Full SH-wavefield modelling and multiple-mode Love wave inversion

Abstract

Full SH-wavefield modelling by the reflectivity method is developed for Love wave inversion. The modelling simulates field tests, first by generating time-distance shot gathers, followed by standard plane-wave transform to extract Love wave phase-velocity dispersion. All SH surface wave modes, plus body waves and wavefront spreading are accurately simulated. When a thin low-velocity layer (LVL) is underlain by a much stiffer layer, Love wave modes are well separated and fundamental-mode identification is more straightforward than the equivalent Rayleigh waves. However, when the LVL is thicker and bounded by equally stiff layers, multiple dominant higher modes propagate, and their correct identification and inversion is essential to resolve the soft layer. A first field test shows how multimode Love wave inversion can resolve a 2 m thick clay layer at 1 m depth in the lithological log when only both the fundamental and first higher modes are incorporated. A second field test, where a much thicker LVL, interpreted as a mud plume, is present, shows how several higher modes are required to resolve the soft layer LVL, whereas use of the fundamental or effective mode only fails.