Wave reflections from sub-marine valleys and canyons

Seismic imaging issues in environments with steep flanks are known to be partly due to the existence of prism waves that are mainly caused by scattering at these steeply dipping structures. More precisely, prism waves are usually doubly-scattered waves that reflect from two nearorthogonal interfaces before reaching the receiver located on the surface. Since they contain a lot of information concerning the steep parts of the reflectors, they are complementary to primary reflections for imaging complex environments. Relying on 3D full-wave simulations and ray theory, we investigate wave reflections from a seafloor canyon of various widths and curvatures in a zerooffset configuration, i.e. when the source and receiver are at the same location. The caustic, related to the primary reflection, occurs at the location predicted by ray theory. In addition to primary reflections, higher-order reflected waves can occur depending on the canyon width, and when the triplication of the primary reflected wavefront hits the canyon walls below their inflection point. Numerical examples illustrate second-, third-, and fourth-order reflections from the canyon walls. As they may significantly contribute to the total reflected wavefield, both in terms of phase and intensity, considering these higher-order reflections could definitely improve seismic imaging.