the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Standardized field methods for fracture-focused surface processes research
Jennifer Aldred
Samantha Berberich
Maxwell P. Dahlquist
Sarah G. Evans
Russell Keanini
Faye Moser
Mehdi Morovati
Steven Porson
Monica Rasmussen
Alex Rinehart
Uri Shaanan
Abstract. Rock fracturing comprises a key component of a broad array of Earth surface processes due to its direct control on rock strength as well as rock porosity and permeability. However, to date, there has been no standardization for the quantification of rock fractures in surface processes research. In this work, we make the case for standardization within fracture-focused research and review prior work to identify various key datasets and methodologies. We then present a suite of standardized methods that we propose as ‘baseline’ for fracture-based research in surfaces processes studies. These methods have been shown in preexisting work from structural geology, fracture mechanics, and surface processes disciplines to comprise best practices for the characterization for cracks, clasts, and outcrops. These practical, accessible and detailed methods can readily be employed across all fracture-focused weathering and geomorphology applications. The wide adoption of a baseline of data, all collected using the same methods, will enable comparison and compilation of data among studies globally, and ultimately will lead to a better understanding of the links and feedbacks between rock fracture and landscape evolution.
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Martha Cary Eppes et al.
Status: final response (author comments only)
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CC1: 'Comment on esurf-2022-61', Stephen Laubach, 12 Dec 2022
A compilation and review of fracture analysis field methods for surface processes research ought to be a valuable contribution and within the scope of this journal.
I enjoyed this MS and I think it’s a valuable contribution.
The paper is well written and clearly illustrated.
There are several places in the text, noted below, where clarifications are needed. My comments are in the attached pdf
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AC2: 'Reply on CC1', Martha-Cary Eppes, 27 Jan 2023
My co-authors and I have reviewed Dr. Laubach's thorough and helpful comments. We are very grateful for this input and look forward to implementing these changes in the manuscript.
The only suggestion of his for which we disagree (albeit not entirely) is the comment regarding the use of the terms 'crack' and 'fracture'. (His comment is copied below for ease of reading this reply).
Though it is suggested that such a distinction has been made in the literature, we can find no consistent use or definition, even in the Anders et al., 2014 paper suggested. In fact, in this paper - there is common interchange of the two terms:
"4.3. Microfractures and fault zones
Faults (shear cracks) are inherently macroscale features. Faults formed by the coalescence of Mode I microcracks (Scholz, 1968b, Lockner et al., 1991) and propagate via a cloud of microfractures formed in the stress concentration at the fault tip; when crack density reaches a critical value, this cloud of microfractures breaks the rock down into a cataclasite that allows shear displacement to occur (Cowie and Scholz, 1992)."The AGI Glossary of Geology also seems to interchange the terms, but simultaneously imply a difference: "fracture: (a) A general term for any surface within a material across which there is no cohesion, e.g. a crack. Fracture includes cracks, joints, and faults. (b) A crack in a rock where the movement of rock separated by the crack is normal to the surface." or ""crack [struc geol] A parting with crack-normal motion. Cf: joint [struc geol]. "We feel that to suggest that there is some size cut-off between when a crack becomes a fracture or vice-versa, or that by bringing a rock from the field to the lab could be interpreted as arbitrary and confusing. From a process standpoint, in the field of geomorphology, both small and large cracks/fractures contribute to the rock properties of interest, namely hydrology and strength.Also, to distinguish a 'crack' measured in the laboratory from a 'fracture' measured in the field, is to in some way discount the lab measurement as relevant to the field. I am sure that is not desirable by any of us.We acknowledge a clear distinction between microcrack/microfracture and crack/fracture being what is visible with the human eye. In this field methods paper, however, we explicitly ignore all microcracks/microfractures.We further acknowledge that despite there being no clear distinction in the literature, many geoscientists appear to have some distinction in their minds between the two terms fracture and crack.We further acknowledge that the word 'crack' has various perhaps negative connotations in popular culture.Thus, in order to be most clear and transparent our plan to address this comment is to 1) clearly define what we mean by the term 2) become consistent in our language in the manuscript, and 3) explicitly explain our decision making-process in the above why we have done so, laying out the argument above about no distinction really that we can find.It has sparked very interesting conversations with geomorphologists, rock physicists and structural geology colleagues!Many thanks again, MCE and co-authorsComment on esurf-2022-61', from his attachment Stephen Laubach, 12 Dec 2022:
