Effective-mode analysis of elastic waves

Surface wave methods offer a cost-effective and non-invasive way to derive the shear wave velocity profile of the subsurface. Traditional surface wave inversions rely on theoretical modal curves derived from free-vibration assumptions, neglecting near-field effects, source–receiver geometry and the influence of other elastic waves. In real-world applications, the observed dispersion data often represent a superposition of multiple modes affected by body waves, leaky and evanescent waves, and acquisition layout. These complexities frequently lead to mode misidentification, particularly in limited acquisition spreads, shallow bedrock conditions and irregularly dispersive media. This study introduces an effective-mode-based elastic wave analysis method, offering a solution to the problem of mode misidentification. The effective mode is computed using higher-order thin-layer method-based active-source modelling, which incorporates complete wavefield effects and source–receiver configuration. Inversion is performed using particle swarm optimisation across diverse field scenarios, addressing the problem of near-field effects, modal osculation, higher-mode skipping and mode jumps due to embedded stiff layers. Synthetic studies demonstrate the consistency of the method across multiple source offsets, enabling the framework to capture offset-dependent behaviour. Synthetic and field investigations demonstrate that the effective-mode inversion reconstructs velocity profiles with improved accuracy, offering a reliable alternative to conventional modal inversion.