On-Site Comparison Calibration of Broadband Seismometers with Temperature Correction

Abstract

Calibration of seismometer sensitivity in observational networks is essential to ensure measurement accuracy. One of the most accurate calibration methods is on-site comparison calibration, an approach developed over the past decade. In this approach, a calibrated seismometer is installed near the target seismometer, and their local vibration signals are compared to calculate the target sensitivity. However, changes in the reference seismometer sensitivity due to temperature dependence degrade comparison calibration accuracy. This study proposes a comparison calibration system incorporating temperature correction. The accuracy of the reference sensitivity was improved by applying correction based on temperature monitoring. Using the proposed system, STS-1 seismometers were calibrated in their operational state. The midband sensitivity around 0.5 Hz was determined with uncertainties of 0.3%–0.5%, which satisfies the requirement of the observational networks. An unidentified delay of 28 ms was also found in the seismometer or the data logger. Furthermore, the relative frequency response down to 0.02 Hz was determined, and the shift of the cutoff frequency was identified. These results will help establish an on-site comparison calibration process.