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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