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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 15:30:56 +0000Time-Frequency Analysis of Systems with Changing Dynamic Properties
https://resolver.caltech.edu/CaltechETD:etd-11292006-214839
Authors: {'items': [{'email': 'samuel.c.bradford@jpl.nasa.gov', 'id': 'Bradford-Samuel-Case-V', 'name': {'family': 'Bradford', 'given': 'Samuel Case, V'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/HMK7-FJ81
<p>The Wigner-Ville Distribution, and related refinements, represent a class of advanced time-frequency analysis tools that are distinguished from Fourier and wavelet methods by an increase in resolution in the time frequency plane. Time-frequency analysis provides a set of exploratory tools for analyzing changing frequency content in a signal, which can then be correlated with damage patterns in a structure.</p>
<p>For systems of interest to engineers, investigating the changing properties of a system is typically performed by analyzing vibration data from the system, rather than direct inspection of each component. Nonlinear elastic behavior in the force-displacement relationship can decrease the apparent natural frequencies of the system - these changes typically occur over fractions of a second in moderate to strong excitation and the system gradually recovers to pre-event levels. Structures can also suffer permanent damage (e.g., plastic deformation or fracture), permanently decreasing the observed natural frequencies as the system loses stiffness. Advanced time-frequency representations provide a set of exploratory tools for analyzing changing frequency content in a signal, which can then be correlated with damage patterns in a structure. Modern building instrumentation allows for an unprecedented investigation into the changing dynamic properties of structures: a framework for using time-frequency analysis methods for instantaneous system identification is discussed.</p>https://thesis.library.caltech.edu/id/eprint/4689Damage Detection in Civil Structures using High-Frequency Seismograms
https://resolver.caltech.edu/CaltechTHESIS:12192013-162221707
Authors: {'items': [{'email': 'VanessaMHeckman@gmail.com', 'id': 'Heckman-Vanessa-Mary', 'name': {'family': 'Heckman', 'given': 'Vanessa Mary'}, 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/NVKN-8D47
<p>The dynamic properties of a structure are a function of its physical properties, and changes in the physical properties of the structure, including the introduction of structural damage, can cause changes in its dynamic behavior. Structural health monitoring (SHM) and damage detection methods provide a means to assess the structural integrity and safety of a civil structure using measurements of its dynamic properties. In particular, these techniques enable a quick damage assessment following a seismic event. In this thesis, the application of high-frequency seismograms to damage detection in civil structures is investigated. </p>
<p>Two novel methods for SHM are developed and validated using small-scale experimental testing, existing structures in situ, and numerical testing. The first method is developed for pre-Northridge steel-moment-resisting frame buildings that are susceptible to weld fracture at beam-column connections. The method is based on using the response of a structure to a nondestructive force (i.e., a hammer blow) to approximate the response of the structure to a damage event (i.e., weld fracture). The method is applied to a small-scale experimental frame, where the impulse response functions of the frame are generated during an impact hammer test. The method is also applied to a numerical model of a steel frame, in which weld fracture is modeled as the tensile opening of a Mode I crack. Impulse response functions are experimentally obtained for a steel moment-resisting frame building in situ. Results indicate that while acceleration and velocity records generated by a damage event are best approximated by the acceleration and velocity records generated by a colocated hammer blow, the method may not be robust to noise. The method seems to be better suited for damage localization, where information such as arrival times and peak accelerations can also provide indication of the damage location. This is of significance for sparsely-instrumented civil structures. </p>
