the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A VLF/LF facility network for preseismic electromagnetic investigations
Patrick H. M. Galopeau
Ashanthi S. Maxworth
Mohammed Y. Boudjada
Hans U. Eichelberger
Mustapha Meftah
Pier F. Biagi
Konrad Schwingenschuh
Abstract. Earthquakes are one of the most frequently occurring natural disasters. Many indications have been collected on the presence of seismo-ionospheric perturbations preceding such tragic phenomena. Radio techniques are the essential tools leading the detection of seismo-electromagnetic emissions by monitoring at very low frequency (VLF, 3–30 kHz) and low frequency (LF, 30–300 kHz) sub-ionospheric paths between transmitters and receivers (Hayakawa, 2015). In this brief communication, we present the implementation of a VLF/LF network for searching earthquake electromagnetic precursors. The proposed system is comprised of a monopole antenna including a preamplifer, a GPS receiver and a recording device. This system will deliver a steady stream of real-time amplitude and phase-measurements and a daily recording VLF/LF data set. A first implementation of the system was done in Graz, Austria, the second one will be in Guyancourt, France, a third one in Réunion, France, and a fourth one in Moratuwa, Sri Lanka. In the near future, we are planning on expanding of our network for enhanced monitoring and increased coverage.
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Patrick H. M. Galopeau et al.
Status: final response (author comments only)
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RC1: 'Comment on gi-2023-1', Jean-Pierre Raulin, 21 May 2023
Authors present the implementation of a new VLF network from a receiver (RX) in Graz, with new RXs in France (metropole and dependency) and Sri Lanka. The paper is well written and deals with an important subject, the search for and detection of earthquakes electromagnetic precursors. My overall impression is that the paper deserves publication GI, however, I have some corrections/suggestions listed below that authors must consider.
- lines 23-24: Few sentences are needed in the introduction to summarize Hayakawa and DEMETER methods, results, and findings on the subject. Terminator Times method for short (Yoshida et al. 2008 NHESS, 8, 129–134, 2008) and long (Samanes et al. 2015, Radio Science, 50, 122-129) VLF propagation paths, VLF nighttime amplitude analysis (trend, dispersion and fluctuations) see Hayakawa & Hobara 2010, Natural Hazards and Risk, 1(2), 115–155; see Hayakawa et al. 2011 Nat. Hazards Earth Syst. Sci., 11, 513–518.
- line 59: UltraMSK needs a reference. Use one of the earlier paper from J. Brundle
- a sentence or 2 will be important for the reader to understand what does UltraMSK. At this time, UltraMSK appears as a black box in Figure 1.
- lines 82-83: I disagree. Many networks exist and are operated outside Europe, some of them since 2 decades: Indian VLF networks (see Chakrabarty papers), Japanese papers (see Hayakawa and Hobara papers), US netwoks (AWESOME etc ...), South American networks were cited.
- distances from TXs, GBZ and TBB to Graz needs to be mentioned in the main text.
- comments/questions related on Figure 2:
* Time unit is UT. Include.
* Indicate (Figure and main text) the GOES Class of the solar flare (C4 I believe)
* why phase from TBB is not shown ?
* amplitude units dB are relative to what ? the output of the sound card once the signal is digitized ? or relative to the absolute E-field of the wave from the transmitter ? Do you (will you) perform an absolute amplitude calibration ?
* mention/explain in the main text the noise (phase and amplitude) increases seen in Figure 2 at specific times during daytime almost every hour. What are they ? local (Graz) interferences, since they are also visible in the TBB amplitude curve ?
- last sentence of Figure caption: change for, "The VLF signal enhancements around 1200 UT coincide with the increase of the solar X-ray flux.". VLF signal changes reflect changes in the electrical conductivity, i.-e. free electron number density (Ne), while Soft X-rays are related to a (photo-) ionization rate, i.-e. rate (change) of Ne. Maxima won't coincide.
- lines 98-99: this is not a correct reference for WWLLN. Use earlier Dowden papers (2000-Australian Patent Office, 2002-JASTP) when the network was known as TOGA or Holzworth (2004; GRL) paper where WWLLN name was used for the first time.
- comments/suggestions on Figure 3: as it is the figure is quite heavy. Please show only TXs which signals your network can detect
* why NLK 24.8 kHz (Western US coast) is not shown ?
* will you use the VLF signal from all theses TXs ? show only TX you will use.
* is UltraMSK able to perform demodulation of FSK signals for example ? this is the case for Japanese JJI and JJY and also ICV.
* what about low power TXs ? I doubt the signal from NAU (Puerto Rico) can reach Europe.
- In case your network cannot treat (deal with) FSK modulation signals, how do you pretend to monitor seismic-electromagnetic precursors in the very active region of Asia (Andaman Sea, Gulf of Thailand, Philippines region, South China sea .....)
