Speaker
Description
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. This identification is only possible when coherent signals are recorded over an extended receiver array. The presence of very local, often station specific, noise sources can severely compromise this coherence of the seismic noise field, preventing the identification of noise sources. Thus, a thorough analysis of the seismic signal coherence over space and time is needed to determine if receivers see the same signal. Time periods with high signal coherence can then be used to identify possible ambient noise sources.
In this study, we show examples of the ambient seismic noise field recorded with different temporary short period surface seismic networks in the Euregio Meuse-Rhine (EMR). 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 beamforming results are compared with results from matched field processing (MFP) to validate the azimuth of noise sources and additionally try to estimate the possible geographic source area.
For larger median station distances, 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 higher coherence values during night. This indicates the prevalence of often very local noise contributions during working hours. For shorter station distances, strong local sources have the potential to locally generate coherent wavefields during working hours. In addition, a clear anti-correlation between wavefield coherence and wind speed is observed. Beamforming results during time intervals with high waveform coherence on the selected receiver array 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. 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.