ASSESSING VIBRATION SUSCEPTIBILITY OVER SHALLOW AND DEEP BEDROCK USING ACCELEROMETERS AND WALKAWAY SURVEYS

Vibration-monitoring instruments that employ standard, velocity-responsive geophones lack the frequency range and sensitivity required to characterize low-level vibration that can degrade the performance of precision laboratory instruments and adversely affect manufacturing processes. High-sensitivity accelerometers have a flat response over a broad seismic frequency range, producing stronger signals (higher signal-to-noise ratio) than standard geophones over much of the spectrum above and below the natural geophone frequency. Accelerometers can be used to examine the vibration susceptibility of sites that are being considered for vibration-sensitive manufacturing, testing, or calibration facilities. Unfortunately, ambient ground motion at proposed, inactive sites is commonly much lower than that at existing facilities and is much lower than would be expected when activity commences at the proposed site. In addition to ambient monitoring at current and proposed laboratory sites, we performed walkaway seismic tests to compare susceptibility to induced ground motion at shallow- and deep-bedrock sites in Central Texas. Ambient monitoring demonstrated that both shallow- and deep-bedrock sites satisfied recommended laboratory guidelines for displacement and acceleration. Walkaway surveys, performed using a weight-drop source at 10-m intervals from 10 to 100 m from the ground-coupled, triaxial accelerometers, showed that (a) accelerations induced over shallow and deep bedrock exceed recommended thresholds to distances of 50 to 80 m from the source, (b) much larger horizontal accelerations are induced close to the source (60 m or less) over deep bedrock, and (c) peak vertical acceleration is comparable or higher over shallow bedrock than over deep bedrock at all distances. Walkaway surveys can be used elsewhere to assess the vibration susceptibility of undeveloped sites and to establish minimum separation distances between potential vibration sources and sensitive equipment.