Spektra CS-18 Seismic Calibration System: Reference Measurement Distortion Reduction

Abstract:

The Facility for Acceptance, Calibration and Testing (FACT) at Sandia National Laboratories has been a valued resource for the U.S. monitoring community for decades. The FACT site hosts a number of capabilities focused on component evaluation, including a Spektra CS-18 seismic calibration system. The system utilizes a laser vibrometer as a means of independently measuring the shake table’s motion. We have observed in seismic sensor calibration, and vibration susceptibility tests of infrasound sensors, distortion of the reference laser measurement increases as excitation frequency decreases at and below approximately 1 Hz. Our Spektra system is housed within an active lab/work area also housing an infrasound source and isolation chamber, environmental chamber and work bench. We suspected the lab’s environment may have negative effects on calibration measurements made with the system, and as a matter of good operating practice we limit work activity and use of the lab during calibration measurements. We hypothesized that circulating air in the lab may be affecting the laser reference. To test our hypothesis we devised a test plan in which calibrations are run on the Spectra S-13 vertical table with a Geotech GS-13 short-period seismometer. Calibrations were run in four different configurations, ranging from those occurring while the system is completely exposed to air circulation induced by operation of the heating, ventilation and air condition system (HVAC), to calibrations occurring with the HVAC off and the laser isolation platform and vertical table enclosed. Average distortion differences between configurations vary with frequency; in particular, below 2 Hz, above 125 Hz and at 3.16 Hz. While distortion is lower at 0.1 Hz in configurations with the enclosure on and HVAC off, at 0.158 Hz these configurations have higher distortion (the difference greater than the standard deviations of the distortion measurements) than configurations with the enclosure off and curtain off.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. SAND2020-7070A.