Improving prediction of particle size with a novel acoustic bedload monitoring system consisting of phased microphone arrays and accelerometer

Chen, Zheng; He, Siming; Badoux, Alexandre; Rickenmann, Dieter

doi:https://doi.org/10.5194/egusphere-2024-2525

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https://doi.org/10.5194/egusphere-2024-2525

Preprints

23 Sep 2024

| 23 Sep 2024

Improving prediction of particle size with a novel acoustic bedload monitoring system consisting of phased microphone arrays and accelerometer

Zheng Chen, Siming He, Alexandre Badoux, and Dieter Rickenmann

Abstract. Accurate measurement of bedload transport flux in rivers remains an important issue in hydraulic engineering. Acoustic-based devices provide a promising way to measure the transport rate with established calibration relationships between the signal and bedload particles. We develop a novel acoustic bedload monitoring system with phased microphone arrays (PMA) and accelerometer to localize the particle impact location and to better determine the particle size. Impact experiments with quartz spheres in a flume setting were performed to investigate the dynamic signal response of the PMA monitoring system for varying particle size and impact location. For a similar virtual set-up, the conventional beamforming method was used to determine the source characteristics of the acoustic wave on the scanning plane of the PMA structure. The model provides a calculation of the cross-power matrix of the recorded pressures generated by bedload which localizes the particle impacts on the plate. The results give correlation relationships between the number of signal impulses per particle mass, the amplitude, and the centroid frequency and the bedload grain size. The findings of this study contribute to the measurement of the bedload transport with the PMA system, which helps to localize the bedload impact positions and improves the predictions of particle size.

Received: 09 Aug 2024 – Discussion started: 23 Sep 2024

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Zheng Chen, Siming He, Alexandre Badoux, and Dieter Rickenmann

Status: final response (author comments only)

RC1: 'Comment on egusphere-2024-2525', Anonymous Referee #1, 18 Nov 2024

comment number	line number	comment
--	--	I want to congratulate the authors on a novel experiment using a microphone array to improve impact-based bedload surrogate systems. Their work represents an incremental, but crucial, step towards an effective instream surrogate system. However, I believe that the manuscript, as written, requires some moderate changes before it is ready for publication. I have detailed those changes below with relevant line numbers.
general comment 1	--	The paper has a number of grammatical errors that should be corrected. I note some of them below in my line-specific comments but also wanted to add a general comment here. This is not at all a criticism of the lead author. Rather, I believe that the coauthors that are responsible for review and supervision should take a more careful review before resubmission of the manuscript. This is particularly noticeable in the introduction and methods sections of the manuscript.
general comment 2	--	Many of the figures in the results section appear before they are referenced in the manuscript. It appears intentional, but it's not typically how papers are structured in my experience. I defer to the preference of the editorial staff, but would like to see the figures referenced in-text before the associated figure is shown.
general comment 3	--	I think the discussion may benefit from some reorganization. My preference would be sections reorganized as follows: 4.4, 4.1, 4.3, 4.2. This would approximately match how the results are organized (number of microphones, microphone location, centroid frequency, grain size).
