Characteristics of surface wave Green’s function for anisotropic ambient seismic noise field — a case study in Limburg, The Netherlands

The Advanced LIGO and the Advanced Virgo gravitational wave (GW) detectors are built on the surface of the earth. Seismic noise being most dominant on the surface limits the low frequency sensitivity of the detectors below 10 Hz. The Einstein Telescope (ET), a European project for constructing a third generation GW detector aims to improve current detector sensitivity by an order of magnitude at frequencies below 10 Hz. One way of reducing the contribution from seismic noise is by building the detector underground. Currently seismic studies are being conducted in Sardinia, Italy and Limburg, The Netherlands, for selecting an appropriate site for constructing the ET. A dense array of 146 wireless vertical component geophones was deployed at Limburg in The Netherlands with the aim of performing passive seismic tomography using the seismic noise in the frequency band 2.5–8.0 Hz. Theoretically for accurate extraction of surface wave Green’s function, an isotropic distribution of noise sources surrounding a station pair is desired. However, in reality the distribution of noise sources manifest azimuthal anisotropy and also vary in source magnitude. In this article, we discuss the results of the accuracy of the extracted surface wave Green’s function when illuminated by an anisotropic distribution of noise sources. We make use of the method of cross-correlation beamforming to extract station pairs that yield accurate surface waves Green’s function and consequently compute the surface wave group velocities for the region.