Nondestructive Testing of Hollowing Deterioration of the Yungang Grottoes Based on THz-TDS
Round 1
Reviewer 1 Report
The authors describe a simple and well-known method to retrieve the layer thickness of a specific layer in a three-layer system. They apply a simple model that completely neglects the material dispersion. There are more sophisticated algorithms known for this kind of problem for more than 20 years. If the authors had at least presented some on-site results from the Yungang Grottoes, this work might be interesting for preservationists. But as a general proof-of-concept, this work comes many years too late. The THz TDS system is not new, the algorithm is not new and not very sophisticated, and the sample is not interesting. Therefore I recommend the rejection of this manuscript.
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Reviewer 2 Report
In this manuscript, the authors report some THz-TDS measurements to acquire the hollowing deterioration width under stone surface. This type of THz-TDS technique has been well investigated. Nevertheless, the specific application seems very interesting and meaningful; there is certain novelty in the investigation.
In general, the presenting quality of the manuscript is relatively poor and need to be significantly improved. I also recommend the authors to re-organize the structure of the manuscript somewhat.
A few more detailed comments:
1). The time domain signal is rather noisy. What is the measurement condition - was it in humid condition or relatively water vapor free condition? THz frequency domain information is always very informative.
2). What is the consideration for choosing 2-mm-thick sheet stone? Will a thinner sheet dramatically improve the S/N quality of the investigation? How absorptive is the stone sheet, and what is the refractive index? These should be very useful information and also relatively easy to measure with the same TDS system.
3) Concern the two major plots, fig5 and fig6, the authors should use error bar to mark the uncertainty of the signal time delay tracing. How precise delta_T (and therefore d_2) can be determined? Given the fact that some of the time domain waveform signals have rather low S/N ratio, the error bar should be very necessary. I would therefore doubt about the 0.99795 linearity as stated in the manuscript.
Overall, major revision is recommended for this manuscript before any further consideration for publishing.
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Reviewer 3 Report
The authors present the THz-TDS Testing of Hollowing Deterioration for the Yungang Grottoes. The idea is interesting, the methodology is sound and the paper is well written. They find that THz testing can efficiently detect hollowing deterioration and its thickness. This method is promising for applications of the THz spectroscopy for the investigation of cultural relics. Some minor spelling and style corrections are possible, e.g. the term linearity in the abstract should be defined as the correlation coefficient or R-squared value, before it is first used.
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Reviewer 4 Report
Authors performed nondestructive testing of hollowing deterioration for the Yungang Grottoes which is useful for cultural relic preservation. Their test method and analysis are reasonable and the submitted paper is suitable to publish in the journal “electronics” after minor revisions described as follows;
- Assumption using Eq. (3) is justified when the thickness of the flaked stone [d_1 in Eq. (2)] is constant. I agree that the authors’ test experiment satisfies this condition. How about the real Yungang Grottoes sample? Is it reasonable to assume that the thickness of all the flaked stone is constant? I think this question is important for real NDT applications.
- The authors should shortly comment on the contribution of T_2 in the THz spectra in Fig. 5. This peak should be between the peaks of T_1 and T_3, and its amplitude may not be so small because the refractive index of the stone sheet is large.
- Authors describe in line 96 that “Moreover, echo energy detection could reveal the presence of a target and its location, size, shape, density and intensity. Therefore, according to the THz time-domain spectra of the hollowing deterioration samples, we can obtain their scale by the reflection THz-TDS method.” However, in the present article, they only analyze the time delay of the echo signal, and do not try the above estimation using the amplitude information. I advise the authors to delete these sentences.
- Line 111: I cannot find discussion in ref. [13] concerning the determination of d_2 (Eq. (1)). Ref. 13 does not describe the THz wave propagation theory. Please check whether the citation is correct or not.
- There are many typographic errors in the texts. Authors should carefully check all the spellings. Examples are as follows;
Line 86: d2-->d_2
Line 107-110: S1-->S_1, T1-->T_1 (also T2, T3), Es1-->E_{s1} (also Es2, Es3)
Line 163: THz domain spectra-->THz time-domain spectra
Line 254: omposites -> composites.
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Round 2
Reviewer 2 Report
I am pleased to see the significant improvements the authors have made. I'd like to recommend publication of the manuscript in the condition that the authors go through the manuscript with fine tooth comb and improve the overall quality.
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