Knock Knock, Who's There?: Identifying the source of signals in the first DAS deployment in a fluvial setting
2023-05
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Knock Knock, Who's There?: Identifying the source of signals in the first DAS deployment in a fluvial setting
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2023-05
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Over the past several decades, many studies have developed traditional seismic methods to monitor pressure fluctuations produced by turbulence and bedload sediment that impact riverbeds. A critical challenge in analyzing and interpreting fluvial processes with seismic signals is the overlap of numerous sources and processes. These signals are recorded by individual seismometers in heterogeneous environments adding further complexity. A new and exciting geoscience method called distributed acoustic sensing (DAS) has been developed that utilizes fiber optic cable as a pressure sensing device to record continuous ground motion, comparable to a large seismic array of single-channel geophones or accelerometers. This study aims to record and study the signal of processes occurring in the creek, but since the cable is not fixed, it is not a passive recorder. Changes in strain along the cable caused by interaction between the cable and the creek are also recorded. In this study, we examine the dataset of the first deployment of DAS in a fluvial setting. The focus is on spatial variations in the signal recorded by a cable submerged along the thalweg of Clear Creek in Golden, CO. Data analysis suggests that there are many different types of signals in the data, with various apparent velocities and frequency content. One such signal reminiscent of repeating “knock” sounds appears to propagate within the cable itself because the depth of the signal is near zero, and the propagation velocity is inconsistent with the propagation velocity of sound in water. We believe a point along the cable is hitting the bottom of the creek, creating these “knocks.” At the same time, we observe spectral gliding in a portion of the data we believe is associated with wave reflection, refraction, and interference phenomena. Another signal is investigated that is reminiscent of bubbles. The signal is localized near two waterfalls and may be present at other waterfalls in the creek but cannot be determined due to substantial amounts of noise at those points. A cross-correlation of each channel with every other channel reveals strong correlation between nearby channels and six regions of interest, associated with similar ambient sound. The cross-correlation matrix demonstrates one method of using DAS data to describe a general layout and other physical properties associated with a fluvial system. The methodology requires refinement to limit unknown variables and improve interpretation. We conclude that a more controlled study is required to utilize and understand DAS fully.
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University of Minnesota M.S. thesis. May 2023. Major: Earth Sciences. Advisor: Maximiliano Bezada. 1 computer file (PDF); vii, 30 pages.
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Tate, Bill. (2023). Knock Knock, Who's There?: Identifying the source of signals in the first DAS deployment in a fluvial setting. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/258614.
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