Local station correlation: large N-arrays and DAS

The use of cross-correlation between seismic stations has had widespread applications particularly in the exploitation of ambient seismic noise. We here show how the effects of a non-ideal noise distribution can be understood by looking directly at correlation properties and show how the behaviour can be readily visualised for both seismometer and DAS configurations, taking into account directivity effects.  For sources lying in a relatively narrow cone around the extension of the inter-station path, the dispersion properties of the correlation relate directly to the zone between the stations.  We illustrate the successful use of correlation analysis for both a large-N array perpendicular to a major highway and DAS cable along a busy road.  For correlation work, the co-array consisting of the ensemble of inter-station vectors provides an effective means of assessing the behaviour of array layouts, supplementing the standard plane-wave array response. When combined with knowledge of the suitable correlation zones for noise sources, the co-array concept provides a useful way to design array configurations for both seismometer arrays and DAS.