general comment 4	--	In additional to general comment #3, I would appreciate some discussion of how this new system may perform under field conditions. How would the PMA system work with more than two impacts at once? In principal, the system should be able to better resolve these impacts based on your laboratory results, but I think a discussion of this is warranted. This technology is intended for deployment in the field to improve our understanding of actual bedload flux. A new subsection should recognize this, discuss the similarities and differences between systems already in place, and how this system may fair.
specific comment 1	1-3	I think the title is a bit misleading. By my understanding, the improved particle size is just one of the improvements described in the manuscript. I would highlight the novelty of PMA system as a whole in the title.
specific comment 2	18-19	This sentence structure is a little confusing - is it a single regression or different regressions between impulses and the rest of the variables?
specific comment 3	15-16	What is the 'conventional beamforming method'? - you describe it below so I think just referencing the section below is sufficient here.
specific comment 4	50	other studies have investigated the effect of grain size when using impact sensors, such as Halfi et al., 2021 or Stark et al., 2024
specific comment 5	89-91	I don't think that specifying the manufacturer is necessary. Keeping it in would be fine though
specific comment 6	100-102	are these calibration coefficients universal across all systems, or is this established for every system prior to use?
specific comment 7	113	The use of eccentric seems unusual here, but is technically correct. I think something like center and off-center may be more intuitive to the reader, rather than centric and eccentric, but accept that there is nothing wrong with the word choice. Still, I would appreciate you explicitly state what a centric and eccentric impact entails here, in the methods, rather than waiting to the results to define it.
specific comment 8	113-114	You note that you conducted the experiments in both air and water, but it isn't clear in my reading of the results which you report. Is it both?
specific comment 9	125-126	Each section (such as this one) often begins with a statement of what the section is intended to accomplish. I don't think this is strictly necessary, and could be removed for brevity but if the authors feel strongly or prefer this, then I think it is fine to keep them!
specific comment 10	216	Are these all the relevant parameters or just some of them? It appears to be all, and if it is not, then the table should be expanded to include the parameter set used in the numerical simulations.
specific comment 11	257	Are centric and centroid frequencies the same thing? I don't think so, based on my understanding of the paper. Please ensure it is centroid frequency and centric impacts
specific comment 12	290-292	I think it would help the reader to include the microphone locations in Figure 6. Perhaps an inset or an overlay somehow? If not, you should reference the Appendix Figure showing the different arrangements here.
specific comment 13	296	Section 3.1.3 -- You only reference the multipole results a single time in your discussion. I do not think this is necessary to be included in this manuscript. To be clear - I think that multiple sources is critical for using this surrogate technology (detailed in another comment), but not necessary for this manuscript.
specific comment 14	298	fig 7 - Several grammatical errors in the figure caption
specific comment 15	328	fig 8 - This caption contains discussion/results content and is unnecessarily long. I would revise it
specific comment 16	338-344	Much of this could be introduced in the methods section, rather than in the results in my opinion.
specific comment 17	408	I believe that figure 11 could be recreated to be more effective. I suggest plotting (a) and (b) by themselves in a vertical orientation, with (c) - (f) in a square orientation separated by a vertical line.
specific comment 18	471	Section 4.1.2 -- The discussion of k_IMP is extremely short, given that this is one of two primary functions of this technology. I think this is a critical part of the manuscript that is generally overlooked throughout. This could also be a section where you discuss how this new system might perform under field conditions.

