the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Sep 2024
Advancing interpretation of incoherent scattering in ice penetrating radar data used for ice core site selection
Abstract. Below the coherent layering in ice penetrating radar data collected in Antarctica and Greenland, incoherent scattering is common. This scattering is signal, not noise, and has the potential to inform our understanding of the structure and dynamics of the bottom 20 % of glaciers and ice sheets. Here, we present a comparison between radar imagery and ice core properties for sixteen ice core sites across Antarctica and Greenland, to identify possible sources for incoherent scattering and evaluate its use in ice core site selection. We find that incoherent scattering is commonly coincident with either gradual changes in crystal orientation fabric or rapidly fluctuating fabrics in deep ice, where strain is localized by strength differences associated with ice grain size. Macro-scale deformation and layer folding at scales below the range-resolution of radar does not seem to result in incoherent scattering or induce an echo free zone, as has been previously hypothesized. Where incoherent scattering is laterally homogeneous in intensity, layering is typically undisturbed in nearby ice cores. But where incoherent scattering is laterally heterogeneous in intensity and the trace of intensity maxima does not appear conformal with subglacial topography, we find multi-meter-scale folding and associated discontinuities in nearby ice core records. Future, higher-resolution sampling of fabric in ice cores would allow for more quantitative interpretation of incoherent scattering and its amplitude, but we show that the qualitative nature of incoherent scattering has the potential to inform us about the continuity of climate records at prospective ice core sites and should be considered when evaluating the nature and quality of basal ice.
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RC1: 'Comment on egusphere-2024-2450', Julien Bodart, 21 Oct 2024
Dear Editor and Authors,
Please find attached my review of Mutter and Holschuh (manuscript number: egusphere-2024-2450) with manuscript title “Advancing interpretation of incoherent scattering in ice penetrating radar data used for ice core site selection”.
This is a really interesting paper, which insightfully combines radar and ice-core data to investigate the nature of incoherent scattering in ice-penetrating radar data. It is particularly interesting to see the link between incoherent scattering and the break in the climate record from ice cores (Figure 4), which may be used as a precursor for ice-core site selection following radar-data acquisitions. I learned a lot and felt that it helped answer some of the questions I often have when sieving through kms of Antarctic radar data myself. I found the paper extremely well structured and for the most part well presented, but there is some margin for improvement, particularly with regards to the figures presented in the paper and the discussion section which could expand a little more on the potential impact of processing on the radar data and the interpretation of discontinuities that ensue from these products.
In general, I would recommend this paper to be published in The Cryosphere with minor revision, and I very much look forward to seeing the updated version soon.With best wishes,
Julien Bodart
General comments
- Discussion section (Section 4): The authors mentioned early in the paper that the scattering or lack of signal may also be related to data acquisition and processing (see end of Section 2). This is again briefly mentioned in the conclusions. This is something that has repeatedly been on my mind whilst reading this paper, and I wonder whether this could be mentioned/discussed again somewhere in the Discussion section as a caveat. Would the authors expect to see more (or less) scattering if the radar data shown in Figure 3 and 4 be processed differently (i.e. using a homogenous processing workflow from the raw data to all the radar products analysed here – i.e. in a similar way to what the Open Polar Radar project aims to do) and thus, if the authors think that the conclusions drawn from Figure 3 in particular may be potentially different as a result? This is seldomly discussed in the paper, but I wonder whether there is scope to add a few sentences on this, and perhaps the discussion section is a good opportunity to add this as a potential caveat.
There is also, of course, the subjectivity in identifying whether the incoherent layering is diffuse, laterally homogeneous, or laterally heterogenous (and how does one set of eye, with one image processed in a certain way to emphasise specific sections or patterns in the ice that may be not be optimised for the type of analysis made in this paper, determines the type of scattering observed as “strictly” as it is done in, for example, Figure 4)? I see this paper as a good opportunity to discuss these in some more details, if possible. - Figures: Overall, I found the figures very interesting but lacking in clarity or additional information in the text/caption that may help the reader understand them. This is particularly true for Figure 3 (see below for specific comments), which has a lot of information, and the reader is left to do a lot of the work to try to piece together all the information that is being presented. The authors may want to consider whether they could split this figure up into several ones, perhaps ordered by region (Greenland vs Antarctica), make labels and legends bigger and much more simplified, and provide a full caption which may help guide the reader. The other figures also need much better captions to explain the different elements being presented (again see my below comments).
- Data availability: There is no mention of where readers can access the data presented in this paper. Could you please add this for all the radar data and other associated datasets presented in this paper?
Line-by-line comments
- Line 23-24: “And while […] future ice coring initiatives hope to build…”. Confusing grammar, please rephrase
- Line 27: “specific ice” – what is meant here? Replace maybe by “stable” or “climatically stable”?
