the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Aug 2023
A Simulation Approach to Characterizing Sub-Glacial Hydrology
Abstract. The structure and distribution of sub-glacial water directly influences Antarctic ice mass loss by reducing basal shear stress and enhancing grounding line retreat. A common technique for detecting sub-glacial water involves analyzing the spatial variation in reflectivity from an airborne ice penetrating radar (IPR) survey. Basic IPR analysis exploits the high dielectric contrast between water and most other substrate materials, where a reflectivity increase ≥ 15 dB is frequently correlated with the presence of sub-glacial water. There are surprisingly few additional tools to further characterize the size, shape, or extent of hydrological systems beneath large ice masses.
We adapted an existing radar backscattering simulator to model IPR reflections from sub-glacial water structures using the University of Texas Institute for Geophysics (UTIG) Multifrequency Airborne Radar Sounder with Full-phase Assessment (MARFA) instrument. Our series of hypothetical simulations modeled water structures from 5 m to 50 m wide, surrounded by bed materials of varying roughness. We compared the relative reflectivity from rounded Röthlisberger channels and specular flat canals, showing both types of channels exhibit a positive correlation between size and reflectivity. Large (> 20 m), flat canals can increase reflectivity by more than 20 dB, while equivalent Röthlisberger channels show only modest reflectivity gains of 8−13 dB. Changes in substrate roughness may also alter observed reflectivity by 3−6 dB. All of these results indicate that a sophisticated approach to IPR interpretation can be useful in constraining the size and shape of sub-glacial water, however a highly nuanced treatment of the geometric context is necessary.
Finally, we compared simulated outputs to actual reflectivity from a single IPR flight line collected over Thwaites Glacier in 2022. The flight line crosses a previously proposed Röthlisberger channel route, with an obvious bright bed reflection in the radargram. Through multiple simulations, we demonstrated the important role that topography and water geometry can play in observed IPR reflectivity. We ultimately conclude the bright reflector from our IPR flight line is more likely a broad area of wide distributed water, such as a series of flat canals or sub-glacial lake, instead of a Röthlisberger channel. The approach outlined here has broad applicability for studying the basal environment of large glaciers, and can aid in constraining the geometry and extent of sub-glacial hydrologic structures.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Status: closed
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RC1: 'Comment on egusphere-2023-1685', Anonymous Referee #1, 01 Sep 2023
The manuscript presents an interesting combination of electromagnetic modeling and radar data analysis across a modeled drainage pathway for Thwaites Glacier. The results have the potential to make an important contribution to our understanding of the glaciers subglacial hydrology, but the analysis, as written, has several issues that would need to be addressed first to reconcile the results, methods, and claims in the paper.
Major Issues:
The range of material properties for the bed and water used in the study seem too narrow given the literature cited in the paper. For example, Peters 2005 included reflectivity differences between water (including groundwater) and frozen bedrock that differ by 26 dB without invoking any change in bed roughness or geometry. If you look at Christiansen 2016 and Tulaczyk and Foley 2020 (https://doi.org/10.5194/tc-14-4495-2020) these values are also in the 25 - 27 dB range. If the manuscript seeks to diagnose the subglacial water configuration by excluding other hypotheses, then the complex permittivity for both water and the bed explored should span the full range of this literature (and reproduce that range of reflectivities).
Similarly, the range of bed roughness considered in the study is also small compared to the literature cited by the paper. Again, Peter 2005 shows roughness-based reduction in reflectivities that are as high as 20 dB. For the reasons described above, I’d expect this paper to explore roughesses losses at that scale as well.
The authors present a simulation that is focusable (and focused) which should allow them to probe the specularity of bed echoes for each of the hypothesized configurations. Since, as the paper mentions, this was the key observable in classifying the downstream water system of Thwaites as concentrated rather than distributed, it would seem incumbent on the authors to address whether their interpretations of the (inherently more ambiguous) reflectivity signal would also explain that larger catchment-wide specularity signature.
