A template to obtain information on gravitational mass movements from the spectrograms of the seismic signals generated
Abstract. We present a template that helps in the classification of gravitational mass motions (snow avalanches, lahars and debris flows) by simply overlaying it graphically with the corresponding spectrogram of the generated seismic signal at the same scale. The template is created with different values of a parameter β' that allows us to analytically reproduce the exponential form of the increase in amplitude of high frequencies in time of the SON (Signal Onset) section of the spectrogram when the mass movement descends a slope and approach a seismic sensor. This increasing shape is a consequence of the appearance of energy at high frequencies as the gravitational mass approaches the seismic sensor. We present a methodology that includes a link between the propagation properties of seismic waves and the results of the application of an image processing using the Hough transform to demonstrate that this shape is related to the speed of the avalanche and the characteristics of the terrain. Seismic data generated by lahars, debris flows, and avalanches are used for the study. Depending on the type of event, differences are obtained in the order of magnitude of the values of β'. The mean value of β' for lahars is around 0.003 s-1, that for debris flows is an order of magnitude greater (0.017 s-1) and an order of magnitude less than that for avalanches (0.12 s-1). Furthermore, differences in β' are observed within each type of event.
Once the appropriated value of β' has been determined, the characteristics of the mass movement must be set according to expert judgement. This must be done for each site and for each type of gravitational mass movement. The application of the templates to the data of lahars and an avalanche recorded in two different places of its trajectory is presented as an example.
Emma Suriñach and E. Leticia Flores-Márquez
Status: final response (author comments only)
RC1: 'Comment on esurf-2022-10', Fabian Walter, 26 Oct 2022
- AC1: 'Reply on RC1', Emma Surinach, 11 Jan 2023
RC2: 'Comment on esurf-2022-10', Anonymous Referee #2, 17 Nov 2022
- AC2: 'Reply on RC2', Emma Surinach, 11 Jan 2023
Emma Suriñach and E. Leticia Flores-Márquez
Emma Suriñach and E. Leticia Flores-Márquez
Viewed (geographical distribution)
The study by Suriñach and Flores-Márquez presents a spectral analysis of continuous seismic data containing the signature of lahars, debris flows and snow avalanches. The authors apply image processing techniques to the corresponding spectrograms in order to automatically recognize and characterize the well-known increase of high-frequency signal content as the mass movement approaches the recording station. With a simple analytical formula, it is possible to model the spectrogram envelopes and thus calculate the mass movement’s velocity.
The presented idea of coupling image processing with a physical understanding of frequency content of mass movement seismograms is clever and provides new perspectives for monitoring. Unfortunately, the manuscript is difficult to read, containing unclear as well as unnecessary explanations and numerous language and grammar mistakes. To be considered for publication, the entire text has to be overhauled and proof-read. Below I elaborate on this point of criticism and provide additional comments.
This manuscript is unnecessarily long. It contains trivial and/or general explanations on spectrograms and computer code in addition to unreferenced assertions and announcements of what is about to be discussed. The corresponding pieces of text should be removed. Moreover, the introduction contains many examples of previous seismological studies of mass movements and their spectral characteristics. However, the investigations by Tsai et al. (2012), Lai et al. (2018) and Wenner et al. (2019), all of which are most relevant to the present study as they also document changes in high-frequency content as a function of source-station distances, are not discussed. It is important to state the underlying assumption adopted by the authors that stochastic superposition of instantaneous particle impacts with infinitely wide frequency spectra are the seismic source processes. Otherwise, a source signature in the spectral content would have to be included (Tsai et al., 2012). The introduction could be shortened to much less than 50% of its current length by discussing the above papers together with a few milestone and review papers. Previous papers should be discussed and cited based on their main point and citations should be complete rather than “examples” of what has been done and could be done. It is also not clear to me what the difference between the current submission and Suriñach et al. (2020) is. Is it simple the inclusion of mass movements other than snow avalanches? If so, then the current manuscript would not require more than 3-4 pages to get across its point. I urge the authors to strictly define the differences between the two studies to avoid accusations of self-plagiarism.
Whereas the authors focus on spectrogram envelopes it should be acknowledged that this is not the only way to characterize a mass movement seismogram. In particular, the authors should consider to couple the envelopes with amplitudes, which also increase as the moving mass approaches the recording station (the basis of amplitude source location mentioned in the introduction). Can the two methods be combined to give a better constraint on mass movement velocity?
