Azimuthal anisotropy at Valhall: The Helmholtz equation approach

We used 6 h of continuous vertical records from 2320 sensors of the Valhall Life of Fields Seismic network to compute 2,690,040 cross-correlation functions between the full set of sensor pair combinations. We applied the "Helmholtz tomography" approach combined with the ambient noise correlation method to track the wave front across the network with every station considered as a virtual source. The gradient of the interpolated phase travel time gives us an estimate of the local phase speed and of the direction of wave propagation. By combining the individual measurements for every station, we estimated the distribution of Scholte's wave phase speeds with respect to azimuth. The observed cosine pattern indicates the presence of azimuthal anisotropy. The elliptic shape of the fast anisotropy direction is consistent with results of previous shear wave splitting studies and reflects the strong seafloor subsidence due to the hydrocarbon reservoir depletion at depth and is in good agreement with geomechanical modeling.