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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2019)

Distributed Acoustic Sensing (DAS) is an emerging tool in array seismology, which uses high frequency interferometry of pulsed laser backscattering inside optical fiber to analyze the axial strain induced on the fiber cable commonly buried in horizontal trenches and vertical wells at the surface of the Earth. This technology was developed over the last decade for hydrocarbon and carbon sequestration reservoir imaging and monitoring, but the focus in this thesis is to explore its application to problems in earth science, broadly defined. Combining DAS with telecommunications optical fiber networks offers meter-scale, long-term observations of ground motion over watershed apertures in sectors of the planet where traditional geophysics has been hindered by cost and field logistics, such as offshore and in urban areas. The thesis is organized as follows. I introduce the motivation for using fiber-optic seismology in Chapter 1. In Chapter 2, I define principles of the methodology and describe how the instrument works. In Chapter 3, I use continuous DAS recordings of ambient vehicular seismic noise generated on a local road to study degrading permafrost over a two-month period of artificial warming. In Chapter 4, I focus on earthquake ground motions recorded on horizontal DAS arrays. Unlike classic inertial seismometers, there is presently a limited amount of information about DAS instrument response, thus in Chapter 5 the aim is to use natural signals to quantify the broadband frequency range of DAS instruments and deduce the related amplitude and phase response functions. Lastly, in Chapter 6, I use fiber-optics on the seafloor of Monterey Bay, CA inside of an unused science cable to investigate the production of nearcoast primary and secondary microseisms, identify unmapped seafloor faults, and observe quasi-geodetic hydrodynamic phenomena in the milli-Hertz frequency range. In Chapter 7, I summarize findings and speculate about future directions in this field.

    Cover page: Fiber-optic Seismology in Theory and Practice
    • Article
    • Peer Reviewed
    LBL Publications (2017)
    Previous magnetotelluric (MT) studies of the high-temperature Coso geothermal system in California identified a subvertical feature of lowresistivity (2-5Ohmm) and appreciable lateral extent (>1 km) in the producing zone of the East Flank field. However, these models could not reproduce gross 3-D effects in the recorded data. We perform 3-D full-tensor inversion and retrieve a resistivity model that out-performs previous 2-D and 3-D off-diagonal models in terms of its fit to the complete 3-D MT data set as well as the degree of modelling bias. Inclusion of secondary Zxx and Zyy data components leads to a robust east-dip (60°) to the previously identified conductive East Flank reservoir feature, which correlates strongly with recently mapped surface faults, downhole well temperatures, 3-D seismic reflection data, and local microseismicity. We perform synthetic forward modelling to test the best-fit dip of this conductor using the response at a nearby MT station. We interpret the dipping conductor as a fractured and fluidized compartment, which is structurally controlled by an unmapped blind East Flank fault zone.
      Cover page: Compartmentalization of the Coso East Flank geothermal field imaged by 3-D full-tensor MT inversion
      • Article
      • Peer Reviewed
      LBL Publications (2019)
      Distributed acoustic sensing (DAS) is a recently developed technique that has demonstrated its utility in the oil and gas industry. Here we demonstrate the potential of DAS in teleseismic studies using the Goldstone OpticaL Fiber Seismic experiment in Goldstone, California. By analyzing teleseismic waveforms from the 10 January 2018 M7.5 Honduras earthquake recorded on ~5,000 DAS channels and the nearby broadband station GSC, we first compute receiver functions for DAS channels using the vertical-component GSC velocity as an approximation for the incident source wavelet. The Moho P-to-s conversions are clearly visible on DAS receiver functions. We then derive meter-scale arrival time measurements along the entire 20-km-long array. We are also able to measure path-averaged Rayleigh wave group velocity and local Rayleigh wave phase velocity. The latter, however, has large uncertainties. Our study suggests that DAS will likely play an important role in many fields of passive seismology in the near future.
        Cover page: The Potential of DAS in Teleseismic Studies: Insights From the Goldstone Experiment
        • Article
        • Peer Reviewed
        LBL Publications (2021)
        The sparsity of permanent seismic instrumentation in marine environments often limits the availability of subsea information on geohazards, including active fault systems, in both time and space. One sensing resource that provides observational access to the seafloor environment are existing networks of ocean bottom fiber optic cables; these cables, coupled to modern distributed acoustic sensing (DAS) systems, can provide dense arrays of broadband seismic observations capable of recording both seismic events and the ambient noise wavefield. Here, we report a marine DAS application which demonstrates the strength and limitation of this new technique on submarine structural characterization. Based on ambient noise DAS records on a 20 km section of a fiber optic cable offshore of Moss Landing, CA, in Monterey Bay, we extract Scholte waves from DAS ambient noise records using interferometry techniques and invert the resulting multimodal dispersion curves to recover a high resolution 2D shear-wave velocity image of the near seafloor sediments. We show for the first time that the migration of coherently scattered Scholte waves observed on DAS records can provide an approach for resolving sharp lateral contrasts in subsurface properties, particularly shallow faults and depositional features near the seafloor. Our results provide improved constraints on shallow submarine features in Monterey Bay, including fault zones and paleo-channel deposits, thus highlighting one of many possible geophysical uses of the marine cable network.
