The impact of isolated noise sources on correlation wavefields

Seismic interferometry gives rise to a correlation wavefield that is closely related to the Green’s function under the condition of uniformly distributed noise sources. Asymmetric correlation wavefields result from the violation of this condition and are commonly observed in field data. In the presence of an additional isolated noise source a second contribution to the correlation wavefield is introduced that emerges from the isolated source location at negative lapse time. The two wavefield contributions interfere, resulting in biased surface wave dispersion measurements. Isolated noise sources that act continuously, such as machinery or ocean microseisms, further have significant impact on the coda of the correlation wavefield. The coda can be dominated by direct waves propagating from the isolated noise source, not by multiply scattered waves originating from the master station. This fundamentally challenges the current understanding of how velocity changes detected in the coda can be measured and interpreted.