Line 180: The authors state that they “confine” themselves to Röthlisberger channels and flat canals for this study. That is a fine choice to support the claim (if it survives addressing the issues raised above) that the bright spot (and downstream water network) is likely not a canonical Röthlisberger channel. However, in order to claim (as the authors do in their abstract and conclusion) that the wanted body is “distributed” (which has a specific subglacial hydrological meaning and implication for modeling) they would need to also address other “concentrated” water geometries and exclude them as well. These include Nye Channels, Creytes & Schoof water sheets (https://doi.org/10.1029/2008JF001215) an other concentrated/efficient water configurations (https://doi.org/10.1098/rspa.2014.0907). Otherwise the authors should limit themselves to falsifying the hypothesis that the bright echo on THW2/UBH0c/X243a is a canonical Röthlisberger channel and remove language like “We ultimately conclude the bright reflector from our IPR flight line is more likely a broad area
20 of wide distributed water, such as a series of flat canals or sub-glacial lake” which cannot be supported by a study that does not consider other “concentrated " water geometries.
Minor Issues:
Abstract Line 1: Depending on its configuration water can either enhance or reduce sliding and/or retreat.
Abstract Line 3: Given the recent paper by Schlegel et al. ( https://doi.org/10.1017/aog.2023.2) you may want to consider the use of IPR here.
Table 1: 1.71 MW seems like an extremely high value. It’s unusual to report a post-processing number for transmit power and presenting it in this way could confuse readers significantly.
Citation: https://doi.org/10.5194/egusphere-2023-1685-RC1 -
AC1: 'Reply on RC1', Christopher Pierce, 22 Nov 2023
We appreciate this reviewer's insightful comments on our manuscript "A Simulation Approach to Characterizing Sub-Glacial Hydrology". These will be very helpful in improving the final product.
In response to the Major Issues raised by the reviewer, we submit the following responses:
- Regarding the range of material properties used in the study:
- As the reviewer correctly points out, we did not explore many possible combinations of bed materials. This was intentional, as the scope of this work was to introduce the conceptual framework for using such a simulation technique in the study of sub-glacial hydrology, then demonstrate its potential usefulness in constraining hypotheses for a single feature seen in a recent IPR survey.
- The emphasis of the work and most important conclusions involve the water geometries considered in the study. Altering the substrate dielectric constant will merely scale power. We acknowledge that definitive diagnosis of the subglacial environment beneath Thwaites may require additional assessment with a broader combination of materials. The nature of this technique may never eliminate all possible combinations of materials and geometry. However, it can be a valuable tool for narrowing the range of likely sub-glacial configurations.
- We propose revisions to acknowledge the range of unexplored material properties more explicitly. This will include stating that we have not completely excluded all other possible hypotheses. Instead, we have found at least one possible match between the observed reflectivity and a hypothetical water structure.
- Regarding the range of bed roughness explored:
- Peters 2005 presented a calculation for theoretical scattering loss due to sub-glacial roughness. In their calculations, small scale RMS roughness up to sigma_bed / lambda = 0.5 was considered, based on observations of RMS roughness calculated from IPR data over Ice Stream C. As the reviewer points out, Peters calculated a scattering loss of 21 dB if sigma_bed / lambda = 0.5.
- However, the Peters paper may have ignored the length scales over which roughness is measured, which is an important consideration. The theoretical scattering loss calculated in Peters assumes correlation length is very small compared to the first Fresnel zone (~100m in our case). However, the range of actual IPR derived roughness observed in Peters beneath Ice Stream C were compiled over a 1km distance, obviously violating this assumption. Peters appears to assume the range of RMS roughness seen with this 1km correlation length translates directly to their calculated theoretical roughness / scattering curve. This logic will result in incorrect understanding of the relationship between scattering and roughness.
- Our method attempts to improve upon this by considering roughness at a meaningful length scale for scattering (correlation length <<100m). We explored a range of roughness up to sigma_bed / lambda = 0.2, with correlation length as small as possible relative to the 1st Fresnel zone. Actual roughness at this length scale is not directly observable by conventional IPR, and therefore the true range of realistic values is unknown. Previous studies such as Peters 2005 or Bingham and Siegert 2009 measure roughness over longer distances, which may be relevant for modeling applications but is insufficient for our purposes. Our range may be smaller than the theoretical range explored in Peters 2005, however we attempt to justify our choice through direct observations of glacier bed roughness matched to our smallest possible correlation length.
