EGU2020-12594
https://doi.org/10.5194/egusphere-egu2020-12594
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Seafloor seismology with Distributed Acoustic Sensing in Monterey Bay

Nathaniel Lindsey1,2, Jonathan Ajo-Franklin2,3, Craig Dawe4, Lise Retailleau1, Biondo Biondi1, and Lucia Gualtieri1
Nathaniel Lindsey et al.
  • 1Stanford University, Geophysics, United States of America (nlindsey@stanford.edu)
  • 2Lawrence Berkeley National Laboratory
  • 3Rice University
  • 4Monterey Bay Aquarium Research Institute

Emerging distributed fiber-optic sensing technology coupled to existing subsea telecommunications cables enable access to meterscale, multi-kilometer aperture, broadband seismic array observations of ocean and solid earth phenomena. In this talk, we report on two multi-day Distributed Acoustic Sensing (DAS) campaigns conducted in 2018 and 2019 with the Monterey Accelerated Research System (MARS) observatory tether cable. In both experiments, a DAS instrument located on shore was connected to a fiber inside the buried MARS cable and recorded a ~10,000-component, 20-kilometer-long, strain-rate array. We use the 8 TB DAS dataset to address three questions:

1. How can seafloor DAS earthquake records inform offshore seismic hazard assessments? Offshore seismic hazards are poorly characterized despite dense coastal populations. The MARS DAS array captured multiple unaliased earthquake recordings, which document phase conversions and abrupt S-wave delays of 0.25 s at mapped (and unmapped) faults that transect the cable. Minor earthquakes in Northern California produce seismic waves in the range 0.5 - 50 Hz, which interact with submarine faults lying just offshore. Spectral ratios and wavefield synthetics are used to explore how seismic waves from well-characterized earthquakes interact with poorly-characterized subsea faults.

2. How are ocean microseisms and other coastal processes recorded by subsea DAS? Horizontal seabed ambient noise recorded with the MARS DAS array matches the expected dispersion of primary microseisms (f~0.05-0.15 Hz) induced by shoaling ocean surface waves, but at a higher band than onshore observations. Separation of incoming and outgoing waves recorded over the DAS array validates the Longuet-Higgins-Hasselmann theory that bi-directional ocean wind-waves undergo nonlinear wave interaction, producing secondary microseisms (f~0.4-1.5 Hz), even when the outgoing energy is observed to be <1% of the incoming energy. Continuous wavelet transforms of sea state observations from buoys, onshore broadband seismometers, and subsea DAS provide insight into the physics of microseism generation and ocean-solid earth coupling. Additionally, DAS provides observation of post-low-tide tidal bores (f~1-5 Hz), storm-induced sediment transport (f~0.8-10 Hz), infragravity waves (f~0.01-0.05 Hz), and breaking internal waves (f~0.001 Hz) consistent with previous point sensor observations in Monterey Bay. 

3. How is the coastal seafloor structure organized from shore to shelf break? The northern continental shelf of Monterey Bay is comprised of allochthonous Cretaceous granite overlain by marine sediments of varying thickness, and is crosscut by abandoned (and subsequently filled) paleochannels. Noise interferometry applied to the full MARS DAS dataset in the 0.25 - 5 Hz range retrieves Scholte waves, which are dispersive and coherent over 2 - 6 kilometers. We apply fundamental mode dispersion (1.5D) imaging to subarray noise correlations in order to understand the sediment thickness distribution across the shelf. Our model is compared with recent seismic reflection profiling conducted by the USGS California Seafloor Mapping Program.

How to cite: Lindsey, N., Ajo-Franklin, J., Dawe, C., Retailleau, L., Biondi, B., and Gualtieri, L.: Seafloor seismology with Distributed Acoustic Sensing in Monterey Bay, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12594, https://doi.org/10.5194/egusphere-egu2020-12594, 2020

Comments on the presentation

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Presentation version 2 – uploaded on 01 May 2020
modified link on slide 7
  • CC1: Comment on EGU2020-12594, Mark Woods, 06 May 2020

    Very interesting, and a good follow-up to a seminar presentation by Verónica Ródriguez Tribaldos last week.

  • CC2: Comment on EGU2020-12594, Alemayehu Jemberie, 06 May 2020

    Thank you for this excellent presentation. The JGR paper (Lindesu et al 2019) has detailed explanations on the method and instrumentation.

    • AC1: Reply to CC2, Nate Lindsey, 06 May 2020

      Hi and thanks for your comment! Just to clarify, I beleive you are referring to Lindsey et al., 2020 in JGR ().

  • CC3: Comment on EGU2020-12594, Alemayehu Jemberie, 06 May 2020

    Yes, sorry for the typo. Is there a publicly accessible data?

    • AC2: Reply to CC3, Nate Lindsey, 06 May 2020

      Yes, ftp to Alaska waveforms here: (if link does not show, check Acknowledgements of paper).

      • CC4: Reply to AC2, Alemayehu Jemberie, 06 May 2020

        I don't see a link here. Do you mean this

        ftp: http://www.ncedc.

        org/ftp/outgoing/Papers/2019_

        Lindsey/

        from the JGR paper?

        Thanks

         

  • CC6: Comment on EGU2020-12594, Tsunehisa Kimura, 06 May 2020

    Dear Nate,

    Thank you very much for your interesting study and impressive results using submarine fiber cable. Thank you for answering my questions as well. We have our own DAS system called "hDVS", mainly used for VSP, flow monitoring and pipeline monitoring in Oil & Gas at this moment. From few years ago we started to use our system for earth monitoring and large stracture monitoring purpose as a research activity. I am leading such activity and we presented two results D1609 and D1611 in this EGU 2020 event.

    We had an experiment of seaquake monitoring using 17km length of submarine fiber cable in Japan offshore Toyohashi (owned by JAMSTEC) in September 2017 and we successfully recorded seaquake occurred in Suruga bay. However, I am a research engineer background of quantum electronics, while seismologist in JAMSTEC is always busy, so that we are not yet interpret the data. Your presentation descrive all major events, so it was impressive.

    I learned separation of one event to other event is quite challanging for the DAS data recorded using submarine fiber cable, but I could demonstrate potential separation by reprocessing with different Gauge Length. Our system records "Native Raw" data, which is basically backscatter data, so that we can re-process the data to enhance one event (e.g. earthquake) from others using selected Gauge Length (and channel length) without using filter. I think this is an unique approach of our system. We can enhance VSP data using variable Gauge Length re-processing method. Anyway, we have a research experiment planed in this year using 120km submarine fiber cable at offshore Sanriku in Japan together with Kyushu university, so hopefully we will find interesting events.

    I hope to discuss about DAS measurement with you in the future face to fact after COVID-19 risk is reduced.

    Thank you.

    Tsune Kimura (tsune@slb.com)

Presentation version 1 – uploaded on 01 May 2020 , no comments