Crustal Imaging across the Princess Elizabeth Land, East Antarctica from 2D Gravity and Magnetic Inversions

Round 1

Reviewer 1 Report (Previous Reviewer 2)

The authors addressed my comments. Please check the English. For example, densitys (please see page 5) should be densities.  Please spell check the entire paper. 

Please report the background density value(s) you used in the inverse solution/interpretation. Please explain that in the paper so that the readers do not get confused. Also please explain  for the readers why the density is negative (see Figures 2 and 3. Perhaps you can indicate the used background density value(s) in the pertinent figure captions). Please make things clear. I recall that this concern was raised in my earlier reviews. 

Author Response

We would like to thank the reviewer for his/her comments which helped improve not only our manuscript but also our understanding of the problem. Please find our point-by-point response in the uploaded PDF file.

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Review of “Airborne Geophysics Reveals Subglacial Bedrock Structural Features Across Princess Elizabeth Land, East Antarctica”. This paper sets out to explore the crustal structure of a sector of the East Antarctic continent using a combination of gravity, magnetic and radar data. The authors have developed a series of four inversions along selected profiles which are interpreted in terms of the underlying lithology. These lithological blocks, along with patterns of faults identified in maps of magnetic data, are used to underpin a relatively complex interpretation of the geological history of the region.

The topic of the paper is of definite interest, and the data and methods used broadly have the potential to reveal the crustal structure as suggested by the authors. However, I have significant concerns about the inversion of the gravity data for crustal density structure. As this underpins the paper I focus on this aspect. I would happily re-review the paper once this section has been clarified and if required the inversions re-run.

I have some additional minor points set out below which the authors may wish to consider in a future draft.

Main points related to gravity data and inversion.

1/ L87-94 describes the gravity data. It is not stated specifically what gravity anomaly is calculated and shown, I suspect it is the Free Air gravity anomaly, but this needs to be stated. Appling the Bouguer correction for the known topography recovered using the airborne radar is a critical step for gravity processing and interpretation, otherwise the gravity anomaly map, shown in Fig. 1f, is simply so dominated by the topographic signal (shown in Fig. 1d) that no significant additional interpretation can be made. It would therefore be good to also show the Bouguer anomaly on Fig. 1.

It would also be useful to add the estimated error of the output Free Air gravity anomalies to this section (often obtained as standard deviation of cross over values, or simply quoted from performance of the sensor in other similar areas) - typically this is 1 to 5 mGal. Other parameters associated with the gravity survey such as flight altitude, any upward/downward continuation and wavelength of the processed gravity anomalies should also be included in this section. Note this wavelength may need to be considered when applying the Bouguer correction.

2/ L101-122 describes the inversion. The displayed output of the inversion (Fig. 2/3) is limited to 10 km depth. Was this the full geometry, e.g. was it padded beneath 10 km? Related to this was any attempt made to remove long wavelength crustal signals? If only the top 10 km is modelled, signals from deeper structures, such as variations in crustal thickness, need to be removed. Even a basic Airy isostatic model of the gravity signal associated with crustal thickness subtracted from the input data would help ensure the signature that is modelled is really in the upper crust, rather than a signal from much deeper.

It is also really important to state how you treated the ice/air layer in your inversion scheme and if the Free Air or Bouguer gravity anomaly was the input. This is my main issue with the paper, and the reason I am not convinced of the output density structures and subsequent interpretation. If Free Air gravity anomalies are used the densities of air and ice need to be fixed at 0 and 0.915 respectivley. Recovered densities will then be 'real' rock densities. For example in the case where geology was uniform and the mountains had a uniform density of 2.67, this is the density value which should be recovered by the inversion where Free Air gravity is the input.

If Bouguer anomalies are used in the inversion (my preferred option as topographic signatures confusing the interpretation have theoretically been eliminated) then air and ice densities can both be set and fixed to zero. The recovered variable density values below the base of the ice sheet are then relative to the standard value used in the Bouguer correction (typically 2.67).

In the case where the Bouguer anomaly is used (and an appropriate correction made to the input gravity anomaly for crustal thickness – see point above) the recovered density structure can be interpreted alongside the recovered magnetic susceptibility as suggested by the authors.

Another point is that the inversion results in Figures 2a and c appear to show the crust under the ice as less dense than the ice - this seems problematic. It may be that the ice layer is simply a fixed colour and density, not related to the colour scale, but this should be clarified.

Minor points:

It would be really good to show the gravity (and magnetic) signals modelled (and the fit of the inversion) above the panels in Fig. 2 and 3. This would allow the reader a better opportunity to be convinced by the interpretation along the individual profiles. Simply seeing the 2D anomaly can allow the reader to judge if the inversion is returning a reasonable result.

