Natural and Anthropogenic Sources of Seismic, Hydroacoustic, and Infrasonic Waves: Waveforms and Spectral Characteristics (and Their Applicability for Sensor Calibration).

The record of seismic, hydroacoustic, and infrasonic waves is essential to detect, identify, and localize sources of both natural and anthropogenic origin. To guarantee traceability and inter-station comparability, as well as an estimation of the measurement uncertainties leading to a better monitoring of natural disasters and environmental aspects, suitable measurement standards and reliable calibration procedures of sensors, especially in the low-frequency range down to 0.01 Hz, are required. Most of all with regard to the design goal of the Comprehensive Nuclear-Test-Ban Treaty Organisation's International Monitoring System, which requires the stations to be operational nearly 100% of the time, the on-site calibration during operation is of special importance. The purpose of this paper is to identify suitable excitation sources and elaborate necessary requirements for on-site calibrations. We give an extensive literature review of a large variety of anthropogenic and natural sources of seismic, hydroacoustic, and infrasonic waves, describe their most prominent features regarding signal and spectral characteristics, explicitly highlight some source examples, and evaluate the reviewed sources with respect to requirements for on-site calibrations such as frequency bandwidth, signal properties as well as the applicability in terms of cost-benefit. According to our assessment, earthquakes stand out across all three waveform technologies as a good natural excitation signal meeting the majority of the requirements. Furthermore, microseisms and microbaroms allow a calibration at very low frequencies. We also find that in each waveform technique man-made controlled sources such as drop weights or air guns are in good agreement with the required properties, although limitations may arise regarding the practicability. Using these sources, procedures will be established allowing calibration without record interrupting, thereby improving data quality and the identification of treaty-related events.