technical comment 1	211-212	Awkward working of this first sentence.
technical comment 2	212	'firstly' is incorrect
technical comment 3	252	extra parentheses in see eq. 14
technical comment 4	278	Mon. is already introduced as an abbreviation of monopole
technical comment 5	305	Mul. is already introduced as an abbreviation of multipole

Citation: https://doi.org/10.5194/egusphere-2024-2525-RC1

AC1: 'Reply on RC1', Zheng Chen, 09 Aug 2025

Dear Reviewer #1,
We sincerely thank you for your positive and constructive feedback. We are grateful for your recognition of the novelty and potential of our acoustic bedload monitoring approach using phased microphone arrays. Following your suggestions, we have made moderate revisions to improve the clarity and completeness of the manuscript. All changes have been highlighted in the revised version, and detailed point-by-point responses are attached.

Sincerely,
Dr. Zheng Chen (on behalf of all co-authors)
Corresponding author
Email: zheng.chen@cdut.edu.cn

Citation: https://doi.org/10.5194/egusphere-2024-2525-AC1

RC2:
'Comment on egusphere-2024-2525', Anonymous Referee #2, 13 Jun 2025

The manuscript by Chen et al. presents a novel approach to river bedload monitoring using a phased acoustic array together with an accelerometer sensor built into a modified version of a Swiss Plate Geophone casing. The study aims to locate bed particles on the cover plate of the device and exploits the wave train content to estimate the particle size. The team presents laboratory aided tests of the principle and performance of the system and compares the results with earlier experiments using the “classic” Swiss Plate Geophone devices.
The study addresses a relevant and timely topic, suggesting indeed a possible step forward in the field of continuous bedload transport research, going beyond what existing approaches can deliver. The concept of using a phased array appears overall suitable to be enrolled for such a customised case. That said, I feel the study remains somewhat premature and incomplete in the methods and tests it presents and is likewise in a not very organised structure in terms of writing and figure design, which prevents the reader from clearly grasping the line of arguments.
I see a main unknown and potential flaw in the design to relate impact amplitudes with grain size while keeping the drop height for each particle somewhat constant. It is hard for me to imagine how under natural conditions there will be constant drop heights for particles of varying size. Or, the other way around, how would the approach successfully estimate the correct grain size if river bed particles hit the instrument with different fall velocities and overall differing horizontal velocity vectors? What about those particles that rather slide and roll instead of jumping? These fundamental questions are neither raised nor implicitly addressed by the study, neither in the experiment design nor in the argumentation.
In writing “somewhat constant” in the above paragraph, I wonder why the height of centre of mass of the particles above the plate was kept constant instead of keeping the full vertical drop height constant. This adds a further unnecessary (or unexplained) variable that changes the experiment results.
Another, major flaw arises from the inability to locate and quantify the effects of multiple particles being in contact with the plate. As nice as it is to isolate single impacts, under natural conditions the norm will be to experience multiple impacts during a bedload transport period, even during the time window for which the first pulse of an impact signal is collected. If it is impossible to reliably locate two or more sources at a time, it will also be impossible to extract reliable information on the particle size and hence the envisioned total bedload flux across the sensor plate. I might have missed some detail in reading the manuscript but from the results section (e.g. Fig. 7) is seems that the two synchronous impacts cannot be deciphered, as one would expect from the capabilities of a phased array setup, and as is discussed in the manuscript further down.
The text contains numerous grammatical glitches, equivocal wording occasions, unclear terminology, and cluttered graphics. I list some of those issues in my detailed comments but think the manuscript is not yet at a stage where a fine screen of such improvements makes sense. I suggest to pay due attention to improvements of the presentation quality of the study both in terms of writing and graphical art.
There are several occasions when the text mixes up theory, methods, results and/or discussion and hence requires reorganisation to be consistent. See details below for examples.
Many figures are awkward to read/interpret. Especially the insets of the reference data (Wyss et al.) make any comparison virtually impossible. Likewise, the semantics of the figures not always add up, which is when I would suggest to split figures to maintain consistent pieces of information rather than collections of fragmented information. See details for examples.
Detailed comments
l. 90-92, in the sensor description section it would help to give more detail on the device characteristics apart from the manufacturer and type ID, e.g. frequency range, sensitivity, digitizer sampling depth, and so on.
l. 108, 2.2 10^1 kg means 22 kg. Check values
Fig. 2, why are there so few validation drop locations and overall, why were not more locations used further towards the margin of the plate? It seem quite unlikely that only the central 2/3 of the plate would be impacted by moving bed particles, and the edge effects appear to be quite significant according to the study results.
The derivation/description of the beam forming technique seems to be or will at least be based on numerous references representing work of generations of scientists. Yet, there is only a single reference included. I suggest the authors give proper credit to the scientists that have built the physical concept, mathematical framework and perhaps computational implementation of the routine – unless all this has indeed been developed by the authors of this study.
Fig. 3 d, R_s needs to be defined
l. 169, the heading (simulation) does not depict what the text below refers to (construction details)
l. 171, 3D grid does not make sense, either it is a grid (2D) or a 3D structure (voxels, cube). Also, pixels is a grid-world term while nodes is a vector-based term. This (and other occasions) need to be corrected throughout.
l. 184, the equation provides rather trivial information, consider removing it
l. 194-196, this is more about phased array theory than actual methods employed in this study. Consider moving this to the introduction