<p>The second SHM method is designed to extract features from high-frequency acceleration records that may indicate the presence of damage. As short-duration high-frequency signals (i.e., pulses) can be indicative of damage, this method relies on the identification and classification of pulses in the acceleration records. It is recommended that, in practice, the method be combined with a vibration-based method that can be used to estimate the loss of stiffness. Briefly, pulses observed in the acceleration time series when the structure is known to be in an undamaged state are compared with pulses observed when the structure is in a potentially damaged state. By comparing the pulse signatures from these two situations, changes in the high-frequency dynamic behavior of the structure can be identified, and damage signals can be extracted and subjected to further analysis. The method is successfully applied to a small-scale experimental shear beam that is dynamically excited at its base using a shake table and damaged by loosening a screw to create a moving part. Although the damage is aperiodic and nonlinear in nature, the damage signals are accurately identified, and the location of damage is determined using the amplitudes and arrival times of the damage signal. The method is also successfully applied to detect the occurrence of damage in a test bed data set provided by the Los Alamos National Laboratory, in which nonlinear damage is introduced into a small-scale steel frame by installing a bumper mechanism that inhibits the amount of motion between two floors. The method is successfully applied and is robust despite a low sampling rate, though false negatives (undetected damage signals) begin to occur at high levels of damage when the frequency of damage events increases. The method is also applied to acceleration data recorded on a damaged cable-stayed bridge in China, provided by the Center of Structural Monitoring and Control at the Harbin Institute of Technology. Acceleration records recorded after the date of damage show a clear increase in high-frequency short-duration pulses compared to those previously recorded. One undamage pulse and two damage pulses are identified from the data. The occurrence of the detected damage pulses is consistent with a progression of damage and matches the known chronology of damage. </p>
https://thesis.library.caltech.edu/id/eprint/8046New Applications that Come from Extending Seismic Networks into Buildings
https://resolver.caltech.edu/CaltechTHESIS:03182014-225151551
Authors: {'items': [{'email': 'minghei.cheng@gmail.com', 'id': 'Cheng-Ming-Hei', 'name': {'family': 'Cheng', 'given': 'Ming Hei'}, 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/STB2-XR07
This thesis describes engineering applications that come from extending seismic networks into building structures. The proposed applications will benefit the data from the newly developed crowd-sourced seismic networks which are composed of low-cost accelerometers. An overview of the Community Seismic Network and the earthquake detection method are addressed. In the structural array components of crowd-sourced seismic networks, there may be instances in which a single seismometer is the only data source that is available from a building. A simple prismatic Timoshenko beam model with soil-structure interaction (SSI) is developed to approximate mode shapes of buildings using natural frequency ratios. A closed form solution with complete vibration modes is derived. In addition, a new method to rapidly estimate total displacement response of a building based on limited observational data, in some cases from a single seismometer, is presented. The total response of a building is modeled by the combination of the initial vibrating motion due to an upward traveling wave, and the subsequent motion as the low-frequency resonant mode response. Furthermore, the expected shaking intensities in tall buildings will be significantly different from that on the ground during earthquakes. Examples are included to estimate the characteristics of shaking that can be expected in mid-rise to high-rise buildings. Development of engineering applications (e.g., human comfort prediction and automated elevator control) for earthquake early warning system using probabilistic framework and statistical learning technique is addressed. https://thesis.library.caltech.edu/id/eprint/8145SteelConverter and Caltech VirtualShaker: Rapid Nonlinear Cloud-Based Structural Model Conversion and Analysis
https://resolver.caltech.edu/CaltechTHESIS:10052015-133333291
Authors: {'items': [{'email': 'chrisjanover@gmail.com', 'id': 'Janover-Christopher-George', 'name': {'family': 'Janover', 'given': 'Christopher George'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9SF2T3V
<p>STEEL, the Caltech created nonlinear large displacement analysis software, is currently used by a large number of researchers at Caltech. However, due to its complexity, lack of visualization tools (such as pre- and post-processing capabilities) rapid creation and analysis of models using this software was difficult. SteelConverter was created as a means to facilitate model creation through the use of the industry standard finite element solver ETABS. This software allows users to create models in ETABS and intelligently convert model information such as geometry, loading, releases, fixity, etc., into a format that STEEL understands. Models that would take several days to create and verify now take several hours or less. The productivity of the researcher as well as the level of confidence in the model being analyzed is greatly increased.</p>
