Multi-modal Surface Waves for Site Characterization - Results from a Unique Marine Shear Wave Experiment

We revisit a unique marine seismic data set collected with a densely populated ocean bottom cable (2.5 m effective channel spacing) and a prototype shear wave vibrator at the Gjøa field, offshore Norway. Whereas the survey was primarly designed for reservoir illumination, multi-modal surface waves stand out on the seismic data (both time-offset and frequency-wavenumber domains), in the frequency band between 3 and 35 Hz. As the source was operated in both in-line and cross-line direction, we identify multiple Scholte and Love waves that were subsequently used for a constrained inversion for obtaining a detailed shear wave velocity with depth, down to 50 m. In addition, a comparison of the velocities yields an estimate on shear wave anisotropy in the shallow sub-surface, with up to 15%. The data are further used to determine attenuation, a second critical parameter affecting wave propagation and dynamic response of seabed-founded structures, which can also be important for soil characterization. Tuned forward modelling of the fundamental surface wave mode yields shear wave damping of less than 1% for the deepwater soft soils. The attenuation coefficient increases linearly with frequency, and indicates that the damping is to a large degree viscous rather than due to hysteresis.