Natural and anthropogenic excitation sources for seismic and infrasonic on-site calibration

In the low-frequency range down to 0.1 Hz suitable and reliable calibration procedures, which include traceability to SI, for seismic and infrasonic sensors are currently missing. Although many events occur whose evaluation is of global interest, much of the low frequency range relevant to these applications is not yet covered by primary measurement standards. A laboratory calibration of sensors results in an interruption of the measurements, just as the use of built-in calibration coils disturbs the measurements. Therefore, with regard to the design goal of the Comprehensive Nuclear-Test-Ban Treaty Organization’s (CTBTO) International Monitoring System (IMS), which requires the stations to be operational 100 % of the time, on-site calibration during operation with a reference sensor previously calibrated in the laboratory is of special interest.

We have assembled sets of both natural and anthropogenic sources of seismic, infrasonic, and hydroacoustic waves with respect to their individual signal characteristics and, as part of the joint research project "Metrology for low-frequency sound and vibration - 19ENV03 Infra-AUV", evaluated their potential use as excitation signals for on-site calibration regarding aspects that include knowledge about the source characteristics, the frequency content, reproducible and stable properties as well as the applicability in terms of cost-benefit. With the aid of these sources, procedures are to be established which will allow permanent on-site calibration without any interruptions of the recordings, thereby improving data quality and consequently the identification of treaty-relevant events.

In that context, man-made controlled sources such as drop weights or loudspeakers exhibit properties that make them an interesting source signal for the calibration of seismometers and infrasound sensors. Among the natural sources, earthquake generated signals in particular stand out because of their highly suitable signal and spectral properties. In addition, microbaroms and microseisms also play an important role for calibration, since they cover the lowest frequency range of interest. In particular, we focus here on sources that may generate both seismic and infrasonic signals. By means of a joint review of the waves’ sources in the solid earth and the atmosphere, parallels and differences are highlighted. Preliminary comparisons performed with IMS stations PS19 and IS26 in Germany show that the frequency response of different excitation sources can be determined using spectral methods and correlation analyses.