High-Frequency Ground Motion and Electrical Calibrations of Seismometers: Application to IMS Stations

Abstract Calibration of seismometers used in networks such as the International Monitoring System (IMS) is important for ensuring confidence in the measurements of ground motion and the resulting analysis that is performed on the waveform data. In this study, six models of seismometers widely used at IMS stations were calibrated using high-precision vertical and horizontal shake tables. The calibration procedure followed the ISO 16063-11 standard for primary vibration calibration by laser interferometry. By following this standard and using shake tables with up-to-date calibrations that are traceable to the International System of Units (SI), the obtained calibrations are themselves fully traceable to the SI, something that many traditional seismometer calibration procedures lack. The sensors were evaluated from 0.1 to 50 Hz, and the calibration results are directly compared to the sensors’ nominal response models provided by the manufacturers. The calibrations were repeated multiple times to allow us to quantify the uncertainty associated with measurement repeatability. We combined these uncertainty estimates with the uncertainties associated with the shake tables and laser vibrometer to obtain an overall estimate for the measurement uncertainty. We find that the calibrations are highly repeatable and the uncertainties that arise from repeated measurements are orders of magnitude smaller than the uncertainties specified by the manufacturer of the shake tables. Electrical calibrations were then performed on the same sensors, which mimics how fielded IMS seismometers are calibrated. The sensors’ response to ground motion, measured with the shake table, was removed from the electrical calibration data so that the performance of the electrical calibration system itself could be determined. This measured electrical calibration system response is then directly compared to manufacturers’ specifications. Results from this study offer insight into the higher-frequency performance of seismometers used in the IMS and demonstrate how primary ground-motion calibrations can be used to verify sensor performance and identify issues that electrical calibration methods may not detect.