<p>It has always been a major goal of Caltech to spread the knowledge created here to other universities. However, due to the complexity of STEEL it was difficult for researchers or engineers from other universities to conduct analyses. While SteelConverter did help researchers at Caltech improve their research, sending SteelConverter and its documentation to other universities was less than ideal. Issues of version control, individual computer requirements, and the difficulty of releasing updates made a more centralized solution preferred. This is where the idea for Caltech VirtualShaker was born. Through the creation of a centralized website where users could log in, submit, analyze, and process models in the cloud, all of the major concerns associated with the utilization of SteelConverter were eliminated. Caltech VirtualShaker allows users to create profiles where defaults associated with their most commonly run models are saved, and allows them to submit multiple jobs to an online virtual server to be analyzed and post-processed. The creation of this website not only allowed for more rapid distribution of this tool, but also created a means for engineers and researchers with no access to powerful computer clusters to run computationally intensive analyses without the excessive cost of building and maintaining a computer cluster. </p>
<p>In order to increase confidence in the use of STEEL as an analysis system, as well as verify the conversion tools, a series of comparisons were done between STEEL and ETABS. Six models of increasing complexity, ranging from a cantilever column to a twenty-story moment frame, were analyzed to determine the ability of STEEL to accurately calculate basic model properties such as elastic stiffness and damping through a free vibration analysis as well as more complex structural properties such as overall structural capacity through a pushover analysis. These analyses showed a very strong agreement between the two softwares on every aspect of each analysis. However, these analyses also showed the ability of the STEEL analysis algorithm to converge at significantly larger drifts than ETABS when using the more computationally expensive and structurally realistic fiber hinges. Following the ETABS analysis, it was decided to repeat the comparisons in a software more capable of conducting highly nonlinear analysis, called Perform. These analyses again showed a very strong agreement between the two softwares in every aspect of each analysis through instability. However, due to some limitations in Perform, free vibration analyses for the three story one bay chevron brace frame, two bay chevron brace frame, and twenty story moment frame could not be conducted. With the current trend towards ultimate capacity analysis, the ability to use fiber based models allows engineers to gain a better understanding of a building’s behavior under these extreme load scenarios. </p>
<p>Following this, a final study was done on Hall’s U20 structure [1] where the structure was analyzed in all three softwares and their results compared. The pushover curves from each software were compared and the differences caused by variations in software implementation explained. From this, conclusions can be drawn on the effectiveness of each analysis tool when attempting to analyze structures through the point of geometric instability. The analyses show that while ETABS was capable of accurately determining the elastic stiffness of the model, following the onset of inelastic behavior the analysis tool failed to converge. However, for the small number of time steps the ETABS analysis was converging, its results exactly matched those of STEEL, leading to the conclusion that ETABS is not an appropriate analysis package for analyzing a structure through the point of collapse when using fiber elements throughout the model. The analyses also showed that while Perform was capable of calculating the response of the structure accurately, restrictions in the material model resulted in a pushover curve that did not match that of STEEL exactly, particularly post collapse. However, such problems could be alleviated by choosing a more simplistic material model. </p>
https://thesis.library.caltech.edu/id/eprint/9198Improving Seismic Collapse Risk Assessments of Steel Moment Frame Buildings
https://resolver.caltech.edu/CaltechTHESIS:06012018-015306089
Authors: {'items': [{'email': 'kennybuyco@gmail.com', 'id': 'Buyco-John-Kenneth', 'name': {'family': 'Buyco', 'given': 'John Kenneth'}, 'orcid': '0000-0002-8182-7119', 'show_email': 'YES'}]}
Year: 2018
DOI: 10.7907/2SFH-WP06
