Why do field seismometers need to be calibrated? Benefits through traceably calibrated seismometers from the laboratory to the field

As part of the EU-funded joint research project "Metrology for low-frequency sound and vibration - 19ENV03 Infra-AUV" laboratory calibration methods for seismometers in the low frequency range down to 0.01 Hz have been developed. The reliable knowledge of the full complex sensor response and its associated measurement uncertainty for both, magnitude and phase, improve data quality and reliability by correctly estimating estimating signal amplitude and phase information. Using newly developed on-site calibration approaches, full-frequency responses are estimated for, and the traceability can be transferred to station seismometers of the Comprehensive Nuclear-Test-Ban Treaty Organization’s (CTBTO) International Monitoring System (IMS) during operation without disturbing their regular measurements. With the on-site methodologies in place for co-located sensors, we show how precisely determined full-frequency response information effects seismogram interpretation and the determination of key parameters such as amplitudes and subsequently magnitudes, as well as first motion polarity or event localisation. For that purpose, we use data from on-site calibration tests performed at IMS station PS19 in Germany with both short-period and broadband seismometers calibrated in the laboratories at PTB. Furthermore, the possibility of an array-wide calibration of seismometers with a number of temporary and stationary reference sensors is assessed using suitable excitation signals and station-wide similarity measures. The primary focus is to estimate the required quantity, spacing, and distribution of reference sensors throughout the array.