimates of ice permeability by Fowler et al, 2023 and 2025, we know that that range is likely outside of what is physically possible at least for interstitial ice. So likely the only way to get water contents of ~8% is if it is held within crevasses or water pockets, but then it is not appropriate to factor it within the fluidity parameter, as that is only sensitive to interstitial water content.
At several places within the paper; for example, in the lines near 330, strain rate units are incorrectly reported as [m/yr] instead of just [/yr]. This may occur at other points within the paper too and so needs to be checked carefully.
In line 340 and in figure 11, I find it quite surprising that there are huge negative strain rates (-40 /yr). In some other instances, people have seen negative strain rates, but it has been attributed to elastic recovery once the stress is removed and in no way are they of the magnitude of the strain rates measured here. A -40 /yr strain rate would indicate, according to the time scale over which it occurred in figure 11 it is viscous movement, that the ice is literally flowing backwards for some period of time. What physical mechanism could produce negative strain rates for such a sustained period of time? I’m left to wonder if these values are accurate and if not, what does that mean for the rest of the measurements and if so, what physical mechanism could possibly do such a thing?
There are likely complexities with the debris, water, ice mixture’s effect on the seismic velocity that are not accounted for, but as the authors state, little is known about them currently. However, those “unknowns” about the multi-phase mixture are likely the cause of some of the nonphysical results reported here. This could be explored further.
Were there measurements of the ice temperature throughout the to confirm it was a temperate glacier?
Minor comments ----------------
Line 10: use a different word than “reflecting” which could be misinterpret in a seismic reflection here.
L26: there is a “?” in the citation
L80: clarify what “red” means after 0.1-98 Hz
L82: clarify what “blue” means after 4.5-98 Hz
Figure 1a. Need label on the x and y axis
Figure 1b. What is the +5.08e6, should that be a multiplication symbol instead of addition
Section 2.2.1 restate what the total thickness of the glacier is in the first part of the paragraph
Section 2.2.2.b before going into how to estimate the “semblance” provide a line of context at the beginning of the paragraph of what it is used for
Line 142. The following text’s meaning is unclear, clarify “window wt, the correlation between M measured signals fm recorded”
Line 145:” enables detection the arrival of different” a word is missing in here. Rephrase
L159. State how many days after drilling 17th of September 2024 was
Figure 3c. Consider dashing the closed crevasse. Readers not familiar looking at televiewer data may not know what to look for in this image
Figure 4a. label time on the top axis
Figure 4c. label depth on the y axis
Figure 4b. Ther there is another high value semblance value around 2000 m/s at time 0.0005-0.00075. What is this result as it is at least as strong as the other Vp and Vst waves. If this is a false positive how can the authors be sure the other values are correct.
L 226 method still yields a measurement.--> method still yields an accurate measurement.
L249-250: How much? and cite the source
Fig 5c, 6c, & 7c. What are the vertical dashed black lines
Figure 8. No label on vertical axis for both a and b
L292: series and set to 17 September 2024 series as 17 September 2024
L293 weaker lower
Figure 9b. What are Pe and Pg. I assume this is the flotation fraction or something similar? Clarify
L312: what is the “effective stress tau_e”? Is this the normal effective stress (overburden minus water pressure) or is this the deviatoric stress or the octahedral stress or something else? Why is it in included and how is it used.
L316. The A value here assumes n=3 which your results show it is not, so how can this be extrapolated
Figure 11. The right axis that simply has +10 does not make sense. I assume this is essentially a scale bar or something but either way it’s confusing the way its included.
L379: there is a ? in the references
L392 “velocity or other heterogeneity counter this effect” this line of text is unclear. Clarify
L418. The presence of debris will lead to a surface energy condition that will create premelt films thus effecting the seismic velocity
Fig A1. Add more labels to the axis.
Fig D1 d=60.98 m has the word angel off set and on top of the neighboring figure. Check that this plot is correct as the values surpass 90 degrees.