<p>It is important to be able to accurately assess seismic risk so that vulnerabilities can be prioritized for retrofit, emergency response procedures can be properly informed, and insurance rates can be sustainably priced to manage risk. To assess the risk of a building (or class of buildings) collapsing in a seismic event, procedures exist for creating one or more mathematical models of the structure of interest and performing nonlinear time history analysis with a large suite of input ground motions to calculate the building's seismic fragility and collapse risk. In this dissertation, three aspects of these procedures for assessing seismic collapse risk are investigated for the purpose of improving their accuracy.</p>
<p>It is common to use spectral acceleration with a damping ratio of 5% as a ground motion intensity measure (IM) for assessing collapse fragility. In this dissertation, the use of 70%-damped spectral acceleration as an IM is investigated, with a focus on evaluating its sufficiency and efficiency. Incremental dynamic analysis (IDA) is performed for 22 steel moment frame (SMF) models with 50 biaxial ground motion records to formally evaluate the performance of 70%-damped spectral acceleration as an IM for highly nonlinear response and collapse. It is found that 70%-damped spectral acceleration is much more efficient than 5%-damped spectral acceleration and much more sufficient with respect to epsilon for all considered levels of highly nonlinear response. Its efficiency and sufficiency compares also compares well with more advanced IMs such as average spectral acceleration.</p>
<p>When selecting input ground motions for nonlinear time history analysis, most engineers select ground motion records from the NGA-West2 database, which are processed with high-pass filters to remove long-period noise. In this dissertation, the extent to which these filters remove actual ground motion that is relevant to nonlinear time history analysis is evaluated. 52 near-source ground motion records from large-magnitude events are considered. Some records are processed by applying high-pass filters and others are processed by record-specific tilt corrections. Raw and NGA-West2 records are also considered. IDA is performed for 9-, 20-, and 55-story steel moment frame models with these processed records to assess the effects of ground motion processing on the calculated collapse capacity. It is found that if the cutoff period (Tc) is at least 40 seconds, then applying a high-pass filter does not have more than a negligible effect on collapse capacity for any of the considered records or building models. For shorter Tc (e.g. 10 or 15 seconds), it is found that the filters sometimes have a large effect on calculated collapse capacity, in some cases by over 50%, even if Tc is much larger than the building’s fundamental period. Of the considered ground motions, simply using the raw, uncorrected records usually yields more accurate results than using ground motions that have been processed with Tc less than or equal to 20 seconds.</p>
<p>For an existing building with unknown design plans, one might perform a collapse risk assessment using an archetype model for which the specific member sizes are assumed based on the relevant design code and building site. In this dissertation, the sensitivity of seismic collapse risk estimates to design criteria and procedures are evaluated for six 9-story and four 20-story post-Northridge SMFs. These SMFs are designed for downtown Los Angeles using different design procedures according to ASCE 7-05 and ASCE 7-10. Seismic risk analysis is performed using the results of IDA with 44 ground motion records and the results are compared to those of pre-Northridge models. It is found that the collapse risk of 9-story SMFs designed according to performance-based design vary by 3x, owing to differences in GMPEs used to generate site-specific response spectra. There is generally less variation in the collapse risk estimates of 20-story post-Northridge SMFs when compared to 9-story post-Northridge SMFs because wind drift limits control the design of many members of the 20-story SMFs. Differences in collapse risk between pre- and post-Northridge SMFs are found to be at least 4x and 8x for the 9- and 20-story models, respectively. Furthermore, in response to four strong ground motion records from large-magnitude events, some of the 9-story and all of the 20-story pre-Northridge SMFs experience collapse and most of the post-Northridge SMFs experience significant damage (MIDR > 0.03).</p>https://thesis.library.caltech.edu/id/eprint/10994Achieving Higher Fidelity Building Response through Emerging Technologies and Analytical Techniques
https://resolver.caltech.edu/CaltechTHESIS:07262017-074416397
Authors: {'items': [{'email': 'massari.8@osu.edu', 'id': 'Massari-Anthony-Thomas', 'name': {'family': 'Massari', 'given': 'Anthony Thomas'}, 'orcid': '0000-0002-6561-4674', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/Z9HH6H7N