L187-189 “These units show an obvious pinch-out trend as they extend inland, with no identifiable geological units interpreted as sedimentary rocks in line 4 (Figure 3c and d)”. I am not convinced that this is not an artefact of using the Free Air anomaly in the inversion. The Free Air anomaly becomes more positive further inland because the topography gets higher. I suspect this is what the inversion is seeing, with denser rocks modelled to match teh more posative Free air anoamly. The input data need to be corrected for both the topographic signal, and the likely impact of regionally changing crustal thickness, before the inversion output of relative densities can be interpreted in this way.

L194 “density contrast is about 2.3” I suspect this is a typo – the density contrast according to the figure is ~0.23 gcm^-3 (note units are also needed).

Fig. 5 refers to the East Antarctic Carton – this should be “Craton”.

We note that there is no data accessibility statement – this would be useful for the wider research community and is considered standard by many journals.

Overall the English was fine. Some very minor points on wording could be amended/simplified, but this is of no real significance.

e.g. "have hindered the exploration and cognition of the basement characteristics" In this case I would use knowlage of the basement.

Author Response

We would like to thank the reviewer for his/her comments which helped improve not only our manuscript but also our understanding of the problem. Please find out point-by-point responses in the uploaded PDF file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report (New Reviewer)

The authors have amended the paper in line with my suggestions related to the scientific method, which has made things clearer.

Unfortunately I found the description of the results remains unclear L138 to 206. Note I had not previously reviewed this section due to my conerns with the method.

e.g. Line 153-156  speaking about Figure 3 states “Zone A associates with low density  contrast  and high susceptibility”, while “Zone B is characterized by high density contrast and low susceptibility”.

In the figure 3b, A is an area of relatively low density, but the corresponding point on the susceptibility panel (3d) susceptibility is relatively low, not high as stated in the text. An area of high susceptibility is marked A in (3d), but it is not clearly associated with an area of low density. Similarly area B in the density result is not clearly associated with low susceptibility – in fact it overlaps an area of high susceptibility.

Another example of the lack of clarity is L190-191 which states “The mountain-shaped bedrocks in zone D exhibit spatial continuity with zone A in line 2 (Figure 4b and d) and are interpreted as magmatic rocks and metamorphic rocks [21,22]”. There are two regions labelled A in Fig 4b and d, making it hard to understand which region is spatially correlated. If only one region correlated it could be stated that D correlates with the western region A.

L193-196 “In survey line 4, we observe a distinct feature in the recovered models, namely a large area of deep negative susceptibility (Zone A in Figure 6d). Further analysis reveals that this negative susceptibility feature is comprised of three distinct geological units (Zones A, B, and D in Figure 6b) with different density contrasts”. Here Zone D in Fig. 6b is nowhere near Zone A in fig 6d. It is therefore hard to understand the interpretation.

L198 “Zones B and C exhibit high positive density contrast” This can’t be correct as C is always the ice.

To clarify this section I have two suggestions:

First - place labels in the same place on both panels. That way a reader can easily judge if a feature really is low density contrast and high susceptibility.

Second – keep consistent labels for features across all profiles, so for example (A) always refers to the same interpreted lithology.

L208 to 244 Section 4.1. Sedimentary units. Following the revision of the paper no sedimentary units are interpreted in Section 3 (interpretation). This entire section can therefore be removed.

L253 “remarkable thick sedimentary units” – see point above.

L257 “This mutation is consistent with the bedrock structure identification of the fracture splicing model in the south pole”. This sentence feels like it needs a reference – what fracture pattern around South Pole?

L399-400 “we have confirmed the deep evidence of the eastward extension of the Rayner orogen and discovered the structural boundary of its extension termination”. Specifically where/what is this? This is the only point the Rayner orogen is mentioned. Does this terminate at the Mt Brown block boundary, i.e. a N/S striking boundary, or between line 3 and 4 ( a coast parallel boundary)? 

English is generally ok, but description feels over complex and un-clear - see points in main review.

Author Response

Dear reviewer,

Thank you for your constructive suggestions and comments, which take our paper to the next level. Please find our point-by-point response and the marked paper in the submission system. 

