Trade-off between signal coherence and ambient noise (3C) beamforming results in a complex noise environment

Ambient seismic noise beamforming tries to identify the direction (traditional one component beamforming) or the direction and wavetype (three component beamforming) of the incoming ambient noise field. If the signal is weak, either taking longer time intervals or stacking several consecutive time windows is employed to obtain a sharper image of the underlying noise sources. However, this also reduces the temporal resolution of the noise sources and might prevent the identification of distinct sources in the case of a temporally highly variable noise field. In addition, the presence of very local, often station specific, noise sources can compromise the coherence of the seismic noise field, preventing the identification of noise sources. The identification and subsequent removal of either time intervals or single stations with exceptionally low signal correlation could thus also lead to an improvement in the temporal resolution of the seismic noise sources and the classification of the ambient seismic noise field.

In this study, we investigate the ambient seismic noise field recorded with different temporary short period seismic networks in the Euregio Meuse-Rhine (EMR). The region is a candidate site for the next generation gravitational wave telescope (Einstein telescope) and characterizing the anthropogenic ambient surface noise field with high spatial and temporal variability containing many different noise sources (e.g. highways, railway lines, industry, wind turbines, urban settlements) is essential. We investigate the spatio-temporal coherence of the seismic wavefield to identify time intervals and regions with high waveform coherence which are then investigated for possible ambient noise sources. The one component and three component (when available) beamforming results are compared with results from matched field processing (MFP) to validate that the sources are outside the recording network and to estimate the possible geographic source area.

We observe a clear diurnal character of the coherence of the ambient seismic noise field over a wide frequency range from about 2 to 20 Hz with significantly higher coherence values during night. This indicates the prevalence of very local noise contributions during working hours. In addition, a clear anti-correlation between wavefield coherence and wind speed also indicates the local character of wind generated noise. Limiting beamforming analysis to time windows and stations with high coherence improves the beamforming results and the temporal resolution during time intervals identified as coherent. Beamforming results during time intervals with high waveform coherence often show stable backazimuth directions indicating persistent ambient noise sources. The source location outside the recording network is confirmed by the results of the MFP analysis. This also holds for time intervals during working hours and can be confirmed by MFP processing. Results from such investigations might be used as best practices for the spatio-temporal characterization of ambient noise sources in the case of highly complex noise fields.