Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Nickschick, Tobias; Flechsig, Christina; Mrlina, Jan; Oppermann, Frank; Löbig, Felix; Günther, Thomas

doi:https://doi.org/10.5194/se-10-1951-2019

Articles | Volume 10, issue 6

https://doi.org/10.5194/se-10-1951-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-10-1951-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 10, issue 6

Research article

|

14 Nov 2019

Research article |

| 14 Nov 2019

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Tobias Nickschick, Christina Flechsig, Jan Mrlina, Frank Oppermann, Felix Löbig, and Thomas Günther

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Final revised paper (published on 14 Nov 2019)
Preprint (discussion started on 06 Mar 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'review of Nickschick et al., "Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures"', Anonymous Referee #1, 19 Mar 2019
- AC1: 'Reply to Referee 1', Tobias Nickschick, 27 May 2019
RC2: 'Review: “Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures”', Anonymous Referee #2, 31 Mar 2019
- AC2: 'Reply to Referee 2', Tobias Nickschick, 27 May 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Tobias Nickschick on behalf of the Authors (18 Jun 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Jun 2019) by Ulrike Werban

RR by Anonymous Referee #1 (09 Jul 2019)

Suggestions for revision or reasons for rejection

Some important clarifications, revisions, and improvements have been made in this version of the manuscript. However, in many cases, it was apparent that “changes” made in response to referee comments did not actually address the point made by the referee. In other words, changes were made, but they were irrelevant or partially relevant to the referee comments. In other cases, referee suggestions appeared to be outright declined. The following is a list of the most offensive examples of comments that were not duly considered and a short explanation of how the response was insufficient for each:

1) Original Referee Comment: “Introduction: the structure of the introduction is awkward, particularly because it immediately jumps into site description…”

Why the response is insufficient: No action was apparently taken in response to this comment. Rather, the author attempts to justify a poorly organized introduction by saying the site is the most important thing. This argument is flawed – a well organized introduction will not detract from the importance of various study components. A well organized introduction will certainly improve readability and comprehension by readers. Placing something “first” in the text has no effect of making it appear “more important.”

2) Original Referee Comment: “Page 2, Line 33-34 & Page 3, Line 1: While I certainly agree that ERT is sensitive to fluids as indicated here, this justification for the ERT method seems incomplete because the several earth properties that control resistivity can be difficult to tease apart to attribute.”

Why the response is insufficient: The response indicates that a table is not included (not requested in the review comment) and logging data has been added (also not requested). The remainder of the response is primarily statements unsupported by evidence. An explanation of how the various material properties can be determined from resistivity as requested was not attempted.

3) Original Referee Comment: “P8-L10-12: As indicated above, the nature of ERT interpretations is that these several properties all affect the measurement together…”

Why the response is insufficient: This is essentially the same comment as in #2 above, and the author response is essentially the same. While saturation, porosity, fluid conductivity, and clay content all contribute to the total observed resistivity, the author only partially addresses porosity. This is central to the interpretation of the manuscript because presumably the gas-phase they a looking for will have an impact on saturation, and pore water conductivity is unknown – these properties would certainly effect the total resistivity, however there is no adequate discussion of the point at all in the revision.

4) Original Referee Comment: “Figure 2: I suggest either merging this with Figure 1 or Figure 3 to make a 2-panel figure, OR perhaps merging all three to make a single 3-panel figure.”

Why the response is insufficient: The only change made was the re-order the figures. The justification given for not addressing the review comment is that “each figure is so important.” To be completely clear: there is NO relationship between the “size or location of the figure” and “the importance of the figure.” How could combining the three separate figures into one figure with three panels possibly detract from their importance? Combining figures that are similar or have similar context is an important tool to improve readability – it will not make the information seem less important.

4) Original Referee Comment: “Page 14, L3-4: What figure does this refer to? I assume #6. “appears significantly smoother” Smoother than what? How do you know it is “significant”? If referring to Fig 6, left panel, then I disagree – if the authors intend to make this argument, then it should be supported by a quantified metric. ”

Why the response is insufficient: The review comment specifically requested a quantified metric to support the argument. This was outright ignored – no quantification is provide in the reformulated sentence.

5) Original Referee Comment: “Page 22, Line 3: “At at least one spot along our profile, the HMF, these fluids can propagate to the surface through the Tertiary sediments, but also at other sites expressions of fluid flow can be observed. ” Please explain how this can be observed in the data measured for this experiment. ”

Why the response is insufficient: The referee comments specifically requests that the author explains how this can be observed in the data measured from this experiment, however the response does nothing more than rephrase the sentence – there is still no indication of how this effect is directly interpreted from data or figures presented in the manuscript. Please explain how the data presented in this manuscript shows that fluids can propagate to the surface through tertiary sediments.

6) Original Referee Comment: “Page 23, Line 3: Figure 11 should be explained in the discussion, not conclusion. The conclusion section contains "summary" content and "discussion" - please rewrite this to focus on only concluding remarks.”

Why the response is insufficient: The response explains a formatting error related to the placement of the figure: this is completely unrelated to the content of the comment which suggests ways to improve organization of the text.

In addition, I remain unconvinced that this manuscript should be cast under the topic of “fluid pathways” as suggested by the Objective 2(p.6), and alluded to in the title. This point was made by both reviewers, but was ultimately not sufficiently addressed by the author. This is claimed to be accomplished in Figure 9, specifically through annotation with arrows that show interpreted fluid flow pathways. However there is no evidence in the geoelectrical image (Figure 8) of either 1) the PPZ/HMF nor 2) anything that could lead to the interpretation of the fluid flow arrows. Of course, the authors state in the abstract that “Distinct, narrow pathways for CO2 ascent are not observed with this kind of setup which hints at wide degassing structures over several kilometers within the crust instead.” However this apparently contradicts what is indicated in the Fig 9 cartoon that shows CO2 movement along specific, narrow conduits.