l. 199-204, there are some references needed to support the statements in this section
l. 204-209, there is some rather trivial information that might be better placed in a general introduction (or just skipped) rather than being an actual topic of “Microphone elements arrangement”.
l. 213, why a sine wave and not the more likely to expect pulse signal? Can you explain/justify this? Will this decision not affect the validity of the test outputs with respect to the envisioned natural settings?
l. 214, why adding white noise? Can you justify, both why adding noise first of all and then also why specifically white noise? I would rather expect to see red noise or some derivative of it to better reflect “real” conditions.
l. 224, replace “centric” by “central”
l. 224, why indexing a repeat experiment run?
Fig. 6, it would help quite a bit if the panels a-d would indicate the locations of the sensors on the coloured matrices, instead of having to look for Appendix A to get that information.
Fig. 7, better split the figure into single impacts and multiple impacts. In addition, in (a) and (b) there are only two locations shown, in (e) there are three locations shown. Similar for c,d and f. Please resolve.
l. 309-310, I disagree with the statement that the locations agree well. This is just an artefact not a real two-source location. Actually we need proper tests here to show the real artefacts arising from the case of more than one impacting particle being relocated. What if two, three, four, … particles impact at the same time? What is the effect of different locations (up-down, oblique, different distances from center)? Here I only see two sources horizontally symmetrically impacting the plate.
l. 316, “indicating that the grain of the noises become weaker…”, this sentence does not make any sense to me
l. 324-326, this is interpretation and should not occur in the results section. Albeit, what have we learnt from that exercise after all? We already know that a phased array cannot locate two synchronous signals from two different locations. Also, what does that imply (later in the discussion) for a real world case when multiple impacts happen?
Fig. 8, any comparison with the inset data is not possible (also in figs. 9 and 10), please add those data points to the main chart, perhaps with clearly distinct colours. Perhaps add a small map that depicts the impact locations to prevent readers from searching back and forth to get the context right. Add the number of samples that are summarised in the box plots. Perhaps remove the x-axis line to make clear this is categorical data and not continuous, because at a first glance one could think the four box plots per diameter are a continuum of sizes. In summery, just make the job easier for readers to correctly interpret the figure.
l. 417-148, this should go to the methods section. In addition, why not just calculating the RMSE between real location and model derived location?
l. 422, “suggesting a good agreement”, first this is interpretation and second it is a weak statement that can and should be quantified.
l. 428, “boxes in green”, I see no green boxes at all
l. 429, what are these values (30 % and 10 %) based on? Are these arbitrarily chosen or based on some reference or some physical meaning?
l. 445-446, this sentence needs some context in which is shall be embedded. Currently, it hangs in the air with no logic
l. 466-470, but this is neither shown nor is this discussed in terms of its consequences for the ultimate goal, to get a reliable bedload flux estimate
l. 476-485, the discussion is weak here, it needs to go for more far reaching thoughts, beyond just interpreting possible reasons why this system deviates from earlier/other SPG devices. The title of the study tells us the new system improves the prediction of particle size for bedload measurements. Yet, this is not clearly presented.
l. 505-510, this part is fairly repetitive, it reiterates the results described earlier but lacks an interpretation
l. 513-536, this reads like methods description, certainly not discussion of results.
l. 534-538, this does not make sense. Ignoring friction any particle that falls from the same height would have the same impact velocity. This may leap back to my very above comment of using the centre of mass to define the fall height instead of the lower margin of the impactor.
l. 540-546, this is a weak interpretation approach, with many “may” and “might” terms but with very little rational arguments that would support the hypothesis. Either add experimental, reference or logical support or skip the attempt of interpretation.
l. 558-560, this is repetitive
l. 564-566, this is repetitive
l. 574, “the decrease of R_s tends to weaken”, what does that mean? I do not understand.
l. 582, “developed” would be quite a bit an overstatement, better use “presented”
The fundamental parts of the conclusion chapter are repetitive, giving a mere summary of details described above. A proper conclusion should step beyond the above presented material and ask “what do we know now, or what can we do now?” It shall feed back to the main research question and answer it.
l. 600, “allowing the spatial location”, yes but only when a single impact is isolated, which I think will be of little help under real world conditions.

Citation: https://doi.org/10.5194/egusphere-2024-2525-RC2
- AC2: 'Reply on RC2', Zheng Chen, 09 Aug 2025
  
  Dear Reviewer #2,
  We sincerely thank you for your constructive and insightful comments on our manuscript. This feedback has helped us to significantly improve the quality, clarity, and structure of the manuscript. We provide a detailed point-by-point response to each comment (see attached). In response to the reviewers’ comments, we have thoroughly revised the original manuscript.
  Sincerely,
  Dr. Zheng Chen (on behalf of all co-authors)
  Corresponding author
  Email: zheng.chen@cdut.edu.cn
  
  Citation: https://doi.org/10.5194/egusphere-2024-2525-AC2

Zheng Chen, Siming He, Alexandre Badoux, and Dieter Rickenmann

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Short summary

We developed a novel bedload monitoring system, which integrates phased microphone arrays and an accelerometer for enhanced performance. This monitoring system can be used to identify bedload particle impact locations on the system plate with precision using beamforming techniques applied to the generated microphone signals. Optimal use of multiple types of signals recorded by the monitoring system improves the accuracy of bedload size prediction.


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