Variability of the horizontal-to-vertical spectral ration (hvsr) method in urban areas
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The Horizontal-to-Vertical Spectral Ratio (HVSR) method is widely employed in geotechnical investigations due to its non-intrusive approach for determining subsurface characteristics such as sediment thickness, bedrock depth, and seismic site response. The primary objective of this paper is to provide both quantitative and qualitative assessments of four potential factors influencing HVSR variability in urban environments: (1) in-situ instrumentation surface coupling (concrete vs. grass), (2) the effect of strong, nearby noise sources on in-situ instrumentation, (3) proximity to buildings, and (4) the influence of time- of-day with respect to building proximity. A total of 126 HVSR measurements across 16 locations at the University of Missouri-Columbia were analyzed, with an emphasis on influences on HVSR variability in urban environments. The results show minimal variability between HVSR measurements conducted simultaneously on concrete and grass surfaces. This finding was in agreement with commonly used HVSR acquisition guidelines but contradicted more recent guidelines provided by an HVSR equipment manufacturer. Furthermore, data indicated that concrete surfaces exhibited greater variability in noisy environments, such as construction zones, possibly due to reduced attenuation of construction vibrations, whereas grass surfaces yielded more consistent measurements. Proximity to buildings was identified as a significant factor contributing to variability, with measurements taken within 50 feet of buildings typically producing low-quality HVSR results, likely due to building-induced vibrations. Time-of-day effects were found to be less significant, with ambient noise levels having a greater impact on measurement quality than noise from building occupancy itself. HVSR frequency peaks identified from manual inspection methods improved result accuracy when compared to automated picking of the highest HVSR peak. The study highlights the importance of considering site-specific conditions and incorporating human oversight in HVSR analysis, particularly in urban environments. Key findings include: (1) concrete is a reliable surface for HVSR measurements, indicating that data acquisition does not need to be exclusively conducted on grass, (2) acquisition near strong construction noise sources can yield accurate HVSR results, and does not necessarily need to be avoided, (3) maintaining a sufficient distance from buildings (preferably >50 ft) will reduce variability in HVSR results (4) manual identification of HVSR peaks can provide improved results over automated picks in difficult environments, and (5) measurements near building sites are best conducted during times when ambient noise levels are high. These insights contribute to the refinement of HVSR guidelines and enhance its application in urban geotechnical and geophysical investigations. This study also contributes to the refinement of HVSR guidelines, offering recommendations for mitigating variability caused by urban factors and suggesting avenues for future research, including exploring the effects of building height, subsurface complexity, and azimuthal variability on HVSR measurements.
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M.S.