the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2026
Seismic Evidence for Increasing Unfrozen Pore Water Heterogeneity from Roadside to Coastal Arctic Tundra
Abstract. Permafrost contains liquid pore water even at subzero temperatures, and the partitioning between pore ice and unfrozen water governs its mechanical and hydrological properties. Resolving this partitioning at depth and across short lateral distances remains challenging because direct subsurface observations are spatially sparse. Here, we apply active-source multichannel analysis of surface waves (MASW) using a long-term distributed acoustic sensing (DAS) cable and conventional geophone arrays at five sites along a ~2 km transect in Utqiagvik, Alaska, that captures a transition from a disturbed roadside environment to the Arctic coast. Shear-wave velocity profiles are converted to seismically inferred unfrozen pore water content (Sw) using a load-bearing effective-medium framework and compared against a depth-dependent reference constructed from local borehole temperatures and laboratory freezing curves measured on permafrost cores. Departures from this reference are spatially organized. The DAS-derived departure metric increases toward the coastal end of the transect, while the geophone-derived profiles show greater variability but also identify the site closest to the coast (Tundra 4) as having the largest departure. At this site, the DAS-derived profile remains higher than the temperature reference through much of the deeper section. These results suggest that temperature and shallow laboratory measurements alone do not fully explain the observed permafrost structure, particularly beneath water-rich coastal tundra surfaces characterized by ponds and lakes. The framework introduced here provides a transferable approach for integrating seismic, thermal, and laboratory data to characterize spatially variable permafrost at depth in locations where the reference estimate process would be expected to deviate most from true subsurface conditions.
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Status: open (until 07 Oct 2026)
- RC1: 'Comment on egusphere-2026-4871', Anonymous Referee #1, 05 Sep 2026 reply
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RC2: 'Comment on egusphere-2026-4871', Anonymous Referee #2, 27 Sep 2026
reply
This manuscript presents an interesting integration of distributed acoustic sensing (DAS), conventional geophone-based MASW, borehole temperature data, laboratory soil-freezing characteristic curves, and rock-physics modeling to investigate spatial variability in Arctic permafrost near Utqiagvik, Alaska. The dataset is potentially valuable, and the seismic observations reveal notable spatial variations in Vs, particularly at Tundra 4. However, there are some substantial concerns regarding whether the available data adequately support the manuscript’s central interpretation of increasing unfrozen pore water heterogeneity from roadside to coastal tundra.
1. Lack of physical data supporting the extrapolation to depth
A fundamental concern is that the laboratory-derived freezing relationships are based on shallow samples from about the upper 1.5 m but are extrapolated to depths of 20-40 m. Although deeper temperature information is available, no corresponding deep physical measurements of lithology, grain-size distribution, porosity, salinity, ground-ice content, or unfrozen water content appear to be available to demonstrate that the shallow relationships remain applicable at depth. The manuscript itself acknowledges that unsampled lithological changes cannot be excluded. Consequently, the inferred deep Sw anomalies, particularly at Tundra 4, rely on an assumption that is not independently supported by physical subsurface data. Without such constraints, it is difficult to determine whether the deep seismic anomaly represents elevated unfrozen pore water or changes in other soil properties.
2. The seismic observations do not uniquely constrain unfrozen pore water content
The primary quantity constrained by the seismic measurements is Vs, whereas Sw is obtained through a model-dependent rock-physics transformation. The manuscript acknowledges that the Vs to Sw relationship is non-unique and depends on the selected rock-physics formulation, porosity, and other assumed parameters. The Supplement further demonstrates that alternative rock-physics models can produce substantially different Sw estimates. Therefore, the seismic data convincingly demonstrate variations in seismic velocity, but the available evidence does not uniquely establish that these variations represent differences in unfrozen pore water content.
3. Evidence for an “increasing” roadside-to-coastal trend
The central claim of increasing unfrozen pore water heterogeneity from the roadside toward the coast is not sufficiently supported by the presented results. The site-to-site metrics do not demonstrate a consistent monotonic increase, and Figure 6 explicitly shows that the geophone-derived anomalies do not exhibit a systematic coastward trend. Instead, the apparent trend is largely driven by the pronounced anomaly at Tundra 4. Therefore, the available data appear to support an anomalous coastal site rather than a general roadside-to-coastal increase in unfrozen pore water heterogeneity. Because this interpretation forms the basis of the manuscript title and one of its principal conclusions, this discrepancy represents a fundamental concern with the current framing of the study. Substantial additional evidence would be required to establish the proposed spatial trend.
4. Comprehensive uncertainty assessment for the inferred SW anomalies
The Supplementary Information provides a porosity-related sensitivity range for the inferred Sw, but this is not a formal uncertainty estimate. Consider additional uncertainties associated with dispersion-curve picking, Vs inversion, rock-physics parameters, porosity regularization, temperature measurements, and laboratory SFCC fitting. Since the central interpretation relies on departures between the seismic-derived Sw and the temperature- and laboratory-informed reference Sw-ref, it is important to determine whether the reported anomalies exceed the combined methodological uncertainty.
