Browsing by Author "Cheng, Feng"

Now showing 1 - 4 of 4
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    Monitoring Water Level of a Surficial Aquifer Using Distributed Acoustic Sensing and Ballistic Surface Waves
    (Wiley, 2024) Sobolevskaia, Valeriia; Ajo-Franklin, Jonathan; Cheng, Feng; Dou, Shan; Lindsey, Nathaniel J.; Wagner, Anna
    Groundwater resources play an increasingly crucial role in providing the water required to sustain the environment. However, our understanding of the state of surficial aquifers and their spatiotemporal dynamics remains poor. In this study, we demonstrate how Rayleigh wave velocity variation can be used as a direct indicator of changes in the water level of a surficial aquifer in a discontinuous permafrost environment. Distributed acoustic sensing data, collected on a trenched fiber-optic cable in Fairbanks, AK, was processed using the multichannel analysis of surface waves approach to obtain temporal velocity variations. A semi-permanent surface orbital vibrator was utilized to provide a repeatable source of energy for monitoring. To understand the observed velocity perturbations, we developed a rock physics model (RPM) representing the aquifer with the underlying permafrost and accounting for physical processes associated with water level change. Our analyses demonstrated a strong correlation between precipitation-driven head variation and seismic velocity changes at all recorded frequencies. The proposed model accurately predicted a recorded 3% velocity increase for each 0.5 m of head drop and indicated that the pore pressure effect accounted for approximately 75% of the observed phase velocity change. Surface wave inversion and sensitivity analysis suggested that the high velocity contrast in the permafrost table shifts the surface wave sensitivity toward the first 3 m of soil where hydrological forcing occurs. This case study demonstrates how surface wave analysis combined with an RPM can be used for quantitative interpretation of the acoustic response of surficial aquifers.
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    Using Dark Fiber and Distributed Acoustic Sensing to Characterize a Geothermal System in the Imperial Valley, Southern California
    (Wiley, 2023) Cheng, Feng; Ajo-Franklin, Jonathan B.; Nayak, Avinash; Tribaldos, Veronica Rodriguez; Mellors, Robert; Dobson, Patrick; Team, the Imperial Valley Dark Fiber
    The Imperial Valley, CA, is a tectonically active transtensional basin located south of the Salton Sea; the area hosts numerous geothermal fields, including significant hidden hydrothermal resources without surface manifestations. Development of inexpensive, rugged, and highly sensitive exploration techniques for undiscovered geothermal systems is critical for accelerating geothermal power deployment as well as unlocking a low-carbon energy future. We present a case study utilizing distributed acoustic sensing (DAS) and ambient noise interferometry for geothermal reservoir imaging, utilizing unlit fiber-optic telecommunication infrastructure (dark fiber). The study exploits two days of passive DAS data acquired in early November 2020 over a ∼28-km section of fiber from Calipatria, CA to Imperial, CA. We apply ambient noise interferometry to retrieve coherent signals from DAS records and develop a bin stacking technique to attenuate the effects from persistent localized noise sources and to enhance retrieval of coherent surface waves. As a result, we are able to obtain high-resolution two-dimensional (2D) S wave velocity (Vs) structure to 3 km depth, based on joint inversion of both the fundamental and higher overtones. We observe a previously unmapped high Vs and low Vp/Vs ratio feature beneath the Brawley geothermal system, which we interpret to be a zone of hydrothermal mineralization and lower porosity. This interpretation is consistent with a host of other measurements including surface heat flow, gravity anomalies, and available borehole wireline data. These results demonstrate the potential utility of DAS deployed on dark fiber for geothermal system exploration and characterization in the appropriate geological settings.
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    Utilizing distributed acoustic sensing and ocean bottom fiber optic cables for submarine structural characterization
    (Springer Nature, 2021) Cheng, Feng; Chi, Benxin; Lindsey, Nathaniel J.; Dawe, T. Craig; Ajo-Franklin, Jonathan B.
    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.
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    Watching the Cryosphere Thaw: Seismic Monitoring of Permafrost Degradation Using Distributed Acoustic Sensing During a Controlled Heating Experiment
    (Wiley, 2022) Cheng, Feng; Lindsey, Nathaniel J.; Sobolevskaia, Valeriia; Dou, Shan; Freifeld, Barry; Wood, Todd; James, Stephanie R.; Wagner, Anna M.; Ajo-Franklin, Jonathan B.
    Permafrost degradation is rapidly increasing in response to a warming Arctic climate, altering landscapes and damaging critical infrastructure. Solutions for monitoring permafrost thaw dynamics are essential to understand biogeochemical feedbacks as well as to issue warnings for hazardous geotechnical conditions. We investigate the feasibility of permafrost monitoring using permanently installed fiber-optic seismic networks. We conducted a 2-month seismic monitoring campaign during a controlled thaw experiment using a permanent surface orbital vibrator (SOV) and a 2D-array of distributed acoustic sensing (DAS) cables, and observed significant (15%) shear-wave velocity (Vs) reductions and approximately 2 m depression of the permafrost table beneath the heating zone. These observations were validated by time-lapse horizontal-to-vertical spectral ratio (HVSR) analysis from three co-located broadband seismometers. The combination of SOV and DAS provided unique seismic observations for permafrost monitoring at the field scale, as well as a basis for design and development of early warning systems for permafrost thaw.