Radial anisotropy in Valhall from ambient noise surface wave tomography of Scholte and Love wave

The ambient noise Scholte and Love wave phase velocity tomography is used to image a few to hundreds of meters of the subsurface at the Valhall Oil Field. Noise is recorded by 4D multi-component ocean bottom cables from the Valhall life of field seismic (LoFS) network. Six and a half hours of continuous recordings are cross correlated between all possible pairs of receivers. The vertical-vertical (ZZ) component cross-correlations (CC) are used to extract the Scholte waves and the transverse-transverse (TT) CC are used to extract the Love waves. The 2D Scholte and Love wave phase-velocity maps are constructed using the eikonal tomography method. These phase velocity maps are inverted jointly by using the neighbourhood algorithm. The local dispersion curve is inverted to get a 1D velocity profile at each point in the geographical cell and then combine them to obtain a 3D anisotropic velocity model. We find significant negative radial anisotropy (VSH < VSV) in shallow layers and positive radial anisotropy (VSH > VSV) in the deeper layers.