- Line 29: not just “accumulation and ice flow” – add basal melting too
- Line 29: Reference to Schroeder et al. 2020 – could add a few more references here. Examples: Bingham et al. 2024 (in review at TC, https://doi.org/10.5194/egusphere-2024-2593); Chung et al. 2023 (https://doi.org/10.5194/tc-17-3461-2023); Karlsson et al. 2018 (https://doi.org/10.5194/tc-12-2413-2018)
- Line 30: “shallow” – replace by “the upper 2/3” as shallow is an understatement and is also a big vague.
- Line 31: “on what incoherent scattering …” – add “on what incoherent scattering in deep ice” to differentiate with the previous sentence which mentions coherent homogenous layering in the top part of the ice column
- Line 35: I would add a few references to seminal work on this topic in the existing list of reference you provide here. For example, Millar (1982), Hammer (1980), Harrison (1973) works would be great here.
- Line 39: Add Chung et al. (2023 – DOI already provided here) as an additional reference to Lilien et al. 2021)
- Line 39: “16 ice cores” – add “across Antarctica and Greenland”
- Line 48: add Bingham et al. 2024 to the Dowdeswell and Evans reference
- Line 53: “to an (up to)...” – should be “a”. Also please provide a reference to this sentence.
- Line 59: add Bingham et al. 2024 to the Fahnestock et al reference already provided.
- Lines 189-194: I was confused when reading this paragraph (and some sentences preceding this) about the lack of figures that would illustrate the description of the patterns found at each ice core locations. I think this is because these are not referred to in the text explicitly. I think that mentions of Figures 3 and 4 throughout the text (with sub panels) would help greatly to guide the reader to these figures. As it stands, I read this paragraph but ask myself why there are no figures showing this in the paper, only to find out later that these exist further down the paper but are not being referred to in the text.
- Line 61: can you provide additional references to the Schroeder et al. 2020 reference here? You provide references to science papers for the previous sentence, but only a review paper for this one. It would help to point the reader to additional science papers that discuss this point.
- Line 64-65: Please refer to Figure 2b here.
- Line 106-108: Add reference here. Perhaps Young et al., 2021 (https://doi.org/10.1029/2020JF006023) is a good starting point.
- Line 204: “time” – what do you mean by this? I think you mean age-depth? Clarify
- Line 210: Again please mention the figures in this sentence and throughout this paragraph
- Line 239: “doesn’t” – replace by “does not”
- Section 3.2: I found this section really interesting – great addition.
- Line 265-273: I wonder if it would be interesting to show the returned echo power graph of a (or several) trace(s) in a figure (perhaps in a modified version of Figure 3 or 4). This would help counteract the problem with the size and colorscale of the radargrams presented which make it often hard to see the pattern of scattering.
- Line 326: replace “radar data” with “ice-penetrating radar data”
- Line 331: replace “to” with “do”
- Line 334: “time is compressed” – again, the use to the word “time” is maybe a bit confusing to me. Perhaps replace by “age-depth”
- Line 335: replace “is” to “to be”
Figure and captions:
- Figure 1 (caption): Specify what the colormap and reliefs show and where these data come from (also the grounding line and IMBIE drainage catchments please).
- Figure 1: you could also add another axis in the “Core length” diagram in Figure 1 which shows the age of each ice cores in combination with the depth axis already provided.
- Figure 2 (caption): Could you add in the caption where the datasets you present are from (source + radar system type)?
- Figure 3: I like this figure a lot, however:
- The caption does not provide many details that could help guide the reader to each part of the figure (e.g. the left-most plots in each subplot are not explained – are these c-axes plots?). And what about the plots with the green line through them? Perhaps the confusion stems from the fact that there is a lot of information on it, which I don’t particularly mind and sometimes I think this is necessary, but it must be properly explained either in the text or in the caption. Having read this multiple times, I am left frustrated that it takes more than a couple of minutes to really get through all the elements presented in the figure.
- The “layer slopes” legend is not clear enough and I can’t see these very well on the plots
- The difference between “no data” and “no visible layering” is too similar and I can’t see the difference between the two
- The difference between the “+” for the thin and thick sections is not very obvious either. Also, what does “Sampling” mean with regards to these two “+” symbols?
- In general, I would say that there is maybe too much information on it, and I would recommend simplifying it a bit but also perhaps making multiple figures from this one, such as by regions or sub-regions. This would also allow for the radargrams to be stretched horizontally a bit so that the patterns are much more visible. Perhaps altering the colorscale or adding some gain to the radargrams would also be beneficial, as I’m left having to trust the authors a lot about what they “see”, when I can’t really see it myself very clearly due to the small size of the figure and the overload of information being presented. This refers also a bit to my general point above with regards to the processing of the radar data that is used to make the interpretations in this paper (of course one could argue this is the case for any dataset, but it would be worth addressing this point in the paper a bit more).