- Further, we note that in our real-world IPR simulations, roughness on long length scales (>~102m) is captured explicitly through our simulation methods. We build a digital elevation model (DEM) derived from IPR ice geometry observations with approximately 5-8m spacing.
- An entire study could be dedicated to sub-glacial roughness and the impact on radar returns. Ideally this will incorporate a range of RMS roughness and correlation lengths on radar scattering. However, such a study is well beyond the scope of this paper.
- We acknowledge that more work should be performed in this realm, which we are willing state explicitly in the manuscript. We will alter language in the introduction, conclusion, and abstract to be clear about scope of the investigation, so it is clear why a more detailed roughness analysis is left to future work.
- Regarding the omission of simulated specularity content:
- The reviewer is correct in their observation that a focused simulation product could, in theory, be used to generate and analyze specularity in the same manner as we have demonstrated for reflectivity. Unfortunately, we acknowledge this as a current limitation of the methodology as presented. Schroeder 2015 suggested aperture lengths of up to 2km to generate specularity content for IPR data from Thwaites Glacier. This is reasonable when focusing real IPR data products, however it is unrealistic when focusing our simulated radargrams.
- As we discuss in the Methodology section, the simulator is bound by a simulation radius “R”, which limits the domain of facets from which to consider transmitted and reflected waves. Increasing R exponentially raises computational cost. Conversely, R must be greater than the aperture length to generate a valid focused product. Even with access to high performance computing resources, simulations with R>2km are unrealistic.
- Due to the above, analysis was necessarily limited to reflectivity. The study seeks to remove some of the ambiguity the reviewer expresses around interpreting this metric. We further limit our interpretation of the reflectivity signal to the transect in question, as a proof of concept for the simulation method. Any attempt to extrapolate our findings to catchment-wide conclusions based on a single transect would be erroneous at this time. Although well outside the scope of this study, it is absolutely an area of future interest.
- We suggest a revision to the discussion section, addressing the above concern and why the study is currently confined to reflectivity alone.
- Regarding limiting the study's scope to Röthlisberger channels and flat canals, coupled with conclusion language such as “We ultimately conclude the bright reflector from our IPR flight line is more likely a broad area of wide distributed water, such as a series of flat canals or sub-glacial lake”
- We concur with the reviewer’s assessment on this point. Similar to this reviewer’s feedback regarding material properties, we have not explored and eliminated all possible geometries. We can be more precise with language regarding what hypotheses our work has excluded, what hypotheses may be compatible with the data, and what remains unexplored.
- As such, our language concluding the structure of the water may be too strongly worded. We will soften this language as the reviewer suggests, stating that the bright echo a) is not consistent with a Röthlisberger channel, b) is consistent with a broad specular water body as we have suggested, and c) other configurations (such as Nye channels, etc) and bed materials (tills, frozen bedrock, clays) remain unexplored. Therefore, it is possible other combinations also match the reflectivity profile in THW2/UBH0c/X243a.
In response to the Minor Issues raised, we submit the following responses:
- Abstract Line 1: Depending on its configuration water can either enhance or reduce sliding and/or retreat.
- Thank you. We agree with this suggested change.
- Abstract Line 3: Given the recent paper by Schlegel et al. ( https://doi.org/10.1017/aog.2023.2) you may want to consider the use of IPR here.
- We thank the reviewer for bringing this to our attention. We will change our nomenclature from IPR to airborne RES, consistent with Schlegel et al.
- Table 1: 1.71 MW seems like an extremely high value. It’s unusual to report a post-processing number for transmit power and presenting it in this way could confuse readers significantly.
- The reviewer’s concern on this point is reasonable. We have coherently pre-summed the radar power for simulation expediency. Therefore, the power in our simulations is significantly higher than MARFA’s 8kW native power. We want to be clear this is NOT the same as post-processing, which may sum power incoherently. Summation happens during data processing after an actual survey.