Citation: https://doi.org/10.5194/gi-2023-1-RC1 -
AC1: 'Reply on RC1', Patrick Galopeau, 22 Jun 2023
We are thankful to the reviewer for the constructive comments and suggestions. The corrected texts are written in blue color in the upgraded version.
Authors present the implementation of a new VLF network from a receiver (RX) in Graz, with new RXs in France (metropole and dependency) and Sri Lanka. The paper is well written and deals with an important subject, the search for and detection of earthquakes electromagnetic precursors. My overall impression is that the paper deserves publication GI, however, I have some corrections/suggestions listed below that authors must consider.
Referee: (1) lines 23-24: Few sentences are needed in the introduction to summarize Hayakawa and DEMETER methods, results, and findings on the subject. Terminator Times method for short (Yoshida et al. 2008 NHESS, 8, 129–134, 2008) and long (Samanes et al. 2015, Radio Science, 50, 122-129) VLF propagation paths, VLF nighttime amplitude analysis (trend, dispersion and fluctuations) see Hayakawa & Hobara 2010, Natural Hazards and Risk, 1(2), 115–155; see Hayakawa et al. 2011 Nat. Hazards Earth Syst. Sci., 11, 513–518.
- Reply (1) Section 1 has been re-considered and re-written taking into considerations the suggestion of the referee.
Referee: (2) line 59: UltraMSK needs a reference. Use one of the earlier paper from J. Brundle, and a sentence or 2 will be important for the reader to understand what does UltraMSK. At this time, UltraMSK appears as a black box in Figure 1.
- Reply (2) we have added, in Section 3, a reference concerning UltraMSK and its capabilities to measure amplitude and phase emitted by VLF/LF radio transmitters.
Referee: (3) - lines 82-83: I disagree. Many networks exist and are operated outside Europe, some of them since 2 decades: Indian VLF networks (see Chakrabarty papers), Japanese papers (see Hayakawa and Hobara papers), US netwoks (AWESOME etc ...), South American networks were cited.
- Reply (3) we agree with the referee about the presence of other networks. However, the sentence concerns the new network (i.e. Guyancourt, Reunion and Sri Lanka) and its link to the actual INFREP localized in Europe. We have re-written this sentence.
Referee: (4) - distances from TXs, GBZ and TBB to Graz needs to be mentioned in the main text.
- Reply (4) In Section 4, we have given the great circle paths for GBZ-GRZ (i.e. 1540 km) and TBB-GRZ (i.e. 1445 km).
Referee: (5) - comments/questions related on Figure 2: (a) Time unit is UT. Include, (b) Indicate (Figure and main text) the GOES Class of the solar flare (C4 I believe), (c) why phase from TBB is not shown? noisy, (d) amplitude units dB are relative to what? the output of the sound card once the signal is digitized ? or relative to the absolute E-field of the wave from the transmitter?, (e) Do you (will you) perform an absolute amplitude calibration?, (f) mention/explain in the main text the noise (phase and amplitude) increases seen in Figure 2 at specific times during daytime almost every hour, What are they ? local (Graz) interferences, since they are also visible in the TBB amplitude curve?, (g) last sentence of Figure caption: change for, "The VLF signal enhancements around 1200 UT coincide with the increase of the solar X-ray flux.". VLF signal changes reflect changes in the electrical conductivity, i.-e. free electron number density (Ne), while Soft X-rays are related to a (photo-) ionization rate, i.-e. rate (change) of Ne. Maxima won't coincide.
- Reply (5) As suggested by the referee:
(a) UT time is now given for the observation time in Figure 2,
(b) C4.8 class of the solar flare is indicated in the legend and in Section 4,
(c) TBB phase is not shown because of the noisy signal on this day,
(d) The amplitude signal is relative to the output of the sound card,
(e) We did not proceed to an absolute amplitude calibration but will be made in the future if required for a certain application, the main goal is to have a stable configuration without drifts
(f) The local intermittent interferences are due to the site location in an urban area. This has been mentioned in Section 4
(g) The sentence, as suggested by the referee, has been added in Section 4 and in the legend of Figure 2.
Referee: (6) - lines 98-99: this is not a correct reference for WWLLN. Use earlier Dowden papers (2000-Australian Patent Office, 2002-JASTP) when the network was known as TOGA or Holzworth (2004; GRL) paper where WWLLN name was used for the first time.
- Reply (6) The reference for WWLN has been corrected in Section 5.
Referee: (7) - comments/suggestions on Figure 3: as it is the figure is quite heavy. (a) Please show only TXs which signals your network can detect, (b) why NLK 24.8 kHz (Western US coast) is not shown?, (c) will you use the VLF signal from all theses TXs ? show only TX you will use.
- Reply (7) (a) It is intended that all transmitters shown in Figure 3 are programmed in the receiver despite the fact that some of them have a S/N not adequate to further process the signals, but this is a site dependent issue, (b) NLK transmitter is not sampled/programmed, (c) All TXs in Figure 3 are sampled but S/N ratio forces to a subset of TXs in real application.
