Characteristics of Deep Groundwater Flow and Temperature in the Tertiary Pohang Area, South Korea

Round 1

Reviewer 1 Report

Dear authors,

The paper deals with the evaluation of 4 deep wells in the Pohang area in South Korea. In particular, the well logs have been analysed and interpreted to evaluate the groundwater flow direction and magnitude through a 1-D steady state model.

I already read a previous version of the paper and this is a reviewed version re-submitted to the Journal. The abstract and introduction are fine, even if some more state of the art can be added and the objectives of the study better clarified. The section 4.1 can be moved to the geological setting in my opinion and all the information about the depth and angle of the fractures, position of the casing and so on could be summarized or even erased. It’s a lot of information that I am not really sure is important for the reader, maybe think of putting it in an appendix.

About your calculation of the groundwater flow and β values, I still do not understand how did you apply the thermal conductivity. I mean did you weight it in your depth intervals? Or you used just one sample in each depth interval you considered? This is an important information for the reader and for the interpretation of the results.

In conclusion, I understand that you did the characterization of the groundwater flow in these wells, but this is a method do attain certain objectives, which I am not sure to understand. It seems to me that this is only a report of geophysical logs, not a scientific paper. A paper should present objectives, methods to attain those objectives, the most important results for the reader and a proper discussion to be helpful for other researchers or professionals around the world. Is there any geothermal potential in this area? Is it interesting for power production or direct use of heat? To me, these can be some useful conclusions that you could draw.

It can be that there is scientific soundness in this paper, but I am sorry to say that I am missing it, and like me other readers can. So I suggest you to better clarify the objectives and draw some interesting conclusions that I am sure you can extrapolate from all this big amount of data.

Specific comments are given in the following:

Line 55: USING rather than uses

125-126: “it should be measured precisely” it’s trivial and not scientific, please reformulate

127: what’s the difference between heat and thermal diffusivity? Please erase one of them

129: the 3% deviation is the accuracy of the instrument I guess? Or it’s the difference between your measurement and literature? It’s not clear. If it’s the second you should put it in the results with more details. But I guess it’s the accuracy. Please comment on that

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Revision satisfactory.

Author Response

첨부 파일을 참조하십시오.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript considers geological measurements of temperature and deep groundwater flow in South Korea through borehole installations. The data analysis is very exhaustive and detailed, nevertheless I have a number of questions and suggestions before approving the paper for publication.

- The English language is quite understandable; I however recommend a full revision of the text.

- It is not clear to me whether the paper has a novelty value regarding the data analysis approach, besides the set of collected data. This is not clearly stated neither in the abstract, introduction or conclusions.

- Maybe I overlooked it, but how do you justify the steady state hypothesis?

- l.56 it seems to me that the claim by Kim about the earthquake has no logical follow up in the text below. Was that an additional information or is your study related to this issue?

- Related to the above, the ending paragraphs of Introduction should state very clearly the motivation of the paper and its main novelty.

- Figure 9: it would be very useful to add the error bars to the experimental data, to visually assess how the predictions deviate from the measurements esp. for 0.2<zD<0.4.

Regarding the latter zD interval, how do you interpret such a large deviation of the BH-2 data from the general trend? I think this relates to the anomalies discussed in the Discussion, however I advise to address the issue immediately in the text referring to the figure in that same section.

- Discussion: anomalies are correctly reported in several paragraphs yet are never analysed. Do you have any conjecture about their origin? What do they imply for the case at hand and for its applications?

- The entire Discussion section is barely reporting results without any proper “discussion” and search for explanation indeed. Here you are supposed to speculate on the origin of deviations, anomalies, correlations (such as in lines 427-429) etc.

- Conclusions: please add a paragraph addressing the limitations of the study, which seem not to be considered anywhere in the present text.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear authors,

my comments to your answers are in the file attached.

 

 

Comments for author File: Comments.pdf

Author Response

We sincerely thank you for your review. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Please find my review attached.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

The paper can be now published after a small revision:

1. add error bars to Figure 9
2. add to the text the following explanation to my question: "The thermal conductivity values of 2.16 - 3.95 W/mK (Table 1) and thermal
   diffusivity values of 0.71-1.60 mm2
   /s seems reasonable by the comparison between the
   reference values (Zoth and Haenel, 1988; Robertson, 1988; Park et al., 2009) and measurement
   values of the thermal diffusivity and thermal conductivity as shown the table below. " and the according table reporting reference values for Thermal diffusivity and Thermal conductivity.

Author Response

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Author Response File: Author Response.docx

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

No specific comments to the authors; review goes to Editor

Comments for author File: Comments.pdf

Reviewer 2 Report

Dear authors,

The paper deals with the evaluation of 4 deep wells in the Pohang area in South Korea. In particular, the well logs have been analysed and interpreted to evaluate the groundwater flow direction and magnitude through a 1-D steady state model.

