On the use of ambient noise surface-wave tomography in marine exploration geophysics context

We present Scholte wave phase velocity maps obtained from cross-correlation of 6.5 hours of noise data from the Valhall Life of Field Seismic (LoFS) network. We computed 2 690 040 vertical-vertical component cross-correlations from the 2320 available sensors, turning each sensor into a virtual source emitting Scholte waves. For every virtual source, we measured the Scholte wave travel times to all other stations and interpolated them on a regular grid to perform a ‚"Helmoltz tomography" of the Valhall oil field subsurface. This tomography technique takes advantage of very dense seismic network like Valhall LoFS network without the need of a formal inversion and automatically accounts for bent rays and finite frequency effects which makes it more accurate comparing with standard straight-ray surface wave tomography methods. Scholte wave isotropic phase velocity maps at periods between 0.65 s and 1.6 s show a coherent geomorphological pattern dominated by paleo-channels in the shallower part. The inversion of these maps in depth allowed us to access the S-wave velocity distribution in the first hundreds of meters of the Valhall subsurface. We also retrieved the azimuthal anisotropy and its lateral variations showing a characteristic elliptical pattern around the central exploitation platform. These results would have important implication for shear wave statics and monitoring of subsidence due to oil extraction.