Traceable Calibration of Seismometers

At present, seismometers are not traceably calibrated. This means that their output sensitivity is not determined in a way that is traceable to the International Systems of Units (SI). The European research project 19ENV03 Infra-AUV, which is part of the EMPIR programme, develops methods and procedures to enable such traceable calibrations.In contrast to many other sensors, seismometers are operated stationary in their typical measurement application, i.e., they must not be moved after their deployment. Conventional calibration approaches which involve a laboratory calibration of the seismometer to be calibrated are therefore not feasible. For this reason, a new concept currently developed by different European partners within the Infra-AUV project proposes an on-site calibration scheme.For the on-site calibration, a reference seismometer is traceably calibrated to the SI in a laboratory. This reference is then used on-site to provide a secondary calibration of other seismometers, e.g. in a seismic station, using natural excitation sources [Schwardt et al., 2022, DOI: 10.1007/s10712-022-09713-4].The calibration of reference seismometers in the laboratory is carried out as a primary calibration. This means that the measured quantity (the velocity-proportional voltage output) is compared to a different quantity, in this case to a dynamic displacement measurement traced back to the units length and time, which can be measured very precisely by laser interferometry. In this calibration, the seismometer is excited with low-frequency mechanical vibrations generated by electrodynamic exciters. These calibrations must be performed for the horizontal and vertical axes. The frequency range of interest is from 20 Hz down to 0.01 Hz, depending on the seismometer under test. Either mono-frequency sinusoidal excitations of different frequencies are applied subsequently, or multiple frequencies are excited simultaneously using a multi-sine approach. The magnitudes and phases of both measured signals, the interferometric reference and the seismometer under test, are determined by using sine approximation algorithms or by applying a discrete Fourier transform (DFT).The results of the laboratory calibration, the transfer function of the reference seismometer, can then be derived from the ratios of the measured magnitudes and the differences of the phase angles for the different excitation frequencies. In addition, the associated measurement uncertainties are estimated and are part of the calibration result. Influences that may change the sensitivity of a seismometer, e.g., temperature effects, electromagnetic sensitivity, or ground stiffness need to be analysed and additionally taken into account for the uncertainty estimation.For the uncertainty of the on-site calibration, differences between the laboratory and the on-site environment also need to be taken into account. This includes, for example, aspects like typically different temperatures or different ground materials.