The introduction misses a comprehensive state of the art about geothermal energy in Korea, EGS systems and geophysical logs (with purposes of each of them). You mention that an earthquake occurred and that this could be related to the use of these wells, but no other information is given so the reader cannot understand the general social context of these geothermal wells and your study as well. Some more information are necessary in the section 3.1, in particular in the estimation of the thermal conductivity. How did you measure the porosity, density and specific heat of your samples? Were your samples core plugs or you just used drill cuttings?

The reliability of the 1-D calculations is in doubt in my opinion, because, if I got it right, you did a general average of all your physical and thermal properties along your boreholes. Apart from missing some information on how the samples were measured and how porosity, density and specific heat were obtained, if you do not do at least a weighted average, your model will never be reliable and sound.

In general, I do not find a good explanation of all the results that you get. There is a long list of all the values of well logs and depths of possible anomalies, but nothing that put them altogether and give the reader a comprehensive view of all the study. The purpose of the study was just to analyse the data and make a 1-D model? Or the purpose of the model was to find possible aquifer zones of geothermal interest? What’s the point of highlighting upward or downward flow in the wells? It means that if we drill a well in a zone with upward flow we will have artesian well and we wouldn’t need to pump the water? This is some discussion you should provide in your paper.

A conclusion section is missing. Actually, this is what you called “Discussion” but then a discussion of your results is missing. Please put your results in comparison with other studies or go more into details of your findings, as before mentioned. What these results can be used for? Put some hints for future activities and perspectives in the conclusions, will you suggest to target a specific aquifer for possible geothermal interest? Maybe not for EGS, if the earthquakes can be related to these boreholes, but for heat production?

I am sorry to say that, in my opinion, the paper is not ready for publication as it is. As before stated (and more in the following comments), some improvements in the structure of the paper should be done. Some better explanation of the methods is necessary and some more detailed discussion of the results should be provided. And most of all, the reliability of your 1-D model should be defended.

Specific comments are given in the following:

Line 18: erase “south Korea” it’s redundant since you already said it before

18 and 21: since you use “calliper” that seems to me a UK term, be sure that you use all UK terms throughout the paper

23-24: it is not clear if your ranges are all negatives or the second value is positive (as I guess). I suggest you to indicate them as, e.g. -1.52 to 4.10 or -1.52 to -4.10 depending on the case, or -1.52 ÷ -4.10

40-41: what do you mean with “there has been a lack of thermal energy generated from it”? from the volcanoes? I don’t get the point

60: erase “properties”, geothermal flow and temperature are not properties

77: I suggest “have increasing diameters with increasing depth”

79-80: what’s the diameter of BH-3

86: fig. 2 does not show the thickness of these rock formations. Are you missing a Figure?

120-166: All the info you give is very difficult to follow. First of all, can you give this info in terms of m a.s.l rather than relative depth? It would help. Moreover, can you make a figure with the stratigraphy of the four wells and put some of the information you give in those lines? It would be much better for the reader to compare the data of the different wells.

167-168: I think that m a.s.l is fine (rather than amsl that I guess means above mean sea level). But really you have the water table 100 m below the sea level? This seems very weird to me. In particular because we are only 10 km away from the sea. Can you comment on that?

175: what do you mean for two phases of rock matrix? What do you mean for liquid rock matrix?

Equation 3: add here what are α and ω. Or put the reference to fig 2

184: “…is the vertical distance BETWEEN z= 0 and z = L…”

185-186: this β value seems interesting but I did not understand if it is something proposed in literature and used in other contexts before, or if it is something you propose.  In the first case, please add references. In the second case, please add more details because some other researchers could be interested in using it. Since you put the results of this value in the abstract as well, it means that it is important

193: the common name is “thermal conductivity”, it’s not only a property of geologic materials, but of all the media.

194-196: please give more information about the instrument. Does it use a transient or steady-state method? What’s the accuracy? What kind of sample do you need for doing analyses with this instrument? Please give also some references

197-200 and equation 8: so the instrument analyses the thermal diffusivity, but how do you infer the density and specific heat of the sample to get the thermal conductivity? Do you need other instruments?

201-202: “…according to the state (dry or saturated) AND THE porosity.”

209: I suggest “The temperature log can show different temperature trends…”

215: I think that this last part of the sentence is useless, redundant

241: not easy to understand “the casing was installed at depths of 6 in to 58 m, 4 in to 366 m…”. you should write “a casing of 6 in diameter was installed down to a depth of 58, then a 4 in diameter to a depth of 366 m etc.. Unfortunately, inches are not of the SI metric system, so it would be advised to avoid them. I know they are commonly used, but it would be better if you used millimetres. Or, at least find a way in order to be clear without misunderstanding as mentioned here

241-250: I am not sure this part belongs to the results, it is more part of the methods. And again, all this info is difficult to read, wouldn’t it be better a figure with these data shown? Or maybe a table. Please think about it. For example, in Figures 4-7 you already give a qualitative info on the casing, if you want you can add some numbers there and that’s it

Figures 4-7: please move the legend of the lithologies from fig 7 to 4. Or add in each caption the meaning of the symbols S-MS, DA etc… In lines 213-214 you say “Temperature logging is mainly used for estimating the groundwater flow and location of aquifers” but then you place some arrows in the EC logs. What do these arrows mean? Is it for groundwater flow? On which criteria you put them? This is something you have to explain in the text

275-282: could you highlight better these anomalies? It’s impossible for the reader to find the 424 or 461 m with the scale available in figures 4-7. Could you also detail better? What’s the purpose of evaluating them in your study?

