High-Frequency Dependence of Acoustic Properties of Three Typical Sediments in the South China Sea

Round 1

Reviewer 1 Report

The authors provide measurements of sound speed and attenuation through three different types of sediment from the South China Sea and one from the Western Pacific at five different frequencies, and calculate the dispersion of these from the measurements. They fit the sound velocity and attenuation dispersion using the Biot-Stoll model of wave propagation employing parameters from the literature and the measurements. 

 

This is an interesting contribution that provides field data and modeling parameters of great interest to those who are focused on wave propagation in sediments. I recommend its publication.

 

Detail

A recent publication on a variant of Biot’s theory, La Ragione, et al. (Wave Motion 99, 102653, 2020),  may be of interest to the authors.

Author Response

Thank you for your advices. We will read the  publication later. thank you

Reviewer 2 Report

This manuscript describes the modeling of the compressional wave speed and attenuation, using the Biot-Stoll model, of sediment cores from four areas - 3 from South China Sea (SCS) sites and one from a Western Pacific (WP) site - in the frequency band 27 to 247 kHz. It was concluded that the Biot-Stoll model provided a good fit to the measurements. 

 

The text says “three fine-grained sediments in the South China Sea (SCS) were statistically analyzed” but there are actually four. The fourth is the Western Pacific (WP) sediment. Why is this not included in the same paragraph?

 

A map of the area from which the cores were taken would be useful, in order to get an idea of the bathymetric features, proximity of rivers, and other nearby factors that may have some influence on the sediment.

 

It is stated that there were “37 stations of silty clay sediment samples, 31 stations of silty sediment samples, and 12 stations of silty sand sediment samples”, yet only single values of speed and attenuation are given. With so many samples, both the average value and the standard deviation of each measurement point should be given. Without the standard deviation, there is no way of estimating the quality of the measurements.

 

The process of fitting the Biot-Stoll model to the measurement needs to be described. Of all the input values in Table 2, it appears that only the porosity and the grain size were measured. The calculation of the permeability, pore size and tortuosity needs to be described. The frame bulk and shear moduli appear to have been adjusted to fit the measured sound speeds, and this process needs to be described in some detail. The process of estimating the imaginary parts of the frame moduli needs to be described. It seems that the attenuation is insensitive to the imaginary part of the frame moduli, because the values do not change much. Perhaps they could be zero. 

 

The reference “Chotiros 2021” is not mentioned in the text, and therefore it should be removed from the reference list.

 

Minor comments:

 

In the abstract and in other parts of the text “liner” should be “linear”.

“long and narrow parallel to the coast” needs to be rewritten.

“different frequency recorded” should be “different frequencies recorded”

A reference for the Biot-Stoll equations (Eqs. 1 – 16) is needed.

“closed related” should be “closely related”.

“quadratic frequency dependence” should be “power-law frequency dependence”

“bule” should be “blue”

 

Author Response

The reply is in the attchment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Review of: Manuscript ID: jmse-1830013 ‘High-frequency dependence of acoustic properties of three typical
sediments in the South China Sea’

This paper presents results of laboratory measurements of acoustic properties (wave speeds) of marine sediment samples. The measurements were made at high frequencies (27 – 247 kHz).   The samples were different sediment types (silt, silty-clay and silt sand) from South China Sea (SCS), and results from these samples are compared with results from measurements of clayey-silt from a Western Pacific site.  The measured values of compression wave speeds and attenuation at the high frequencies are interpreted using the Biot-Stoll model of sound propagation in porous media.  The main findings include: sound speed dispersion is very weak in the three SCS sediment types;  the variation of attenuation with frequency is not exactly consistent with predictions from the Biot-Stoll model; an inflection in the sound speed and attenuation dispersion curves is noted and the frequency of the inflection is related to the permeability of the sediment type.

I have several questions that need to be addressed in a revised version of this paper.  The questions on technical and interpretation issues are listed below.  There are many corrections that need to be made to improve the English usage, and these are provided as annotations in the pdf file of the paper.  There are far too many to list separately.

My recommendation is that the authors need to address the issues raised in my review, and make appropriate revisions to the text.  The article is not ready for publication at this stage.

Major issues:

I have a fundamental problem with measurements such as the ones presented in this paper that are made in laboratory conditions.  Although the authors describe the acoustic measurement system, they do not provide sufficient description about the measurements themselves or the condition of the sediment samples in relation to the in situ conditions. 

1. Apparently there were many samples available, so the question is did the authors use just one sample of each type of sediment, or were many samples of each type used?  If many, how many?

2. What type of sample was used?  Was the sample from a sediment core?  If so, what was the depth of the core beneath the sea floor?  And at what depth in the core were the acoustic measurements made?  This is important.  The measurements are related to sediment properties at some depth beneath the sea floor.  Readers will want to know the depth beneath the sea floor.

