Near-surface dual-wave imaging from repurposed 2D reflection data: P-wave traveltime and Rayleigh-wave inversion

Integrating 3D P-wave traveltime tomography and Rayleigh-wave inversion enables characterization of shallow fractured bedrock using conventional 2D reflection seismic data, originally acquired for deep structural imaging. Repurposing these datasets for near-surface analysis resolves geomechanical transitions within the crystalline terrain of the Revell Site, Canada’s proposed Deep Geological Repository for spent nuclear fuel. Despite challenges from wavefield scattering and acquisition geometry limitations, a dual-wave approach reconstructs P- and S-wave velocity models that delineate the transition from fractured to competent bedrock at a median depth of approximately 48 m. Convergence of refracted body wave and dispersive surface wave responses, supported by borehole velocity and fracture data, confirms the presence of a geologically and mechanically significant interface separating the upper fractured domain from the underlying competent crystalline bedrock. A multivariate elastic indicator, the Relative Compliance Indicator (RCI), serves as a proxy for mechanical degradation in the fractured domain. The RCI integrates elastic attributes, including modulus reductions and seismic velocity ratios, through a data-driven weighting scheme based on Principal Component Analysis (PCA). The PCA-optimized RCI produces a more spatially focused compliance pattern that is more consistent with mapped fracture zones, outperforming equal-weight formulations. The resulting compliance distribution reveals subtle stiffness variations not evident in single-attribute or uniformly weighted approaches. This integration of wave-based seismic imaging with data-driven elastic indicators provides an effective framework for near-surface mechanical characterization in crystalline rock. The methodology is scalable to 3D seismic datasets, where volumetric imaging would enhance the delineation of fractured domains, and is broadly applicable to other crystalline terranes where repurposing legacy reflection data can improve near-surface geomechanical assessments.