"On the use of ‘crack’ and ‘fracture’ interchangeably. Although this usage is widespread it has the potential to cause confusion, particularly where these may be language barriers. The text jumps back and from between ‘fracture’ and ‘crack’ and I found this distracting. In brittle structural geology a case has been made for restricting ‘crack’ to experimental and theoretical applications, and ‘fracture’ for features observed in the field. I believe this convention is stated in Anders et al. 2014, Microfractures: a review, J. Struct. Geol.) Maybe field-monitored examples you have described on fracture propagation in outcrops or clasts would fall into the category of ‘cracks’ by this convention. My advice is to make a distinction between these two terms along these lines and revise the MS accordingly. Even if the distinction has not been made in the past in this field, it would be useful to do so now "
Citation: https://doi.org/10.5194/esurf-2022-61-AC2 -
CC2: 'Reply on AC2 fracture versus crack', Stephen Laubach, 27 Jan 2023
On the terminology or ‘fracture’ versus ‘crack’, I’m not sure that there really is any disagreement here. I do not think that such an arid topic as terminology is in any case worth disagreeing about. The distinction in Anders et al. was more a recognition that usage of these terms does vary within disciplines, and consequently different parts of Anders et al primarily use ‘fracture’ (for observational studies) or ‘crack’ (in theoretical or lab contexts). The crack usage cited above from Anders et al. is in the latter category (cites experimental studies). In writing Anders et al, we did try to consistently use the terms with those distinctions in mind. In making the original comment I did not mean to imply a size cut off, or say that only one term should be used, or to slight field observations (I’m primarily a field geologist). But the terminology in use to describe fractures can be confusing, and notwithstanding the frequently unhelpful definitions in the AGI glossary, it can aid comprehension to define terms and keep usage as consistent and simple as possible. Based on MCE and co-author’s comment, it seems we agree.
Citation: https://doi.org/10.5194/esurf-2022-61-CC2
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CC2: 'Reply on AC2 fracture versus crack', Stephen Laubach, 27 Jan 2023
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AC2: 'Reply on CC1', Martha-Cary Eppes, 27 Jan 2023
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AC1: 'Comment on esurf-2022-61', Martha-Cary Eppes, 14 Dec 2022
Dr. Laubach's detailed suggestions and comments are exactly what we were hoping to recieve in this public forum. Addressing these comments will strengthen the manuscript and its utility considerably.
Currently, however, a majority of us are at AGU, and/or have school aged children and family that require our undivided attention over the holidays.
We will provide a more detailed response to Dr. Laubach's suggestions - and any other comments received through the end of the year - starting no later than Jan 5, earlier if we are able. We thank you for your understanding and patience with this choice.
Eppes et al.,
Citation: https://doi.org/10.5194/esurf-2022-61-AC1 -
RC1: 'Comment on esurf-2022-61', Anonymous Referee #1, 17 Feb 2023
This manuscript wishes to deliver a standardization approach of how to measure fractures in the field, with application to Earth surface related researches. I appreciate this effort to define a standardization or at least a guidance (I am not that convinced by the need for standardization - see my first main comment) on how fracture measurement should be performed. This can be extremely valuable as a starter guide for young or more experience researchers or students who need to measure fractures in the field. Most of the advices seem justified, and the guidance is quite thorough and exhaustive. I congratulate the authors for that, and I am convinced this will be useful to the community. However, in its current form I have some strong doubts about the publication of this manuscript in Esurf, as it is a paper that develops a standardization approach without bringing new results. My recommendation to the Editor is therefore to suggest the authors to consider another, more technical/methodological outlet (see my last main comment).
Here are my main criticisms:
- Standardization or Guidelines - In my opinion, the paper should read more as a series of guidelines or good practices than really the standardization of an approach. The phase of standardization generally occurs after there has already been extensive research in a particular field so that 1) it is quite clear what are the best practices, and 2) there is a clear need to make datasets comparable, in particular for applied sciences. In geomorphology, studying fractures related to geomorphology is almost exclusively limited to fundamental research, and the topic remains a niche topic investigated by only a few researchers or groups worldwide. This is also highlighted by the large number of self-citations (13) of the main author in this manuscript. Please do not get me wrong, I am not here criticizing these self-citations, which are indeed pertinent ones, but I believe this shows that the study of fractures in geomorphology still remains a niche topic (despite being a very interesting and promising one). I also believe a more community-wide approach to standardization is required to prevent secondary effects, such as studies rejected because of a lack of consistency (despite being sound) with the methodology described here. So, at this stage, providing guidelines is useful, but defining a standardization might be unnecessary or too early.