Cheers,

All authors

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This manuscript applies the gravity and magnetic data to obtain the structural features in the Mountain Brown region. It is logical and well-structured. The discussion in section 4 is very detailed, especially in analyzing the data results from different perspectives, e.g., sedimentary units and Indo-Antarctic. The topic is relevant in the field and it helps to understand the processes of the supercontinent’s breakup and convergence cycle in Princess Elizabeth Land, East Antarctica, which is a specific gap in the field. All inversions of the gravity and magnetic data are performed using the software SimPEG. Thus, there are no technical improvements from a technical point of view. However, the adequate explanations are given in section 4 from a data interpretation perspective. The conclusions from the interpretation of the results are consistent with the evidence and the main question is answered. The references are appropriate. The pictures are well presented, nothing to add.

There are some comments as follows:

1. On line 5, during -> from.

2. On line 6, magnetism -> magnetic.

3. On line 16, what the fullname of the I-A-A-S?

4. On line 31, become -> became.

5. On line 106, what is the '?]'?

6. On line 309, what is the 'halpin2017strike'?

7. On line 321, is -> was.

Please see the comment above.

Author Response

We would like to thank three Reviewers as well as Editor and AE for their comments, questions and suggestions which help improve our manuscript. Below we provide point-by-point responses to the comments that reviewers and editors raised. Our responses are in red. 

This manuscript applies the gravity and magnetic data to obtain the structural features in the Mountain Brown region. It is logical and well-structured. The discussion in section 4 is very detailed, especially in analyzing the data results from different perspectives, e.g., sedimentary units and Indo-Antarctic. The topic is relevant in the field and it helps to understand the processes of the supercontinent’s breakup and convergence cycle in Princess Elizabeth Land, East Antarctica, which is a specific gap in the field. All inversions of the gravity and magnetic data are performed using the software SimPEG. Thus, there are no technical improvements from a technical point of view. However, the adequate explanations are given in section 4 from a data interpretation perspective. The conclusions from the interpretation of the results are consistent with the evidence and the main question is answered. The references are appropriate. The pictures are well presented, nothing to add.

Thank you for your time to review our manuscript and for your positive comments! The manuscript has been revised regarding your general concerns and specific comments.

There are some comments as follows:

1. On line 5, during -> from.

Done! Thank you for your comment.

2. On line 6, magnetism -> magnetic.

Done! Thank you for your comment.

3. On line 16, what the fullname of the I-A-A-S?

Thanks for your comment! I-A-A-S indicates Indo-Australo-Antarctic Suture. We noted that in our revision.

4. On line 31, become -> became.

Thanks for your comment! We would like to friendly note that “has become” is grammatically correct. We have double-checked the English dictionary.

5. On line 106, what is the '?]'?

Sorry for the typo. We added the correct reference in the revised manuscript.

6. On line 309, what is the 'halpin2017strike'?

Sorry for the invalid citation of the reference. We corrected the citation in the revised manuscript.

7. On line 321, is -> was.

Done! Thank you for your comment.

Reviewer 2 Report

Please see my two pdf files (report and annotated manuscript).

Comments for author File: Comments.zip

Author Response

We would like to thank three Reviewers as well as Editor and AE for their comments, questions and suggestions which help improve our manuscript. Below we provide point-by-point responses to the comments that reviewers and editors raised. Our responses are in red. 

The manuscript has a lot to recommend. However, the current manuscript mostly reads like A report rather than a research paper. The rationale of the work are not fully indicated. The manuscript needs work in terms of layout and ow. Please make sure that all figures and references are cited correctly in the text.

Thank you for your time to review our manuscript! The manuscript has been revised regarding your general concerns and specific comments.

Are you doing separate inversion or gravity and magnetic data joint inversion? Not clear. The author should make this clear right in the beginning of the manuscript.

Thanks for your comments! We performed two separate inversions using gravity and magnetic data, respectively. We clearly noted this in the revised manuscript.

In the geophysical literature the depth is chosen positive downward. Please correct all figures of the entire manuscript.

Thanks for your comments! We implemented geophysical inversions using SimPEG, which is an widely used open source package. The coordinate system of SimPEG follows the right-hand rule, namely, x is east, y is north, and z is positive upward. We don’t think the direction of depth affects our results. We would like to keep our implementation consistent with SimPEG where z is positive upward. Thanks for your understanding.

Line 92 Page 4: This statement is incorrect. The magnetometer does not really measure the magnetic eld of the bedrock. It measures an integrated response from deep and shallow structures. Please see a text book on magnetic methods.

Thanks for your comments! We agree! We have re-phrased the sentence in the revised manuscript.

Line 117 Page 4:

Section 3. Geophysical and geological interpretation This section is not organized well. First, you should tell the reader how you did inversion. Are you doing separate inversion? or joint inversion? What is the starting/initial model you used for inversion?

