LONG-TERM STABILITY OF BOREHOLE SEISMOMETER TRANSFER PROPERTIES

Abstract

Borehole seismic stations in the Slovenian National Seismic Network are equipped with Quanterra Q730 data loggers and Güralp CMG-40TBH broadband seismometers. Although the manufacturer of the seismometer guarantees the long-term stability of the sensor transfer properties, a question regarding this guarantee arises, and thus sensor calibration needs to be performed periodically. Typical parameters for broadband seismometers at low frequencies are damping and its natural period. On the calibration sheet we found that for CMG-40T seismometers the natural period is 30 seconds and the damping factor is 0.707. In order to check the stability of the seismometer it is sufficient to evaluate these two parameters only. We will present the stability of these parameters for borehole seismometers, which are in a stable thermal environment. Analysis shows that the parameters change over time, but that these changes are not critical, even for a period longer than 10 years.