 3.  How were the acoustic measurements made?  I assume from the photo of the equipment that the sample was placed between the transmitter and the receiver.  What was done to ensure good contact between the sensors and the sample?  This is important; if there is not good contact (for instance air spaces) the measurements are distorted.  It would be useful to know how each type of measurement (sound speed and attenuation) was made.  For instance, I assume the sound speed was inferred from travel time?  Over what distance?  Same question with attenuation.  I assume the waveform amplitude or energy was measured after transmission through the sample?  This is a very difficult measurement to make.  Readers will want to know how it was done.  One suggestion is to show examples of the attenuated waveform compared to the transmitted waveform.

4.  Another important question that applies to laboratory measurements is the actual condition of the sediment sample.  This is important, because the laboratory conditions could be significantly different from the conditions in the sea bed.  This is very important, because the authors relate one feature of their measurements (the inflection point of the dispersion curve) to the permeability of the sediment.  I assume that the laboratory conditions were very different, but it depends on what was done to preserve the samples.

5.  The authors relate one of their findings to the permeability of the sediment.  It is very difficult to measure permeability.  Was the permeability measured in the laboratory?  I assume the permeability used in the interpretations was the permeability inferred for the type of sediment.  The authors should discuss whether the laboratory conditions properly represented the permeability of the sediment in its natural conditions.

Interpretation using the Biot-Stoll model. 

1.  There are other models of sound propagation in marine sediments that could have been used to interpret the results, for instance the Buckingham viscous grain shearing model (VGS).  The VGS model has been used by many authors to interpret their results, and the comparisons with measurements over a wide frequency band are very good.  I do not require that the authors extend their work to compare with the VGS model, but they should at least make a reference to the model and state that it is an alternative approach.  And perhaps in future research, the authors could compare their results to VGS, for comparison with the interpretation using the Biot-Stoll model.  A reference to Buckingham’s model is:

M. J. Buckingham, “On pore-fluid viscosity and the wave properties of saturated granular materials including marine sediments,” J. Acoust. Soc. Am. 122, 1486 (2007).

Corrections to the text:

There are far too many corrections and revisions to list separately, they are provided in the attached annotated file.  One issue that I will state here is about the incorrect use of ‘velocity’.  In water saturated sediment material there is no preferred direction to the sound propagation.  It is a scalar, not a vector.  So the technically correct term is sound speed.  This should  be corrected throughout the text.  I have highlighted all the instances in the annotated file.

 

 

Comments for author File: Comments.pdf

Author Response

The reply is in the attchment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Most of the comments were addressed. I can now recommend acceptance.

Author Response

thank you for your comments

Reviewer 3 Report

Review of revised version of ‘High-frequency dependence of acoustic properties of three typical
sediments in the South China Sea’

The authors have adequately addressed the concerns I raised in my review of the original version of the paper, and the revised version is more complete.  The authors also provided an extensive description of the experimental method in their response letter.  Although it would have been useful to provide this information in the revised version, I am satisfied that readers can consult the reference to the authors’ previous paper that explains the method. 

There are a few corrections that need to be made in the revised version, but they are all very minor edits.  I have also provided an annotated version of the pdf file.

I would like to suggest to the authors that in future work in which they report the results of measurements over a number of samples that they provide the standard deviation of the averaged results, as well as the mean.  I realize the authors have provided the maximum and minimum values over the set of measurements for each type of sediment.  However, in addition, the standard deviation gives a sense of the uncertainty or experimental error in the results.

My recommendation is that the paper can be accepted with the minor corrections.

Page 2 line 68:  Change to ‘..widespread attention..’  (‘very great’ attention is not a good choice).

Page 2 line 81:  Change to: ‘Ragione et al. (2020)…reported a study based upon…’

Page 2 line 83: delete ‘has’ ; and insert ‘..developed the viscous grain shearing…’

Page 2 line 84: make two sentences:  ‘…sediments.  The VGS…’

Page 3 line 128:  change to: ‘measured in the top 0.3..’

Page 6 line 210:  it would be useful to change this to ‘’different sediment types including the ones from the three study areas…

Page 6 line 215 and page 8 line 251: ‘It is..’  (capitalize ‘It’)

Page 8 line 251: change to ‘…follows a power law..’

Page 10 line 331: change ‘is’ to ‘are’

Page 11 line 346: delete ‘is’

Page 11 line 372:  change to ‘In the low frequency band..’

Page 11 line 374: change to ‘In the high frequency band…’

 

 

 

 

 

Comments for author File: Comments.pdf

Author Response

thank you for your comments, we have corrected all the errors you have mentioned, thank you.

Author Response File: Author Response.docx