As transportation infrastructure continues to age, new methods of noncontact sensing should be evaluated and, if found suitable, used for bridge monitoring and structural health assessment. This study highlighted the use of infrasound monitoring, a geophysical technique utilizing acoustics below 20 Hz, as one possible solution for noncontact, nonline-of-sight bridge health monitoring. The study focused on the technique of infrasound for infrastructure monitoring with a detailed case study involving a steel, two-girder bridge in northern California. Infrasound was used to detect natural modes of the structure from a distance of 2.6 km. The frequencies detected infrasonically were validated with data collected by on-structure accelerometers. The noncontact nature of this structural assessment approach has potential to supplement traditional structural assessment techniques as affordable, remote, persistent monitoring of transportation infrastructure. Implications for use of this technology were also discussed alongside specific applications for scour monitoring and postdisaster assessment.
To date, the infrasound community has avoided deployments in noisy urban sites because interests have been in monitoring distant sources with low noise sites. As monitoring interests expand to include low-energy urban sources only detectable close to the source, case studies are needed to demonstrate the challenges and benefits of urban infrasound monitoring. This case study highlights one approach to overcoming urban challenges and identifies a signal's source in a complex acoustic field. One 38 m and one 120 m aperture infrasound arrays were deployed on building rooftops north of downtown Dallas, Texas. Structural signals in the recorded data were identified, and the backazimuth to the source determined with frequency-wavenumber analysis. Fourteen days of data were analyzed to produce 314 coherent continuous-wave packets, with 246 of these detections associated with a narrow range of backazimuth directions. Analyzing the backazimuths from the two arrays identified the Mockingbird Bridge as the probable source which was the verified with seismic measurement on the structure. Techniques described here overcame the constraints imposed by urban environments and provide a basis to monitor infrastructure and its conditions at local distances (0–100 km).
Summary Infrasound data contain contributions from incoherent noise, coherent noise, and signals of interest. The design of an infrasound array to target sources of interest requires a quantification of array response, individual sensor response, propagation effects (topography and meteorological conditions), signal spectrum, and the noise environment. The Comprehensive Nuclear-Test-Ban Treaty community has spent significant effort in quantifying the acoustic field in rural environments for frequencies up to 7 Hz. Given that the nuclear monitoring and tactical infrasound community have growing interests in monitoring sources in or near populated regions, there is an emergent need to measure and understand acoustic fields in these environments as well. This paper focuses on quantification of the acoustic field in three different urban environments: (1) arrays installed within Dallas, TX, a metropolitan area, (2) an array installed at the rural-suburban interface near San Diego, CA, and (3) an array installed in Vicksburg, MS, a small city with multiple major transportation corridors. A minimum of five months of data was recorded and used for each site characterization. The analysis focuses on frequencies from 0.1–45 Hz. The quantification of the data from these three sites is accomplished with statistical noise models that capture the total ambient acoustic field, separation of the coherent portion of the field, and trend analysis to link temporal and seasonal variations to wind speed and anthropogenic activities. The resulting physical interpretation of the data demonstrates that the total acoustic field in urban regions is overall higher and inversely related to the number of coherent detections observed by an array. Furthermore, this study presents an analysis framework for characterizing additional urban arrays and provides a basis for future array site selection and installation.
Historically, infrasound arrays have been deployed in rural environments where anthropological noise sources are limited. As interest in monitoring sources at local distances grows in the infrasound community, it will be vital to understand how to monitor infrasound sources in an urban environment. Arrays deployed in urban centers have to overcome the decreased signal to noise ratio and reduced amount of real estate available to deploy an array. To advance the understanding of monitoring infrasound sources in urban environments, we deployed local and regional infrasound arrays on building rooftops of the campus of Southern Methodist University (SMU) and collected data for one seasonal cycle. The data was evaluated for structural source signals (continuous-wave packets) and when a signal was identified the back azimuth to the source was determined through frequency wavenumber analysis. This information was used to identify hypothesized structural sources; these sources were verified through direct measurement, structural numerical modeling and/or full waveform propagation modeling. Permission to publish was granted by Director, Geotechnical & Structures Laboratory.
Infrastructure in the continental United States is often used beyond its design life due to budgetary constraints and logistical hurdles. New structural health monitoring techniques have been developed to mitigate the increasingly constrained resources available by identifying the potential need to repair and retrofit aging infrastructure from modal signatures. Large structures, such as bridges and dams, emit infrasound (acoustic energy below that of human perception) at their natural modes of vibration, which can be related to structural health and capacity; this infrasonic energy can propagate tens of kilometers from the infrastructure. Currently, the determination of structural health requires intensive hands-on measurements of individual elements at repeated intervals. Persistent, remote infrasound assessment provides a method for standoff assessment of the modal behavior of structures for structural health monitoring and damage assessment. Case studies and demonstrations of this technology are presented for water control infrastructure systems, transportation infrastructure systems, and riverine infrastructure systems.
This report summarizes results of the basic research project “Infrasound Propagation in the Arctic.” The scientific objective of this project was to provide a baseline understanding of the characteristic horizontal propagation distances, frequency dependencies, and conditions leading to enhanced propagation of infrasound in the Arctic region. The approach emphasized theory and numerical modeling as an initial step toward improving understanding of the basic phenomenology, and thus lay the foundation for productive experiments in the future. The modeling approach combined mesoscale numerical weather forecasts from the Polar Weather Research and Forecasting model with advanced acoustic propagation calculations. The project produced significant advances with regard to parabolic equation modeling of sound propagation in a windy atmosphere. For the polar low, interesting interactions with the stratosphere were found, which could possibly be used to provide early warning of strong stratospheric warming events (i.e., the polar vortex). The katabatic wind resulted in a very strong low-level duct, which, when combined with a highly reflective icy ground surface, leads to efficient long-distance propagation. This information is useful in devising strategies for positioning sensors to monitor environmental phenomena and human activities.
Geophysical techniques have the potential to comprehensively assess the urban landscape, including the human-built environment. The U.S. Army Engineer Research and Development Center (ERDC) has developed methods to persistently and remotely monitor critical infrastructure by using infrasound (subaudible acoustics below 20 Hz). Infrasound can propagate tens of kilometers or more depending on source characteristics and meteorological conditions. Large bridges, dams, and other industrial sources are considered critical infrastructure. They emit infrasound since they exhibit characteristics of modal motion in the infrasound passband. While the motion of these structures may not be perceptible to humans, instrumentation can detect small air-coupled movements, providing real-time information about infrastructure behavior and condition. Techniques originally developed by the nuclear monitoring community for detecting large impulsive events from permanently emplaced instrumentation located in quiet isolated locations had to be adapted to the complex and dynamic urban space. The infrasound arrays and associated processing schema developed by the ERDC do not need to be in close proximity to the structures, are omnidirectional, and are capable of assessing multiple structures simultaneously. They continually record structural information to provide real-time information to infrastructure owners. However, several unique challenges must be addressed to successfully accomplish no-contact, persistent, structural monitoring in cluttered and noisy urban areas. This article describes recommendations for urban-appropriate instrumentation deployment methods and elevated noise floor concerns based on three field experiments representing metropolitan (Dallas, Texas), suburban (San Diego, California), and industrial (Sutter County, California) areas.
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