Thanks for your comments! In our work, we perform separate inversions using gravity and magnetic data, respectively. We use uniform initial models with zero density contrast and susceptibility value. According to your comments, we clearly noted these details in our revision.

How did you separate the residual component of your gravity and magnetic data prior to inversion?

Thanks for your comments! We would like to note that we did NOT use residual data. For gravity data, we have completed free air correction, elevation correction, and terrain correction. For magnetic data, we use total magnetic intensity data. The gravity and magnetic residual data can better reflect the surface structural variation due to the removal of the regional gravity and magnetic trend. In Antarctica, the corrected gravity and TMI data is good enough to reflect geological structures, so we did not use residual data.

What were the regularization parameters values you used in inversion? How did you select them?

Thanks for your comments! We use the beta-cooling strategy, implemented in SimPEG, to find the appropriate regularization parameter.

Please provide the computation time for your calculations.

Thanks for your comments! Since we perform 2D inversions, it is fast. Each inversion took less than 2 minutes.

Figure 2: I think this is not the density. It is the density contrast.

Thanks for your comments! Yes, it is density contrast. We changed the figures and main text in our revision.

I think this is also the susceptibility contrast.

Thanks for your comments! No, susceptibility is not the contrast value. It is a scalar value to describe the ability of rock to be magnetized.

Please correct all of these in all figures in the entire manuscript.

Thanks for your comments! Done!

Most of the figure captions are inappropriate. See for example, the caption of Figure 8. Please fix the figure captions of the entire paper.

Thanks for your comments! Done!

Please check all figures and their citation. Please see above and the annotated text.

Thank you for bringing the new significant references to our attention. We have cited all the references according to your comments!

Some references of the text are not listed in the List of references. Please check.

Thanks for your comments! Done!

Reviewer 3 Report

The paper presents the geologic and tectonic results deducted from the analysis of aerial geophysical measurements: GPR data are used as a constraint for the 2D inversion of gravity and magnetic datasets acquired over four parallel profiles in the bordering areas of the Princess Elizabeth Land, East Antarctica. Inverted sections are commented against the physical properties of possible geological materials, and, from that, against the existing knowledge on tectonics and land formation in East Antartica and in continental lands originally attached to this region. Sedimentary, bedrock, and faults setups are reconstructed across the inspected area and the most probable interpretations are given.
The paper is well-written and -organized, the conclusions are supported by experimental results and critical analysis of the literature, and the topic and the results are of potential interest to the readers of Remote Sensing.

The language is generally good and fluent.


Please check:

- at line 97, attitude?

- at line 106: Similar to the work of ? ]

- in many parts of the manuscript:  recovered,  please check if retrieved better expresses what you mean


- at line 309: halpin2017strike... please check if the sentence is correct

Author Response

We would like to thank three Reviewers as well as Editor and AE for their comments, questions and suggestions which help improve our manuscript. Below we provide point-by-point responses to the comments that reviewers and editors raised. Our responses are in red. 

The paper presents the geologic and tectonic results deducted from the analysis of aerial geophysical measurements: GPR data are used as a constraint for the 2D inversion of gravity and magnetic datasets acquired over four parallel profiles in the bordering areas of the Princess Elizabeth Land, East Antarctica. Inverted sections are commented against the physical properties of possible geological materials, and, from that, against the existing knowledge on tectonics and land formation in East Antartica and in continental lands originally attached to this region. Sedimentary, bedrock, and faults setups are reconstructed across the inspected area and the most probable interpretations are given.
The paper is well-written and -organized, the conclusions are supported by experimental results and critical analysis of the literature, and the topic and the results are of potential interest to the readers of Remote Sensing.

Thank you for your positive comments! The manuscript has been revised according to your comments.

The language is generally good and fluent.

Please check:

- at line 97, attitude?

Sorry for the typo. We corrected the typo in our revision.

- at line 106: Similar to the work of ?

Sorry for the typo. We added the correct reference in the revised manuscript.

- in many parts of the manuscript:  recovered,  please check if retrieved better expresses what you mean

Thanks for your comments! “Recovered” is an appropriate word to describe the process of model reconstruction, which has been commonly used in papers regarding geophysical inversion. We would like to keep using the word “recovered” in our manuscript. Thanks for your understanding.

- at line 309: halpin2017strike... please check if the sentence is correct

Sorry for the invalid citation of the reference. We corrected the citation in the revised manuscript.

Round 2

Reviewer 2 Report

Please see the two attached pdf files (report and annotated revised manuscript). 

Comments for author File: Comments.zip

Author Response

The reply is attached.

Author Response File: Author Response.pdf