The discussion of the analytical function (Equation 1 or a subsequent version thereof) is incomplete and confusing. First of all, what is the physical basis? There exist different kinds of waves for which attenuation laws can be formulated (e.g., surface waves or body waves). Which wave type is assumed? Depending on the answer it may or may not be legitimate to apply the same kind of equation to infrasound waves. This requires justification! Some more comments on Equation 1: why is t in the denominator primed? When fitting this equation to the spectrogram envelopes, I believe that it should be squared according to the definition of a power spectrum. See Stein and Wysession (2003), Equation 30 on Page 384: the power spectrum is proportional to the SQUARED norm of the Fourier components. Finally, it makes no sense to refer to a power spectral density (“frequency transect” as used by the authors) at a specific frequency (e.g., Figure 1f): by definition, this is a density residing between two frequencies.
Even though I believe I understand the application of the analytical expressions to the spectrogram envelope, I did not follow the author’s explanations. The explanations on Line 274ff make no sense to me (although this may be a fault on my side). As an alternative, I would suggest the following approach: One can rearrange Equation 1 such that the right-hand side contains the exponential and 1/sqrt(r) terms, only. The left hand side then contains the ratio A(t,f)/AS(f_s,t_s). To define an envelope, one can use a specific value of this ratio, say 80%, whose shape in the spectrogram follows the right-hand side (multiplied by some constant factors).
When analyzing the different events, it seems that the authors simply aim at determining the beta and K values. It would be more convincing to compare the velocities measured with the spectrogram method against independent measurements. At least for those sites hosting more than one event, this should be possible.
The avalanches from the Vallée de la Sionne are classified differently from the ones from Ryggfonn (powder, transitional, wet as opposed to dry-dense, dry-mixed). Why is this so? Do Norwegian avalanches differ from Swiss avalanches? What is a transitioning and mixing avalanche?
It seems that one important aspect of this work is the potential automatic classification of mass movements. The usefulness of this should be further discussed: for instance, what are the situations when a station records a lahar and a snow avalanche and one has to automatically discriminate between the two?
The fitting of the envelope in the Hough transform makes no sense to me, although this may be a problem on my side: how do the lines in Figure 5e constrain the beta value? I did not understand the corresponding explanations. How are the uncertainties determined?
In its current state the Conclusion is not a conclusion but actually a summary.
Avoid 1-sentence paragraphs.
There exist many undefined acronyms.
Lines 114-115: This sentence needs justification.
Line 206: Give the equation number in Aki and Richards (1980). Ideally use a more recent version of this textbook.
Equation 2: I thought that the quality factor is defined in a way that it does NOT depend on frequency.
Line 232: I do not know how to read the expressions for Q(f) and c(f). At least for the latter there is a dimensional error. Also, a frequency dependence of c implies dispersion, which should be commented on.
Lines 242ff: I have to admit that I did not follow the variable changes between r’, R and r. Why is this necessary? According to Line 246, r is negative, which does not seem to agree with Figure 2. Similarly, the sentence on Line 259 (constant exponential term) makes no sense to me.
Lines 268-270: Unclear.
Line 287: Specify “physical processes”.
Line 329: To argue for a constant beta^prime, is it not possible to simply assume a constant velocity?
Lines 392-393: This sentence is trivial. However, the exact coupling is more complicated than suggested since the source mechanisms generating seismic and infrasound waves are different.
Line 401: eigenvalue ï natural frequency
Lines 455-456: This agreement is not clear to me.
Lines 475-478: Define the “same considerations” and explain/justify the statement in the latter sentence.
Line 485: Define “shot point”.
Lines 488-490: This sentence is unclear to me.
Lines 509ff: I cannot confirm the shifting effect of K. Not sure I get the point of Figure 16. Perhaps this could be shown better?
It seems that some figures do not appear in the order that they are mentioned.
All figure panels need labels.
Figure 3: Superscript “-1” is missing in the bottom right panel.
Figure 4: What unit does the color scale have?
Figure 5: Y-axis and color bar labels are missing in some panels. I would remove “SON” since it appears in all panels. What are the dashed and solid lines?
Lai, V. H., Tsai, V. C., Lamb, M. P., Ulizio, T. P., & Beer, A. R. (2018). The seismic signature of debris flows: Flow mechanics and early warning at Montecito, California. Geophysical Research Letters, 45(11), 5528-5535.
Tsai, V. C., Minchew, B., Lamb, M. P., & Ampuero, J. P. (2012). A physical model for seismic noise generation from sediment transport in rivers. Geophysical Research Letters, 39(2).
Suriñach, E., Flores-Márquez, E. L., Roig-Lafon, P., Furdada, G., & Tapia, M. (2020). Estimation of avalanche development and frontal velocities based on the spectrogram of the seismic signals generated at the Vallée de la Sionne Test Site. Geosciences, 10(3), 113.
Wenner, M., Walter, F., McArdell, B., & Farinotti, D. (2019). Deciphering debris-flow seismograms at Illgraben, Switzerland. Association of Environmental and Engineering Geologists; special publication 28.