- For additional clarity, we suggest putting native MARFA values in parentheses next to the simulator parameters. In the table description, we can state that native instrument values are in parentheses where they deviate from the simulation parameters, and the reason for this difference is because the simulator approximates pre-summed power and PRF for computational efficiency.
Citation: https://doi.org/10.5194/egusphere-2023-1685-AC1 - Regarding the range of material properties used in the study:
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AC1: 'Reply on RC1', Christopher Pierce, 22 Nov 2023
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RC2: 'Comment on egusphere-2023-1685', Anonymous Referee #2, 03 Nov 2023
The manuscript offers a compelling blend of electromagnetic modelling and radar data analysis in the context of Thwaites Glacier's subglacial drainage pathway. While the results hold promise for enhancing our knowledge of glacier subglacial hydrology, however it needs to be improved considering the suggestions below.
Some of the Major issues to be addressed, they are.
- If the manuscript aims to identify the subglacial water arrangement by eliminating alternative hypotheses, it is imperative that the comprehensive permittivity values for both water and the glacier bed cover the entire spectrum found in existing literature, thus reproducing the full range of reflectivity associated with these parameters.
- The paper primarily focused on the regional scale of Thwaites' downstream water system. However, it is imperative to acknowledge the broader applicability of the model for a more extensive catchment when considering its practical utility.
Minor comments:
- Line 25: Better to add more recent citations.
- Line 32: Missing citations
- In the introduction section where the objectives are outlined, it is important to include the potential applications of the model to other catchments, thereby emphasising its versatility beyond the specific case of Thwaites Glacier.
- Providing a dedicated "Study Area" section, rather than including it in the introduction, would offer a more comprehensive understanding of the research area.
- Additionally, separating the "Results" and "Discussion" sections would enhance the clarity and structure of the paper. It would improve the presentation if all figures and tables were enclosed by borders for a more organized and visually clear layout.
- In the conclusion section, it would be beneficial to provide an overview of future work, offering the potential research directions and developments to be pursued.
-
AC2: 'Reply on RC2', Christopher Pierce, 22 Nov 2023
We would like to thank this reviewer for their comments on our manuscript "A Simulation Approach to Sub-Glacial Hydrology". Their insight will be very helpful in improving the final product.
In response to the reviewer's "Major Issues", we submit the following responses:
- Regarding the range of permittivity values for the bed material used in the study:
- As the reviewer correctly points out, we did not explore many possible combinations of bed materials. This was intentional, as the scope of this work was to introduce the conceptual framework for using such a simulation technique in the study of sub-glacial hydrology, then demonstrate its potential usefulness in constraining hypotheses for a single feature seen in a recent IPR survey.
- The emphasis of the work and most important conclusions involve the water geometries considered in the study. Altering the substrate dielectric constant will merely scale power. We acknowledge that definitive diagnosis of the subglacial environment beneath Thwaites may require additional assessment with a broader combination of materials. The nature of this technique may never eliminate all possible combinations of materials and geometry. However, it can be a valuable tool for narrowing the range of likely sub-glacial configurations.
- We propose revisions to acknowledge the range of unexplored material properties more explicitly. This will include stating that we have not completely excluded all other possible hypotheses. Instead, we have found at least one possible match between the observed reflectivity and a hypothetical water structure.
- Regarding the broader applicability of the model beyond the Thwaites water system:
- We appreciate and agree with the reviewer on this point. The choice of the Thwaites water system was a convenient example of how this method can be used, but of course the method could be applied more broadly to examine other regions and hypotheses. We propose additional language in the abstract, introduction and conclusions discussing the broader applicability of the technique, and also clarify that our Thwaites use case is just one example.
Regarding the minor comments raised by this reviewer:
- Line 25: Better to add more recent citations.
- We agree to add updated citations for the relationship between hydrology, shear stress, and ice rheology as suggested.
- Line 32: Missing citations
- We agree to add citations establishing IPR as a technique in the study of hydrology, as the reviewer suggests.
