Temperature Dependence of Broadband Seismometer Sensitivity

Broadband seismometers are widely used in global observation networks deployed for geophysics research. Recently, the calibration of their sensitivity has become an important factor for ensuring observation accuracy. One of the limiting factors of calibration uncertainty is the temperature dependence of the sensitivity because seismometers operate in a wide range of temperatures. However, systems that accurately measure the temperature coefficient of sensitivity in seismometers have not been established. Herein, we develop such a system using a triaxial vibration exciter combined with a thermostatic chamber. Using this system, we calibrated several broadband seismometers (Trillium Compact 120s, Trillium Horizon 360, and CMG-3T-360) at various temperatures, ranging from −15°C to + 45°C. The power supply and digitizer were kept at room temperature (23°C) so that only the sensor itself was investigated. We found that the temperature coefficient of the midband sensitivity was (0.11 ± 0.01) %/°C, which can be attributed to the internal magnet of the seismometers. The variation of the phase delay corresponded to a change of less than 2 ms in output time delay. In addition, we found that the relative frequency response below 1 Hz was stable against temperature variations. These values are useful for evaluating the measurement accuracy of seismometer observation networks.