Finally, the conceptual Figure 9 is essentially a redrawn version of the borehole cross section with resistivity values added for each lithology. Confusingly, the spatial variability of resistivity is noted in the labels (apparently following the associated comments in the Interpretation section), but each lithology is drawn in the same color so it is not possible to actually tell how the resistivity values are distributed as shown in the Figure 8 imaging. This leads back to another reviewer comment that was not adequately addressed: what value has the ERT image actually brought to the conceptual subsurface understanding? It seems that everything the authors want to interpret is taken from the borehole cross section (e.g., discrete CO2 flow pathways that are not visible on the ERT image; overall subsurface structure and lithology that is better resolved by boreholes than from ERT; a 100 ohm m block is inferred on the conceptual model that was not observed in drilling, however it apparently has very little to do with fluid flow).

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RR by Anonymous Referee #2 (13 Jul 2019)

ED: Reconsider after major revisions (17 Jul 2019) by Ulrike Werban

AR by Tobias Nickschick on behalf of the Authors (27 Jul 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Jul 2019) by Ulrike Werban

RR by Marceau Gresse (26 Aug 2019)

Suggestions for revision or reasons for rejection

This article presents several 2-D ERT surveys performed in the Cheb Basin. One profile, 6.5 km long, allowed to reach a depth of investigation of 1 km, which remains unusual with D-C methods. Authors have done a careful work when processing/filtering the raw data which is appreciable.

This work brings interesting results that are quite consistent with boreholes data. In addition, the authors performed a gravity survey along a 2-D ERT profile and the anomaly seems reliable with the degassing area.

Authors are aware of ERT limitation, and globally made a good interpretation of the resistivity structure. However, there are 3 points I have raised about the inversion process where I request additional information. If these problems are cleared, I recommend to publish this work in SE.

Marceau Gresse

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Main points:

>> 1- During inversions, authors modified the classic vertical to horizontal smoothness factor. I wonder if the vertical conductive structures (so-called plume by authors) observed in Fig .6-2 are the results of the modified vertical to horizontal smoothness factor or not. What kind of structure is observed if a “classic smoothness factor” is used? What would be the RMS of a “regular inversion” VS the modified vertical smoothness factor? I request more explanations/results to justify the reliability of these vertical conductive structures.

>> 2- Authors do not provide information about the RMS (or chi-square) evolution during inversions. It would be interesting to show the convergence of models in successive iterations, especially for the long 2-D ERT profile.

>> 3- Page 15, line 30: I recommend to add additional information about the 1D sensitivity analysis used to determine the maximum depth of resistivity models.

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Minor points:

>> Abstract
- Page 1, line 8: I would suggest to change “the presence of low-resistivity rock fraction… » by « The presence of conductive rock fractions…”.
- Page 1, line 19: Authors explain that “gravity clearly shows the deepest part of the basin”, but they do not present how (in the abstract). I think authors could add few words in this sentence to show the main result of this gravity survey. Example “A negative gravity anomaly of **** is observed along the long ERT profile and is explained/consistent by/with ***** … showing the deepest part of the basin…”

>> 1-Introduction
- Page 2, lines 5-7: I think authors could add few references about CO2 sequestration experiments.
- Page 2, lines 7-9: I fully agree with this sentence, but once again, I think the authors need some references to justify it. I suggest to cite some work combining electrical resistivity images and CO2 degassing surveys on volcanoes and/or geothermal areas (e.g. Gresse et al., 2017, Revil et al 2010).

>> Figure
- Figure 1: Please add the coordinate system and its unit in this figure.
- Figure 6: It would be interesting to show the direction of each ERT profile. Information about faded colours (related to the depth of investigation) in the figure caption is also needed such as the final RMS of each model.
The maximum resistivity value reported on the scale is 200 ohm m. However in the text, “4.1 -Small-scale ERT: Page 19, line 1. Authors wrote “Profile 3 (Fig. 6 bottom) reveals a 10-15 m thick layer with resistivities above 300 ohm m”. Please correct the figure and/or the text.
- Figure 7: It would be better to show the direction of each profile. Information about the faded colours in the figures caption is also needed such as the final RMS of each model.
- Figure 8: Authors wrote “m a. s. l.” but they also use negative elevation values (below the sea level). Please change this unit.

>> Other minor comments:
- Small Scale ERT, page 17, line 32 “…strong degassing at surface in previous studies…” should be replaced by “…strong degassing as reported at the surface in previous studies …”
- “Ground water level” should be written as “Groundwater level” or “Water table”.
- Few “))” appear throughout the text.

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RR by Anonymous Referee #1 (11 Sep 2019)

ED: Publish subject to minor revisions (review by editor) (25 Sep 2019) by Ulrike Werban

AR by Tobias Nickschick on behalf of the Authors (29 Sep 2019) Author's response Manuscript

ED: Publish as is (30 Sep 2019) by Ulrike Werban

ED: Publish as is (06 Oct 2019) by CharLotte Krawczyk (Executive editor)

AR by Tobias Nickschick on behalf of the Authors (12 Oct 2019) Manuscript

Short summary

An active CO₂ degassing site in the western Eger Rift, Czech Republic, was investigated with a 6.5 km long geophysical survey using a specific large-scale geoelectrical setup, supported by shallow geoelectrical surveys and gravity measurements. The experiment reveals unusually low resistivities in the sediments and basement below the degassing area of less than 10 Ω and provides a base for a custom geological model of the area for a future 400 m deep research drilling in this area.