- In the introduction section where the objectives are outlined, it is important to include the potential applications of the model to other catchments, thereby emphasising its versatility beyond the specific case of Thwaites Glacier.
- This comment appears similar to the reviewer’s second concern under “Major Issues”. We believe our proposed resolution addresses both points.
- On providing a dedicated "Study Area" section:
- We appreciate this comment and agree to create a dedicated “Study Area” section to describe the target region of Thwaites glacier.
- On separating "Results" and "Discussion" to improve clarity:
- We will address the reviewer’s concern by separating the Results and Discussion for clarity.
- On adding table and figure borders:
- We acknowledge that authors and readers have differing stylistic preferences. However, we believe our figures are well aligned with the Cryosphere style guidance and prefer to keep them as is.
- With respect to adding a discussion of future research directions in the Conclusions:
- We agree to add language as suggested by the reviewer.
Citation: https://doi.org/10.5194/egusphere-2023-1685-AC2 - Regarding the range of permittivity values for the bed material used in the study:
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1685', Anonymous Referee #1, 01 Sep 2023
The manuscript presents an interesting combination of electromagnetic modeling and radar data analysis across a modeled drainage pathway for Thwaites Glacier. The results have the potential to make an important contribution to our understanding of the glaciers subglacial hydrology, but the analysis, as written, has several issues that would need to be addressed first to reconcile the results, methods, and claims in the paper.
Major Issues:
The range of material properties for the bed and water used in the study seem too narrow given the literature cited in the paper. For example, Peters 2005 included reflectivity differences between water (including groundwater) and frozen bedrock that differ by 26 dB without invoking any change in bed roughness or geometry. If you look at Christiansen 2016 and Tulaczyk and Foley 2020 (https://doi.org/10.5194/tc-14-4495-2020) these values are also in the 25 - 27 dB range. If the manuscript seeks to diagnose the subglacial water configuration by excluding other hypotheses, then the complex permittivity for both water and the bed explored should span the full range of this literature (and reproduce that range of reflectivities).
Similarly, the range of bed roughness considered in the study is also small compared to the literature cited by the paper. Again, Peter 2005 shows roughness-based reduction in reflectivities that are as high as 20 dB. For the reasons described above, I’d expect this paper to explore roughesses losses at that scale as well.
The authors present a simulation that is focusable (and focused) which should allow them to probe the specularity of bed echoes for each of the hypothesized configurations. Since, as the paper mentions, this was the key observable in classifying the downstream water system of Thwaites as concentrated rather than distributed, it would seem incumbent on the authors to address whether their interpretations of the (inherently more ambiguous) reflectivity signal would also explain that larger catchment-wide specularity signature.
Line 180: The authors state that they “confine” themselves to Röthlisberger channels and flat canals for this study. That is a fine choice to support the claim (if it survives addressing the issues raised above) that the bright spot (and downstream water network) is likely not a canonical Röthlisberger channel. However, in order to claim (as the authors do in their abstract and conclusion) that the wanted body is “distributed” (which has a specific subglacial hydrological meaning and implication for modeling) they would need to also address other “concentrated” water geometries and exclude them as well. These include Nye Channels, Creytes & Schoof water sheets (https://doi.org/10.1029/2008JF001215) an other concentrated/efficient water configurations (https://doi.org/10.1098/rspa.2014.0907). Otherwise the authors should limit themselves to falsifying the hypothesis that the bright echo on THW2/UBH0c/X243a is a canonical Röthlisberger channel and remove language like “We ultimately conclude the bright reflector from our IPR flight line is more likely a broad area
20 of wide distributed water, such as a series of flat canals or sub-glacial lake” which cannot be supported by a study that does not consider other “concentrated " water geometries.
Minor Issues:
Abstract Line 1: Depending on its configuration water can either enhance or reduce sliding and/or retreat.
Abstract Line 3: Given the recent paper by Schlegel et al. ( https://doi.org/10.1017/aog.2023.2) you may want to consider the use of IPR here.