The integration of sensor technology into the built environment has created an opportunity for a new approach to infrastructure development and management. Using collected data and principles of general physics, we discuss means and methods of using low cost dense instrumentation to perform damage detection, structural identification, and the benefits of cyber physical systems to community resilience. A nonlinear damping strategy for braced frame structures is introduced incorporating capped levels of damping forces. The study shows the effect of having control of damping forces in nonlinear analysis and the importance of limiting energy dissipation to rational levels. The issue of sliding mass is also studied to determine the contribution to energy loss and the effect to overall response. The results indicate a need to incorporate this effect in stiff structures with intentionally decoupled mass such as data centers. Finally, a discussion on dual system structures under plastic deformation in a post event deformed configuration is presented. A suggested displacement based method for design is suggested for implementation into future editions of the building code.https://thesis.library.caltech.edu/id/eprint/10352Identification of Structural Damage, Ground Motion Response, and the Benefits of Dense Seismic Instrumentation
https://resolver.caltech.edu/CaltechTHESIS:11052020-043034327
Authors: {'items': [{'email': 'filip_os@outlook.com', 'id': 'Filippitzis-Filippos', 'name': {'family': 'Filippitzis', 'given': 'Filippos'}, 'orcid': '0000-0001-8377-4914', 'show_email': 'NO'}]}
Year: 2021
DOI: 10.7907/x0sf-pq18
<p>This study explores the problems of identifying structural damage in steel frame buildings, through the use of dense instrumentation over the height of the building, and of characterizing the ground motion response in urban Los Angeles following the 2019 Ridgecrest earthquakes, through the use of dense instrumentation from available seismic networks, including the very dense Community Seismic Network.</p>
<p>First we explore the possibility of tracing possible nonlinear behavior of a structure by updating an equivalent linear system model in short time segments of the earthquake-induced excitation and response time histories, using a moving time window approach. The stiffness and damping related parameters of the equivalent linear model are estimated by minimizing a measure of fit between the measured and model predicted response time histories for each time window. We explore the effectiveness of the methodology for two example applications, a single-story and a six-story steel moment frame building. For the single-story building, the methodology is shown to be very effective in tracing the nonlinearities, while the six-story building is designed to also reveal the limitations of the methodology, mainly arising from the different types of model errors manifested in the formulation.</p>
<p>Next, we investigate the problem of structural damage identification through the use of sparse Bayesian learning (SBL) techniques. This is based on the premise that damage in a structure appears only in a limited number of locations. SBL methods that had been previously applied for structural damage identification used measurements related to modal properties and were thus limited to linear models. Here we present a methodology that allows for the application of SBL in non-linear models, using time history measurements recorded from a dense network of sensors installed along the building height. We develop a two-step optimization algorithm in which the most probable values of the structural model parameters and the hyper-parameters are iteratively obtained. An equivalent single-objective minimization problem that results in the most probable model parameter values is also derived. We consider the example problem of identifying damage in the form of weld fractures in a 15-story moment resisting steel frame building, using a nonlinear finite element model and simulated acceleration data. Fiber elements and a bilinear material model are used in order to account for the change of local stiffness when cracks at the welds are subjected to tension and the model parameters characterize the loss of stiffness as the crack opens under tension. The damage identification results demonstrate the effectiveness and robustness of the proposed methodology in identifying the existence, location, and severity of damage for a variety of different damage scenarios, and degrees of model and measurement errors. The results show the great promise of the SBL methodology for damage identification by integrating nonlinear finite element models and response time history measurements.</p>
