Calibration of Seismic Sensors Traceable from the International Metrology System

Seismic sensors can be considered as a special type of vibration transducer having very high sensitivity and capable to measure very low frequency range. However, the current calibration process for seismometer has been separately operated from the international standard system of vibration measurement. From this reason, the quantity of seismic measurement cannot be traceable from the international metrology system. Recently, the necessity for connecting to international metrology system has been raised because the importance of reliability assurance is increased due to the increase of seismic activities and extension of the international monitoring system (IMS) operated by various independent institutes and countries. In this study, the international standard procedure of vibration transducer is applied to calibrate the seismic sensors and the consideration to ensure the traceability of seismic sensor is investigated. For the reference sensor, the primary calibration method by laser interferometry is applied and the comparison calibration with an excitation table is conducted for the device under test (DUT). The change of sensitivity according to the frequency and uncertainty are evaluated for several types of seismic sensors. The experiment shows that the sensitivity of seismic sensors has noticeable changes according to the excitation frequency and direction. Therefore, the results show that the conventional method, just notifying the single number of nominal value, widely used in the seismology area is not sufficient to describe the characteristics of sensor. Up to now, the traceability of proposed approach can only assure the frequency range down to 0.1 Hz. To cover the whole frequency range of interest for the seismic observation, the development of excitation system for the lower frequency range and the international cooperation of metrology area are essential for assuring the international equivalency of quantity.