Table 1: 1.71 MW seems like an extremely high value. It’s unusual to report a post-processing number for transmit power and presenting it in this way could confuse readers significantly.
Citation: https://doi.org/10.5194/egusphere-2023-1685-RC1 -
AC1: 'Reply on RC1', Christopher Pierce, 22 Nov 2023
We appreciate this reviewer's insightful comments on our manuscript "A Simulation Approach to Characterizing Sub-Glacial Hydrology". These will be very helpful in improving the final product.
In response to the Major Issues raised by the reviewer, we submit the following responses:
- Regarding the range of material properties used in the study:
- As the reviewer correctly points out, we did not explore many possible combinations of bed materials. This was intentional, as the scope of this work was to introduce the conceptual framework for using such a simulation technique in the study of sub-glacial hydrology, then demonstrate its potential usefulness in constraining hypotheses for a single feature seen in a recent IPR survey.
- The emphasis of the work and most important conclusions involve the water geometries considered in the study. Altering the substrate dielectric constant will merely scale power. We acknowledge that definitive diagnosis of the subglacial environment beneath Thwaites may require additional assessment with a broader combination of materials. The nature of this technique may never eliminate all possible combinations of materials and geometry. However, it can be a valuable tool for narrowing the range of likely sub-glacial configurations.
- We propose revisions to acknowledge the range of unexplored material properties more explicitly. This will include stating that we have not completely excluded all other possible hypotheses. Instead, we have found at least one possible match between the observed reflectivity and a hypothetical water structure.
- Regarding the range of bed roughness explored:
- Peters 2005 presented a calculation for theoretical scattering loss due to sub-glacial roughness. In their calculations, small scale RMS roughness up to sigma_bed / lambda = 0.5 was considered, based on observations of RMS roughness calculated from IPR data over Ice Stream C. As the reviewer points out, Peters calculated a scattering loss of 21 dB if sigma_bed / lambda = 0.5.
- However, the Peters paper may have ignored the length scales over which roughness is measured, which is an important consideration. The theoretical scattering loss calculated in Peters assumes correlation length is very small compared to the first Fresnel zone (~100m in our case). However, the range of actual IPR derived roughness observed in Peters beneath Ice Stream C were compiled over a 1km distance, obviously violating this assumption. Peters appears to assume the range of RMS roughness seen with this 1km correlation length translates directly to their calculated theoretical roughness / scattering curve. This logic will result in incorrect understanding of the relationship between scattering and roughness.
- Our method attempts to improve upon this by considering roughness at a meaningful length scale for scattering (correlation length <<100m). We explored a range of roughness up to sigma_bed / lambda = 0.2, with correlation length as small as possible relative to the 1st Fresnel zone. Actual roughness at this length scale is not directly observable by conventional IPR, and therefore the true range of realistic values is unknown. Previous studies such as Peters 2005 or Bingham and Siegert 2009 measure roughness over longer distances, which may be relevant for modeling applications but is insufficient for our purposes. Our range may be smaller than the theoretical range explored in Peters 2005, however we attempt to justify our choice through direct observations of glacier bed roughness matched to our smallest possible correlation length.
- Further, we note that in our real-world IPR simulations, roughness on long length scales (>~102m) is captured explicitly through our simulation methods. We build a digital elevation model (DEM) derived from IPR ice geometry observations with approximately 5-8m spacing.
- An entire study could be dedicated to sub-glacial roughness and the impact on radar returns. Ideally this will incorporate a range of RMS roughness and correlation lengths on radar scattering. However, such a study is well beyond the scope of this paper.
- We acknowledge that more work should be performed in this realm, which we are willing state explicitly in the manuscript. We will alter language in the introduction, conclusion, and abstract to be clear about scope of the investigation, so it is clear why a more detailed roughness analysis is left to future work.
- Regarding the omission of simulated specularity content:
- The reviewer is correct in their observation that a focused simulation product could, in theory, be used to generate and analyze specularity in the same manner as we have demonstrated for reflectivity. Unfortunately, we acknowledge this as a current limitation of the methodology as presented. Schroeder 2015 suggested aperture lengths of up to 2km to generate specularity content for IPR data from Thwaites Glacier. This is reasonable when focusing real IPR data products, however it is unrealistic when focusing our simulated radargrams.