- Figure 4 (caption): here and in the main text, it would be great if you could refer to your Appendix A, which describes whether a break in the climate record is visible in ice cores and hence can be seen in the radargrams.
- Figure 4: what is the dotted red or black lines in some radargrams (e.g. for GISP2?)
- Figure S2 (caption): there is no “(c)” in the figure, but 2x “(a)”
Citation: https://doi.org/10.5194/egusphere-2024-2450-RC1 - Discussion section (Section 4): The authors mentioned early in the paper that the scattering or lack of signal may also be related to data acquisition and processing (see end of Section 2). This is again briefly mentioned in the conclusions. This is something that has repeatedly been on my mind whilst reading this paper, and I wonder whether this could be mentioned/discussed again somewhere in the Discussion section as a caveat. Would the authors expect to see more (or less) scattering if the radar data shown in Figure 3 and 4 be processed differently (i.e. using a homogenous processing workflow from the raw data to all the radar products analysed here – i.e. in a similar way to what the Open Polar Radar project aims to do) and thus, if the authors think that the conclusions drawn from Figure 3 in particular may be potentially different as a result? This is seldomly discussed in the paper, but I wonder whether there is scope to add a few sentences on this, and perhaps the discussion section is a good opportunity to add this as a potential caveat.
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RC2: 'Comment on egusphere-2024-2450', Anonymous Referee #2, 06 Dec 2024
Dear editor,
Dear authors,
please find my review of the manuscript “Advancing interpretation of incoherent scattering in ice penetrating radar data used for ice core site selection”, DOI 10.5194/egusphere-2024-2450.
The manuscript provides a comprehensive compilation of ice penetrating radar (IPR) data measurements near deep ice core drill sites and, where published, fabric data of the ice cores. Aim of this compilation is to gain insight into the nature of incoherent scattering in radar data. The title has to be considered as a call for further detailed studies in this field. Even though, system characteristics of the deployed RES systems and kind of processing of the data have not been considered in the presented work, respectively discussed in detail, the author can show the potential of incoherent scattering in radar data for extracting more information on the bottom part of ice sheets.
Thus, I would like to recommend to publish the manuscript with revisions.
Comments
Since the figures are essential for the conclusions derived, they should be improved by enlarging them and making the colour bars showing layering and fabric observations as well as scattering information wider. It might reasonable to use the height of a page in landscape mode as height of a subpanel shown in figures 3 and 4.
L 33: Amend “All radio-wave scattering originates from electrical contrasts.“ All radio-wave scattering originates from electrical contrasts in conductivity and permittivity
L 53: Please provide reference for your statement „… with transitions in c-axis fabric leading to an (up to) 1% contrast in the polarization-dependent bulk permittivity.“
L 54: reference of inset (i)?: …(as in the upper half of fig. 2.a, respectively marked example i), …
L 68: “Incoherent scattering typically occurs at rough interfaces …“ please define in physical units
L 77/78: The statement „each pixel typically represents backscattered energy from only a
single subsurface target.“ is only correct if the transmitted signal and the sample interval are short enough to resolve the layering. This is for airborne RES system usually not true.
L 119 … „within the ~8 cm diameter ice cores.“ – doesn’t make much difference, but isn’t the diameter of m
L137: (first occurrence): I suggest to replace NorthGRIP by NGRIP when referring to the ice core.
L 177: (first occurrence): When referring to the ice core drilled at Dome C use EDC, similar to EDML.
L 331 Correct “… interfaces to not manifest …” Correct “… interfaces do not manifest …”
L 492 Oatm atm should be subscript
L 493, 495: ky kyr
References, L695-697 Wang et al 2023 not cited in main manuscript
References in Supplement:
- List is incomplete
- L55: reference Dansgaard 1982 is incomplete
- L60: Eichler 2013: what kind of thesis; how published?
Fig. 2:
- Captions for a-c should be below the figure
- Sublabel should be before the respective text (same for other figures)
- Labels of examples in 2a are difficult o read and boxes are too small
- 2c – it should be clearly stated that the two RES sections are not mapped on the same profile
Fig. 3:
- The sub- panels of the figure should be larger
- It seems to be counter-intuitive to use a bold “+” for thin section and a normal sized one for thick sections.
- The depth exis should be of equal length for all panels
Fig. 4
- The sub- panels of the figure should be larger
- The depth axis should be of equal length for all panels
Fig. S2:
- label (c) is not readable in the figure
- the feature marked with (c), diffuse, banded scattering, looks very much like 2a i coherent scattering might not be a typical example; please choose a better one
Fig. S3:
- Even though it is stated that only examples of strong laterally heterogeneous incoherent scattering are indicated in the figure, the choice of makred features, respectively their distribution is odd.
Citation: https://doi.org/10.5194/egusphere-2024-2450-RC2
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