- Accounting for automated measurements - I also feel that in terms of timing, it might be too late (sorry for the apparent contradiction with previous sentence) to define a standardization approach simply based on manual measurements in the field. There are now plenty of – and at least a few good - algorithms and softwares that can help to automatically identify, and measure fractures or sediment grains based on 2D images or 3D point clouds at all scales. These methods are more and more routinely used by research teams and are generally successful to limit or remove operator bias and to lead to reproducible measurements. I agree that they cannot be used in all conditions and that hand manual measurements remain a complementary and more polyvalent approach, as it brings confidence to the automatic approaches. But I also believe that the need for standardization has clearly changed since the last century (when most standardization approaches were defined in Earth sciences). The manuscript ignores or does not account for these more automated and more objective approaches, while they will probably represent a universal approach to fracture analysis in the following years or decades.
- A too long paper - The paper is well written, but it is also too long with too many details, and it is sometimes hard to follow the logic of its organization. The authors therefore need to make a clear effort to explain these guidelines in a more synthetic way, and even more importantly to motivate the readers to read it and to better justify why these guidelines are important (section 1 does not fully succeed to do that). This is critical. Indeed, if the authors wish the general audience to follow these guidelines, they need to make sure that most researchers - interested in fractures for Earth surface related research – read thoroughly the paper. And I have strong doubts this will be the case with the present form of the manuscript.
- Arbitrary choices while developing a standardization approach - Some choice of how to measure fractures are not sufficiently justified. As an example (but there are several other examples), it is mentioned in section 5.4.1 that “If a seemingly continuous crack (Fig. 2b, left) is in fact separated by bridges of solid rock (Fig. 2b, right inset), then these should be measured as two different cracks and their lengths should terminate at the rock bridges”. This statement (as some others) is not - or not sufficiently - justified or motivated. For instance, in this case, can’t there be some long fractures with some small rock bridges (which are quite common due to fracture roughness) that mechanically behaves as long fractures and not as a series of smaller fractures? Then why and on which scientific basis should we separate the fracture in smaller segments? This is problematic as it gives an impression of arbitrary choices, while defining a standardization approach that can be useful only if there is a community agreement, obtained after a logical explanation, about these apriori best practices.
- Hierarchy of State Factors - The presentation of the State Factors is interesting, but probably lacks a bit of hierarchy. Currently, all the State Factors as cl,o,r,p,t,T (climate, organisms, relief, parent material, time and Tectonics) are presented at the same level without a clear hierarchy, as if each of these factors had the same weight in controlling fracture growth, which is likely not the case. I agree that considering subcritical growth – which is something important and often neglected - brings some complexity. But it also leads to some confusion about the role of each of these factors. Some factors mainly lead to a global – almost static on human timescale - stress field (tectonics and relief), some induce some local temporal stress variations (e.g., pore pressure – related to hydrogeology and climate / thermal expansion – related to heat and insolation), and some lead to favorable conditions for subcritical crack growth (e.g., time, water chemistry, organisms). I suggest presenting these State Factors with a more explicit hierarchy and more explicit link to either critical or subcritical growth.
- I must finish by stating that I am not used to review a manuscript dedicated to developing a standardization approach, so my evaluation might not be relevant. Yet, I also question the suitability of Esurf for publishing a manuscript (that clearly deserves to be published somewhere, as mentioned earlier these guidelines are useful) that is technical and does not really bring new results (section 8 presenting a case example is not really a thorough demonstration of the need for standardization). Esurf is supposed to publish either Research articles (which report substantial and original scientific results within the journal scope) or Review articles (which summarize the status of knowledge and outline future directions of research within the journal scope) or Short communications. The editorial team will have better assessment than mine, but my opinion is that this manuscript does not correspond to any of these article types.
Last, I note that I have not checked in details potential syntax issues or less minor issues than the one mentioned, as I believe we first need to clarify these more important comments that I have mentioned above.
Citation: https://doi.org/10.5194/esurf-2022-61-RC1 -
RC2: 'Comment on esurf-2022-61', Claire Bossennec, 20 Feb 2023
Dear authors,
The submitted manuscript provides a very useful summary and synthesis of good practices for the quantification of fracture networks for the purpose of surface process studies.
It is useful for this community but not only, and thus I recommend the acceptance after minor revision.
Where I have the most trouble with is the mixed use of the terms cracks/fracture, which is for me more confusing than picking one over the other. As the title of the article refers to fracture, I would suggest using this term only throughout the manuscript and mentioning the reasons for this choice in the introduction.
Moreover, the language and writing style could be revised in some sections with a more nuanced and neutral tone and a bit less of a "we" form.
Some paragraphs also need to be rephrased and revised accordingly but don´t understand me in a bad way, the overall quality is really good.
I attached here the annotated manuscript with specific comments.
Congratulations again to the authors, I hope to see this work published soon.
Best regards,
Dr. Claire Bossennec
Martha Cary Eppes et al.
Martha Cary Eppes et al.
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