- As we discuss in the Methodology section, the simulator is bound by a simulation radius “R”, which limits the domain of facets from which to consider transmitted and reflected waves. Increasing R exponentially raises computational cost. Conversely, R must be greater than the aperture length to generate a valid focused product. Even with access to high performance computing resources, simulations with R>2km are unrealistic.
- Due to the above, analysis was necessarily limited to reflectivity. The study seeks to remove some of the ambiguity the reviewer expresses around interpreting this metric. We further limit our interpretation of the reflectivity signal to the transect in question, as a proof of concept for the simulation method. Any attempt to extrapolate our findings to catchment-wide conclusions based on a single transect would be erroneous at this time. Although well outside the scope of this study, it is absolutely an area of future interest.
- We suggest a revision to the discussion section, addressing the above concern and why the study is currently confined to reflectivity alone.
- Regarding limiting the study's scope to Röthlisberger channels and flat canals, coupled with conclusion language such as “We ultimately conclude the bright reflector from our IPR flight line is more likely a broad area of wide distributed water, such as a series of flat canals or sub-glacial lake”
- We concur with the reviewer’s assessment on this point. Similar to this reviewer’s feedback regarding material properties, we have not explored and eliminated all possible geometries. We can be more precise with language regarding what hypotheses our work has excluded, what hypotheses may be compatible with the data, and what remains unexplored.
- As such, our language concluding the structure of the water may be too strongly worded. We will soften this language as the reviewer suggests, stating that the bright echo a) is not consistent with a Röthlisberger channel, b) is consistent with a broad specular water body as we have suggested, and c) other configurations (such as Nye channels, etc) and bed materials (tills, frozen bedrock, clays) remain unexplored. Therefore, it is possible other combinations also match the reflectivity profile in THW2/UBH0c/X243a.
In response to the Minor Issues raised, we submit the following responses:
- Abstract Line 1: Depending on its configuration water can either enhance or reduce sliding and/or retreat.
- Thank you. We agree with this suggested change.
- Abstract Line 3: Given the recent paper by Schlegel et al. ( https://doi.org/10.1017/aog.2023.2) you may want to consider the use of IPR here.
- We thank the reviewer for bringing this to our attention. We will change our nomenclature from IPR to airborne RES, consistent with Schlegel et al.
- Table 1: 1.71 MW seems like an extremely high value. It’s unusual to report a post-processing number for transmit power and presenting it in this way could confuse readers significantly.
- The reviewer’s concern on this point is reasonable. We have coherently pre-summed the radar power for simulation expediency. Therefore, the power in our simulations is significantly higher than MARFA’s 8kW native power. We want to be clear this is NOT the same as post-processing, which may sum power incoherently. Summation happens during data processing after an actual survey.
- For additional clarity, we suggest putting native MARFA values in parentheses next to the simulator parameters. In the table description, we can state that native instrument values are in parentheses where they deviate from the simulation parameters, and the reason for this difference is because the simulator approximates pre-summed power and PRF for computational efficiency.
Citation: https://doi.org/10.5194/egusphere-2023-1685-AC1 - Regarding the range of material properties used in the study:
-
AC1: 'Reply on RC1', Christopher Pierce, 22 Nov 2023
-
RC2: 'Comment on egusphere-2023-1685', Anonymous Referee #2, 03 Nov 2023
The manuscript offers a compelling blend of electromagnetic modelling and radar data analysis in the context of Thwaites Glacier's subglacial drainage pathway. While the results hold promise for enhancing our knowledge of glacier subglacial hydrology, however it needs to be improved considering the suggestions below.
Some of the Major issues to be addressed, they are.
- If the manuscript aims to identify the subglacial water arrangement by eliminating alternative hypotheses, it is imperative that the comprehensive permittivity values for both water and the glacier bed cover the entire spectrum found in existing literature, thus reproducing the full range of reflectivity associated with these parameters.