5. DAS–geophone differences cannot be uniquely attributed to small-scale spatial heterogeneity
A major interpretation of the manuscript relies on differences between nearby DAS- and geophone-derived profiles as evidence of subsurface heterogeneity. However, the two datasets were acquired in different years and used different sensing systems, acquisition geometries, frequency characteristics, and lateral separations of approximately 5–45 m. The manuscript itself acknowledges that spatially heterogeneous interannual changes cannot be excluded. The Roadside split-DAS experiment in the Supplement demonstrates that laterally separated measurements can produce different dispersion curves, but it does not quantitatively separate spatial variability from sensor, acquisition, or temporal effects at the sites where the largest anomalies occur. Therefore, the experimental design does not uniquely support interpreting the DAS–geophone discrepancy as a direct measure of increasing small-scale heterogeneity.
Citation: https://doi.org/10.5194/egusphere-2026-4871-RC2
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- 1
The paper presents methodological work conducted in Utqiaġvik on the relationship between seismic geophysics, temperature, and unfrozen water content in sediments, with the aim of directly connecting Vs with Sw. The difference between modeled Sw and the reference measurements demonstrates that temperature is not the only factor determining the amount of unfrozen water at the studied sites.
The study includes two years of data from five sites, combined with previously conducted laboratory measurements of unfrozen water content under controlled conditions.
The paper presents interesting research; however, there are several issues related to the methodology, state of the art, and presentation of the results that are important to address:
1. Introduction
The Introduction does not sufficiently cover previous studies of unfrozen water and the different factors influencing its variation, including not only temperature but also sediment characteristics. Previous laboratory and field studies on unfrozen water content should be included to better highlight the importance of the present work and provide readers with a clearer overview of the state of the art. This would also help clarify the different factors controlling unfrozen water content.
Factors other than temperature are discussed later in the Results and Discussion (for example, Section 3.4, lines 241–242, and Section 4.3), but it would be beneficial to introduce these factors earlier, especially considering the amount of experimental research available on the factors affecting unfrozen water content. I would recommend considering, for example:
Chuvilin, E., Sokolova, N., & Bukhanov, B. (2022). Changes in unfrozen water contents in warming permafrost soils. Geosciences, 12(6), 253.
2. Study goal and motivation
The Introduction lacks a clear definition of the goal of the study, and the overall motivation is not fully developed. It is understandable that unfrozen water content is difficult to determine without drilling and that geophysical methods could provide an alternative approach, but the broader motivation should be explained more clearly. Why exactly do we need to determine unfrozen water content? Why is knowledge of unfrozen water content important? Clarifying these points would help establish the significance of the study and its potential applications.
3. Methods
The Methods section could be substantially improved:
Instrument information, including the equipment name/model, precision, measurement limitations, and particularly any limitations associated with measurements in cold regions, should be provided in Section 2.2. There is methodological information currently presented in other sections (for example, Section 2.1 and Section 3.2, lines 170–177, among other places). It would be beneficial to review the manuscript and move the relevant methodological information into the Methods section.
The site description in Section 2.1 contains information about the geophysical methodology but does not provide enough information about the sites themselves. It would be valuable to include sediment composition based on core data from previous research, since sediment composition is directly related to unfrozen water content. Information on ice content would also be useful. In addition, the authors could describe any relevant permafrost or landscape features observed in the field. For example: What are the road conditions and road material? Is the road elevated? Is there ponded water near the measurement locations? Are there other site characteristics that may be important for interpreting unfrozen water content?
Figure 1a: It would be useful to include higher-resolution satellite imagery for each study site to show local landscape variations.
Figure 1b: From which borehole was this temperature profile obtained? Please identify and mark the borehole on the map.
Figure 1c: Are any of the water-content measurements associated with the boreholes shown in Figure 1a? If so, please include the borehole names/numbers on the map and corresponding labels on the graph so that the measurements can be connected spatially.
Figures 1d and 1e: Are these examples from a specific study site? If so, please indicate which site they represent.
In Section 2.2, it would be beneficial to clearly describe which sites were investigated in each year. From the current text, I assume that all five sites were investigated in both 2022 and 2024 using both methods, but this is not entirely clear. If not all five sites were measured in both years, it is important to specify which sites were measured in each year and distinguish them accordingly on the map.
Figure 2: Please increase the size of all text in the figure, including axis titles, legends, and numbers, to improve readability.
Figures 2a, 2c, and 2g: Are these examples from a specific site? If so, please indicate which site they represent.
4. Conclusions
Lines 335–337: The influence of salinity and sediment characteristics on unfrozen water content has been well studied experimentally. However, to the best of my knowledge, a direct and unique relationship between VsV_s and SwS_w cannot be established without accounting for other sediment properties and state variables. I suggest revising this statement to more clearly distinguish what is already established experimentally from what remains unresolved in terms of using seismic measurements to estimate unfrozen water content.
5. Organic matter
There is no discussion of the potential influence of organic matter on unfrozen water content, although organic-rich sediments are also present in Utqiaġvik. The potential role of organic matter should therefore be considered, particularly when discussing differences between measured and modeled unfrozen water content.
Overall, the manuscript presents interesting and potentially important research. However, I recommend revising the paper to strengthen the connection between the study goals, methodology, findings, and their broader significance. In particular, the manuscript would benefit from a clearer explanation of what controls unfrozen water content, why its estimation is important, and what the presented geophysical approach allows us to understand beyond existing methods.
Nevertheless, I find Figure 7 particularly interesting, and the amount of field, and analytical work conducted for this study is good. The dataset has strong potential, and addressing the points above would substantially improve the manuscript and clarify the significance of the findings.