          Cover page: Utilizing distributed acoustic sensing and ocean bottom fiber optic cables for submarine structural characterization
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2017)
          Our understanding of subsurface processes suffers from a profound observation bias: seismometers are sparse and clustered on continents. A new seismic recording approach, distributed acoustic sensing (DAS), transforms telecommunication fiber-optic cables into sensor arrays enabling meter-scale recording over tens of kilometers of linear fiber length. We analyze cataloged earthquake observations from three DAS arrays with different horizontal geometries to demonstrate some possibilities using this technology. In Fairbanks, Alaska, we find that stacking ground motion records along 20 m of fiber yield a waveform that shows a high degree of correlation in amplitude and phase with a colocated inertial seismometer record at 0.8–1.6 Hz. Using an L-shaped DAS array in Northern California, we record the nearly vertically incident arrival of an earthquake from The Geysers Geothermal Field and estimate its backazimuth and slowness via beamforming for different phases of the seismic wavefield. Lastly, we install a fiber in existing telecommunications conduits below Stanford University and show that little cable-to-soil coupling is required for teleseismic P and S phase arrival detection.
            • Article
            • Peer Reviewed
            LBL Publications (2018)
            Global climate change has resulted in a warmer Arctic, with projections indicating accelerated modifications to permafrost in the near future. The thermal, hydrological, and mechanical physics of permafrost thaw have been hypothesized to couple in a complex fashion but data collection efforts to study these feedbacks in the field have been limited. As a result, laboratory and numerical models have largely outpaced field calibration datasets. We present the design, execution, and initial results from the first decameter-scale controlled thawing experiment, targeting coupled thermal/mechanical response, particularly the temporal sequence of surface subsidence relative to permafrost degradation at depth. The warming test was conducted in Fairbanks, AK, and utilized an array of in-ground heaters to induce thaw of a ~11 × 13 × 1.5 m soil volume over 63 days. The 4-D temperature evolution demonstrated that the depth to permafrost lowered 1 m during the experiment. The resulting thaw-induced surface deformation was ~10 cm as observed using a combination of measurement techniques. Surface deformation occurred over a smaller spatial domain than the full thawed volume, suggesting that gradients in cryotexture and ice content were significant. Our experiment provides the first large field calibration dataset for multiphysics thaw models.
              Cover page: Permafrost Degradation and Subsidence Observations during a Controlled Warming Experiment
              • Article
              • Peer Reviewed
              LBL Publications (2018)
              The Tatun geothermal system, located in northern Taiwan, is hosted by the Plio-Pleistocene Tatun volcanic group, consisting dominantly of andesitic lavas, domes and pyroclastic flows. During the late 1960s and early 1970s, geologic mapping, geochemical sampling, geophysical surveys, and the drilling of numerous exploration and temperature gradient wells were conducted at Tatun under the leadership of the Mineral Research and Service Organization (MRSO), the predecessor to the geothermal group at the Industrial Technology Research Institute (ITRI). Deep (1–2 km) wells encountered commercial temperatures (200–300 °C) in the Matsao area, but these initial exploration efforts were discouraged by the presence of very acidic (pH < 3.5) and highly corrosive fluids. Numerous geoscience studies conducted over the past 15 years at Tatun have shown that the central portion of the system is dominated by corrosive volcanic fluids, as evidenced by abundant SO2 as well as HCl. More reduced gases are encountered on the flanks of the system, suggesting that water-rock interaction may have at least partly neutralized the volcanic-derived fluids on the margins of this geothermal system.
                Cover page: Conceptual model of the Tatun geothermal system, Taiwan
                • Article
                • Peer Reviewed
                LBL Publications (2019)
                We present one of the first case studies demonstrating the use of distributed acoustic sensing deployed on regional unlit fiber-optic telecommunication infrastructure (dark fiber) for broadband seismic monitoring of both near-surface soil properties and earthquake seismology. We recorded 7 months of passive seismic data on a 27 km section of dark fiber stretching from West Sacramento, CA to Woodland, CA, densely sampled at 2 m spacing. This dataset was processed to extract surface wave velocity information using ambient noise interferometry techniques; the resulting VS profiles were used to map both shallow structural profiles and groundwater depth, thus demonstrating that basin-scale variations in hydrological state could be resolved using this technique. The same array was utilized for detection of regional and teleseismic earthquakes and evaluated for long period response using records from the M8.1 Chiapas, Mexico 2017, Sep 8th event. The combination of these two sets of observations conclusively demonstrates that regionally extensive fiber-optic networks can effectively be utilized for a host of geoscience observation tasks at a combination of scale and resolution previously inaccessible.
                  Cover page: Distributed Acoustic Sensing Using Dark Fiber for Near-Surface Characterization and Broadband Seismic Event Detection
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