- The paper primarily focused on the regional scale of Thwaites' downstream water system. However, it is imperative to acknowledge the broader applicability of the model for a more extensive catchment when considering its practical utility.
Minor comments:
- Line 25: Better to add more recent citations.
- Line 32: Missing citations
- In the introduction section where the objectives are outlined, it is important to include the potential applications of the model to other catchments, thereby emphasising its versatility beyond the specific case of Thwaites Glacier.
- Providing a dedicated "Study Area" section, rather than including it in the introduction, would offer a more comprehensive understanding of the research area.
- Additionally, separating the "Results" and "Discussion" sections would enhance the clarity and structure of the paper. It would improve the presentation if all figures and tables were enclosed by borders for a more organized and visually clear layout.
- In the conclusion section, it would be beneficial to provide an overview of future work, offering the potential research directions and developments to be pursued.
-
AC2: 'Reply on RC2', Christopher Pierce, 22 Nov 2023
We would like to thank this reviewer for their comments on our manuscript "A Simulation Approach to Sub-Glacial Hydrology". Their insight will be very helpful in improving the final product.
In response to the reviewer's "Major Issues", we submit the following responses:
- Regarding the range of permittivity values for the bed material used in the study:
- As the reviewer correctly points out, we did not explore many possible combinations of bed materials. This was intentional, as the scope of this work was to introduce the conceptual framework for using such a simulation technique in the study of sub-glacial hydrology, then demonstrate its potential usefulness in constraining hypotheses for a single feature seen in a recent IPR survey.
- The emphasis of the work and most important conclusions involve the water geometries considered in the study. Altering the substrate dielectric constant will merely scale power. We acknowledge that definitive diagnosis of the subglacial environment beneath Thwaites may require additional assessment with a broader combination of materials. The nature of this technique may never eliminate all possible combinations of materials and geometry. However, it can be a valuable tool for narrowing the range of likely sub-glacial configurations.
- We propose revisions to acknowledge the range of unexplored material properties more explicitly. This will include stating that we have not completely excluded all other possible hypotheses. Instead, we have found at least one possible match between the observed reflectivity and a hypothetical water structure.
- Regarding the broader applicability of the model beyond the Thwaites water system:
- We appreciate and agree with the reviewer on this point. The choice of the Thwaites water system was a convenient example of how this method can be used, but of course the method could be applied more broadly to examine other regions and hypotheses. We propose additional language in the abstract, introduction and conclusions discussing the broader applicability of the technique, and also clarify that our Thwaites use case is just one example.
Regarding the minor comments raised by this reviewer:
- Line 25: Better to add more recent citations.
- We agree to add updated citations for the relationship between hydrology, shear stress, and ice rheology as suggested.
- Line 32: Missing citations
- We agree to add citations establishing IPR as a technique in the study of hydrology, as the reviewer suggests.
- In the introduction section where the objectives are outlined, it is important to include the potential applications of the model to other catchments, thereby emphasising its versatility beyond the specific case of Thwaites Glacier.
- This comment appears similar to the reviewer’s second concern under “Major Issues”. We believe our proposed resolution addresses both points.
- On providing a dedicated "Study Area" section:
- We appreciate this comment and agree to create a dedicated “Study Area” section to describe the target region of Thwaites glacier.
- On separating "Results" and "Discussion" to improve clarity:
- We will address the reviewer’s concern by separating the Results and Discussion for clarity.
- On adding table and figure borders:
- We acknowledge that authors and readers have differing stylistic preferences. However, we believe our figures are well aligned with the Cryosphere style guidance and prefer to keep them as is.
- With respect to adding a discussion of future research directions in the Conclusions:
- We agree to add language as suggested by the reviewer.
Citation: https://doi.org/10.5194/egusphere-2023-1685-AC2 - Regarding the range of permittivity values for the bed material used in the study:
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Chris Pierce
Christopher Gerekos
Mark Skidmore
Lucas Beem
Don Blankenship
Won Sang Lee
Ed Adams
Choon-Ki Lee
Jamey Stutz
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