Effect of Airgun Source Depth on Scholte and Guided P-waves for Near Offshore Subsurface Characterization

Accurate estimation of shear wave velocity (Vs) in marine sediments is crucial for offshore geotechnical and seismic applications. Scholte and guided P-waves offer valuable insights due to their sensitivity to near-seafloor mechanical properties. This study investigates the influence of airgun source depth on the excitation and dispersion of Scholte and guided P-waves on field measurements and numerical data. A seismic survey was conducted offshore Concarneau (France), where airgun shots were performed at three depths (5 m, 13 m, and 22 m) and recorded using 50 four-component ocean-bottom sensors. Parallely numerical simulations with Spectral Element Method (Spec2DY tool) were conducted on multi-layered models representative of the marine subsurface. The results demonstrate that deeper source near the seabed significantly enhances the resolution, amplitude, and bandwidth of Scholte wave modes, and improves the continuity of guided P-waves in the dispersion diagrams. The higher frequency of Scholte wave modes detected in the dispersion diagram, increases with source depth and is inversely related to the source-seafloor distance. These findings highlight the key role of source depth in optimizing the retrieval of seismic parameters from Scholte and guided waves for marine subsurface characterization.