<p>The final part of the thesis involves studying the ground motion response in urban Los Angeles during the two largest events (M7.1 and M6.4) of the 2019 Ridgecrest earthquake sequence using recordings from multiple regional seismic networks as well as a subset of 350 stations from the much denser Community Seismic Network. The response spectral (pseudo) accelerations for a selection of periods of engineering significance are calculated. Significant spectral acceleration amplification is present and reproducible between the two events. For the longer periods, coherent spectral acceleration patterns are visible throughout the Los Angeles Basin, while for the shorter periods, the motions are less spatially coherent. The dense Community Seismic Network instrumentation allows us to observe smaller-scale coherence even for these shorter periods. Examining possible correlations of the computed response spectral accelerations with basement depth and Vs30, we find the correlations to be stronger for the longer periods. Furthermore, we study the performance of two state-of-the-art methods for estimating ground motions for the largest event of the Ridgecrest earthquake sequence, namely 3D finite difference simulations and ground motion prediction equations. For the simulations, we are interested in the performance of the two Southern California Earthquake Center 3D Community Velocity Models (CVM-S and CVM-H). For the ground motion prediction equations, we consider four of the 2014 Next Generation Attenuation-West2 Project equations. For some cases, the methods match the observations reasonably well; however, neither approach is able to reproduce the specific locations of the maximum response spectral accelerations, or match the details of the observed amplification patterns.</p>https://thesis.library.caltech.edu/id/eprint/13992Modeling and Parameterization of Basin Effects for Engineering Design Applications
https://resolver.caltech.edu/CaltechTHESIS:03312022-021127047
Authors: {'items': [{'email': 'peyman.ayoubi@gmail.com', 'id': 'Ayoubi-Peyman', 'name': {'family': 'Ayoubi', 'given': 'Peyman'}, 'orcid': '0000-0001-6795-4923', 'show_email': 'YES'}]}
Year: 2022
DOI: 10.7907/4e61-q346
<p>The term "Basin effects" refers to trapped and reverberating earthquake waves in soft sedimentary deposits overlying convex depressions of the basement bedrock, which significantly alter the frequency content, amplitude, and duration of seismic waves. This has played an important role in shaking duration and intensity in past earthquakes such as the M<sub>w</sub> 8.0 1985 Michoácan, Mexico, M<sub>w</sub> 6.9 1995 Kobe, Japan, and M<sub>w</sub> 7.8 2015 Gorkha, Nepal earthquakes. While the standard practice is to perform a 1D analysis of a soil column, edge-effect and surface waves are among the key contributors to the surface ground motion within a basin. This thesis studies basin effects in a 2D medium to help better understand the phenomena, better parameterize them, and suggest a path to appropriately incorporate them in ground motion prediction equations and building design codes. After the introduction in Chapter 1, I present the results in three main parts as follows:</p>
<p>In Chapter 2, we perform an extensive parametric study on the characteristics of surface ground motion associated with basin effects. We use an elastic idealized-shaped medium subjected to vertically propagating SV plane waves and examine the effects of basin geometry and material properties. We specifically study the effects of four dimensionless parameters, the width-to-depth (aspect) ratio, the rock-to-soil material contrast, a dimensionless frequency that quantifies the depth of the basin relative to the dominant incident wavelength, and a dimensionless distance that quantifies the distance of the basin edges relative to the dominant wavelength. Our results show that basin effects can be reasonably characterized using at least three independent parameters, each of which can significantly alter the resultant ground motion. To demonstrate the application of dimensional analysis applied here, we investigate the response of the Kathmandu Valley during the 2015 M<sub>w</sub> 7.8 Gorkha Earthquake in Nepal using an idealized basin geometry and soil properties. Our results show that a simplified model can capture notable ground motion characteristics associated with basin effects.</p>
<p>Chapter 3 uses the identified parameters from the previous chapter to estimate surface acceleration time-series given earthquake frequency content, basin geometry and material properties, and location inside a basin. This is of practical use when the amount of available data is limited or the fast estimation of time-series is desirable. For that, we train a neural network to estimate surface ground acceleration time-series across a basin. Three input parameters are needed for the estimation: basin-to-bedrock shear wave velocity ratio, aspect ratio of the basin, and dimensionless location. These parameters define an idealized-shaped basin and the location at which the time-series are to be computed. It will be shown that the model performs with high accuracy in comparison to the result of a full-fidelity Finite Element (FE) simulation (ground truth) and generalizes reasonably well for input parameters outside of the training set. Moreover, we will also use the model for the case of Kathmandu Valley, Nepal during the 2015 M<sub>w</sub> 7.8 Gorkha earthquake and compare the results of NN versus recordings of the mainshock, similarly to Chapter 2.</p>
