ASSESSMENT OF SPATIALLY VARYING GROUND MOTION COHERENCE BASED ON MICROSEISMIC FIELD MEASUREMENTS AND NUMERICAL ANALYSES

Abstract eng:
The influence of the subsurface soil structure and in particular its fundamental frequency on the evolution of the spatial coherence of ground motion, a measure of importance in high sensitivity accelerator facilities such as synchrotrons and linear colliders, is studied based on extensive field measurements of micro-seismic activity and numerical analysis of wave propagation in layered media. Specifically, the evolution of the micro-seismic activity at the surface of the NSLS II site consisting of ~600m deep formation of silt, clay, sand and sandy gravels over bedrock with a water table interface near the surface has been assessed over an extensive period of daily and seasonal variations. Along with the micro-seismic amplitudes and spectra, coherence measurements were made both at the said site and other sites around the world where stability sensitive synchrotrons were being constructed. Observed characteristic behavior of the spatial coherence was explored using two- and three-dimensional, numerical models representing the site were field measurements were conducted. The field data indicated loss of coherence even at small separation distances attributed to the resonant frequency of the deposit. This elaborate study was conducted in an effort to understand the impact of the soil layer (s) as well as the spatial variation of soil properties on coherence and help explain the observed behavior. Results of the field study on micro-seismic background which includes natural and cultural noise (ocean wave action and man-made sources) and of the analytical/numerical effort aiming to predict the coherence spatial evolution will be presented.

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