Referee: (8) (a) is UltraMSK able to perform demodulation of FSK signals for example ? this is the case for Japanese JJI and JJY and also ICV, (b) What about low power TXs ? I doubt the signal from NAU (Puerto Rico) can reach Europe.
- Reply (8) (a) No demodulation is performed by UltraMSK. JJI, JJY (long distance paths) and ICV transmitters are all sampled. The ICV transmitter (20.27 kHz, 820 km great circle path to the Graz receiver) is regularly recorded with a good S/N, it leads us to monitor the earthquake prone area in the Italian northern Apennine mountain range, (b) The NAU transmitter (40.8 kHz) has not sufficient S/N ratio.
Referee: (9) - In case your network cannot treat (deal with) FSK modulation signals, how do you pretend to monitor seismic-electromagnetic precursors in the very active region of Asia (Andaman Sea, Gulf of Thailand, Philippines region, South China sea...)
- Reply (9) The two new VLF/LF receiving sites in Réunion Island and Sri Lanka will be capable to monitor certain active regions of Africa and Asia (i.e. the paths shown in Figure 3) provided that a suitable S/N can be achieved. The full performance of the system in facts and figures can only be evaluated in the commissioning phase on site. If the VLF/LF method to monitor seismic-electromagnetic precursors fails, alternatives have to be considered, for a brief review see Chen et al. (2022). Chen, H., Han, P., Hattori, K.: Recent Advances and Challenges in the Seismo-Electromagnetic Study: A Brief Review, Remote Sens., 14, 5893. https://doi.org/10.3390/rs14225893, 2022.
Citation: https://doi.org/10.5194/gi-2023-1-AC1
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AC1: 'Reply on RC1', Patrick Galopeau, 22 Jun 2023
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EC1: 'Comment on gi-2023-1', Lev Eppelbaum, 22 Aug 2023
The MS "A VLF/LF facility network for preseismic electromagnetic investigations" by Galopeau et al. presents a useful and practical methodology for the forecasting dangerous geodynamic events using the very low frequency (3–30 kHz) and low frequency (30–300 kHz) paths between transmitters and receivers. These observations are low-expensive and can be done during long periods. The high observation density will provide the increase accuracy and reliability of the measurements. The publication of this MS will motivate the authors for further studies. I recommend this MS for publication.
Citation: https://doi.org/10.5194/gi-2023-1-EC1 -
RC2: 'Comment on gi-2023-1', Anonymous Referee #2, 28 Aug 2023
In this communication, the authors described their VLF/LF system for preseismic electromagnetic and presented an observation case. The work fits the scope of the journal. I suggest the authors consider the following problems in revision:
- In Introduction, the contribution and novelty are not clearly described, please clarify it.
- Section 4, you only presented the observation results, could you add corresponding discussion?
- Could you add a photo of your new system?
Citation: https://doi.org/10.5194/gi-2023-1-RC2 -
AC2: 'Reply on RC2', Patrick Galopeau, 15 Sep 2023
We are thankful to the reviewer for the constructive comments and suggestions. The corrected texts are written in blue color in the upgraded version.
In this communication, the authors described their VLF/LF system for preseismic electromagnetic and presented an observation case. The work fits the scope of the journal. I suggest the authors consider the following problems in revision
Referee: (1) In Introduction, the contribution and novelty are not clearly described, please clarify it.
- Reply (1) The Section 1 has been re-considered and re-written taking into considerations the suggestions of the first referee. The new text (blue colour) concerns a review of the terminator times (TT) methods (Hayakawa et al., 1996) and those used in the investigations of DEMETER VLF electric measurements. In this context, we have insisted on previous and recent studies that reported about the sunrise and sunset shifts in time of the phase and amplitude of some VLF transmitter signals. Such changes in time occurred before earthquake occurrences. In addition, we have reported about the VLF radio propagation in the Earth-ionosphere waveguide and the role of the D- and E-layers. In the upgraded version, five new references have been included to the Section 1, and three others to Section 3 and Section 5.
Referee: (2) Section 4, you only presented the observation results, could you add corresponding discussion?
- Reply (2) In Section 4, we have emphasised on the capability of the VLF new system showing one example as recorded by the reception system detailed in Section 3. The intention is to show amplitude (i.e. VLF GBZ and TBB transmitters) and phase (i.e. VLF GBZ transmitter) signal variations emitted by two stations located at great circle paths of more than 1400 km from Graz. In addition, we display a particular case where a solar flare is detected by the new system to highlight the synergy between the ground-based (Graz facility) and space (GEOS satellite) observations. New references about the preliminary results of the new reception system have been added to the Section 4.
Referee: (3) Could you add a photo of your new system?
- Reply (3) As suggested by the referee, a new figure has been considered with photos of the new reception system.
Citation: https://doi.org/10.5194/gi-2023-1-AC2
Patrick H. M. Galopeau et al.
Patrick H. M. Galopeau et al.
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