Application of controlled vibration sources for traceable on-site calibration of seismometers

In the context of the joint research project "Metrology for low-frequency sound and vibration - 19ENV03 Infra-AUV" we evaluated natural, anthropogenic and controlled sources of seismic, infrasonic, and hydroacoustic waves with respect to their potential use as excitation signals for on-site calibration of the respective sensors in the range of 0.01 to 20 Hz. In that context, man-made controlled sources such as drop weights, hammer blows or vibrator sources exhibit properties such as broad frequency content and high repeatability that make them an interesting source signal for the calibration of seismometers.

At IMS station PS19 in Germany we conducted an excitation experiment using both a portable electrodynamic seismic vibrator source and simple hammer blows on the ground for the purpose of covering the higher frequencies of interest from 8 to 20 Hz and above for on-site seismometer calibration. As previous on-site calibration experiments have shown, insufficient coherent natural excitation signals within the relevant high frequency range have been recorded, leading to missing information in the frequency response estimation. Using the seismic vibrator source either P- or S-waves could be excited for example as monofrequent (18 Hz) or sweep (10-100 Hz) signals of 10 s time duration. The distance between excitation source and station/reference sensors as well as the direction of signal arrival at the sensors was varied.

We present calibration results for the conducted excitation experiment using a comparison between station and laboratory-calibrated instruments, showing that the frequency response can be determined for the higher frequencies of interest using the different signals from the excitation experiment.