284-285: “an accurate temperature log was critical in determining the geothermal gradient and thermal conductivity”. Did you use the temperature log to get the thermal conductivity? if so, what value of heat flow did you use? And what about the thermal conductivity analyses with the instrument LFA-447 NanoFlash?

308-309: “In this study, the geothermal gradients of the boreholes were estimated to examine groundwater flow pattern”. So what are the results? Here you presented some values that we don’t know if they are your calculations or if these are data of reference 7 (Song, 2006)

311-324: please refer to the arrows in figures 4-7. Please explain how you discriminate between an inflow or an outflow, this is not clear.

316: here you say that the EC ranges from 800 to 3,700 in BH”, but few lines before you say that the maximum EC in BH2 is 1,162. Where is the error?

341-348: can you please detail the lithologies that you analysed? You have rather different geological formations, you cannot do an average with all of them. You could make a weighted average once you know the thermal conductivity of each lithology and then weigh it based on the their thickness

344: how did you get these porosities from a geometric mean? Of what?

345-346: so the samples that you measured were completely dry? Did you oven-dried them before the analyses?

347: the harmonic mean should be used when you have values of minerals and pore fluids and you adopt a mixing law. How did you used this harmonic mean?

349: please be consistent with the unit measure. Either you write W/mK and J/kgK or W/(m·K) and J/(kg·K). also, once you write the specific heat as kJ/kgK and another time as J/gK, please be consistent

349-358: why did you do a harmonic mean for the density? Why do you also have values of thermal conductivity for BH-2 and BH-3 when few lines before you say that you just have samples for 1 and 4?

359-362: I am not really sure that it’s useful to make an average of those properties all along the borehole. As I said before, the geological formations are pretty different, it’s not scientific unless you do a weighted average as I said before.

Table 1: Specific heat and density are misplaced in the headers

Figure 8: this figure should be better explained. What do you have on the x axis? What do you mean for function of β and z/L? if β is fixed (e.g. to 3.95 in the left graph) what is changing if the parameter z/L is already in the y axis?

398-399: “…and field hydraulic tests on deep geothermal boreholes…” you did not talk about hydraulic tests in the paper…

Nicolò Giordano

Reviewer 3 Report

In their manuscript "Geothermal Water Flow and Temperature Characteristics of Tertiary Pohang Area in South Korea", Lim et al. present data concerning 4 wells in the Pohang area (South Korea).The analyses include well log integration and extensive measurements taken on the temperature, electrical conductivity, caliper and natural gamma logs from boreholes at different depth. From the compilation of the information, the authors attempt explaining how geophysical logging can help for characterize geothermal properties (for sure, these information are relevant and are essential for geothermal prospections). However, syntheses, conclusion and outlooks of the work presented are inexistent. They also argue that the tendency of the temperate decreasing from the surface to a depth of 100m indicates the influence of the surface air temperature on the underground temperature due to global warming (citing several papers). What is the point to refer to such conclusion?

Concerning the form, the paper is poorly structured, and it is poorly written. It would benefit from a thorough revision from a native English speaker.  All figures don’t have a correct caption description, the table presented has mistakes (e.g. units) and most of the references are rather old (I assume that today (2020), the authors could cite more recent work related to the topic). Moreover, several of the figures are overly simplistic and not sufficiently descriptive, as if they were prepared for a poster or short PowerPoint presentation. In several cases, the figure captions don’t contribute to the intelligibility of the figures as they are nearly directly copy/pasted from the text.

Concerning the content, the paper makes describe geophysical methods which are used for geothermal borehole prospection in general. There is no detailed description of its hydro-thermo-mechanical physical characteristics in the paper because the data presented do not permit to do so. No real groundwater dynamics, heat transfer or geomechanics is presented. Geothermal gradient, hydrogeological recharge are presented but not really discuss and the description of thermal fluids are also rambling. The manuscript would greatly benefit from a deeper presentation and discussion of the data, and how these data can explain the hydrogeology and the geothermal potential of the Pohang area.

Writing a paper on geothermal processes without touching seriously on any of these aspects indicate to me that the authors didn’t make their homework in terms of literature review of what was already published on geothermal borehole characterization.  For this reason, I advice to reject this manuscript, but encourage the authors to pursue their effort in geothermal analyses in the region of Pohang area, by performing a serious thermo-hydro-mechanical analyses of their data.