<p>Once we have studied basin behavior in a homogeneous case in previous chapters, we focus on material representation inside a basin in Chapter 4. Here, we study basin effects for the cases where high-frequency response and realistic material representation are desirable. However, the lack of sufficient information about the material properties and stratigraphy of a basin prevents accurate simulation of the phenomena. To do that, we perform a stochastic analysis using the Monte Carlo technique, where a random field represents basin material. Similarly to the previous chapters, we use a 2D FE model with an idealized basin subjected to vertically propagating SV plane waves and investigate the spatial variation of surface ground motion (SGM) associated with basin effects by assuming different realizations of the correlated random field. We then study various correlation lengths, coefficients of variations, and autocorrelation functions to evaluate their contribution to SGM. We show that the coefficient of variation is the most influential parameter on SGM, followed by correlation lengths and type of autocorrelation function. Increasing the coefficient of variation not only affects the mean surface amplification, but also results in a dramatic change in the standard deviation. Correlation lengths and autocorrelation functions, on the other hand, are of less importance for the cases we examine in this study.</p>https://thesis.library.caltech.edu/id/eprint/14535Probing Solid-Earth, Ocean, and Structural Dynamics with Distributed Fiber-Optic Sensing
https://resolver.caltech.edu/CaltechTHESIS:11172022-023850059
Authors: {'items': [{'email': 'ethanfwilliams@gmail.com', 'id': 'Williams-Ethan-Francis', 'name': {'family': 'Williams', 'given': 'Ethan Francis'}, 'orcid': '0000-0002-6471-4497', 'show_email': 'NO'}]}
Year: 2023
DOI: 10.7907/vehm-dd85
Observational geophysics conventionally relies on point sensors to document and monitor Earth’s dynamic processes, from locating earthquakes and imaging subsurface structure with seismometers to forecasting coastal wave heights and detecting tsunamis with buoys. Distributed acoustic sensing (DAS) offers a fundamentally different paradigm: distributed instead of point sensing. DAS converts fiber-optic cables into dense arrays of broadband, linear strainmeters, with spatial resolution as fine as one meter and temporal resolution up to several thousand samples per second. The first four chapters of this thesis concern ocean-bottom DAS, repurposing pre-existing telecommunications and power cables as distributed seafloor sensing networks for seismology and physical oceanography. In Chapter 2, we analyze one of the first ocean-bottom DAS datasets, demonstrating that seismic and ocean waves observed on the same array are related by a classic theory of double-frequency microseism generation. We also extract the principal body-wave phases of a M8.2 deep earthquake, demonstrating the earthquake detection capabilities of DAS even in a shallow water environment. In Chapter 3, we apply ambient noise interferometry to a one-hour of ocean-bottom DAS data and derive a shallow shear-wave velocity model. We also isolate spurious arrivals in noise cross-correlations associated with nearby offshore wind turbines, suggesting potential for remote monitoring. In Chapter 4, we adapt ambient noise interferometry to the ocean surface gravity wavefield, and estimate the tidal current velocity along a short cable segment in the Strait of Gibraltar with a waveform stretching method. In Chapter 5, we explore the application of DAS as a temperature sensor at long periods, documenting temperature signals up to 4 K associated with internal wave and boundary layer dynamics. We demonstrate that while ocean-bottom DAS exhibits sufficient strain sensitivity to record seafloor geodetic processes, oceanic temperature transients may overprint such signals. The last part of this thesis concerns a different frontier in geophysical instrumentation: long time-series. With a 20-year continuous record of ambient vibrations from a single accelerometer located on the ninth floor of a concrete building, we document long-term, passive changes in the building’s natural frequencies as well as complex, time-dependent nonlinear elasticity during earthquakes.https://thesis.library.caltech.edu/id/eprint/15065