Rayleigh-wave tomography at Coronation Field, Canada: The topography effect

W reflection seismology, surface waves or ground roll, are often considered a form of unwanted sourcegenerated noise. Unlike body waves, surface waves propagate exclusively in the lateral direction and are virtually insensitive to structure deeper than one wavelength. For a nominal frequency of 5 Hz and phase velocity of 500 m/s, this means that a surface wave of the Rayleigh or Love type only feels the upper 100 m of the subsurface. As a result, surface waves cannot be used for imaging deep reflectors; however, they can be used to estimate near-surface properties (Xia et al., 1999; Ross et al., 2008), in particular the shear-wave velocity. Knowledge of near-surface velocity structure in turn can be used to estimate shear-wave statics in reflection seismology. Estimating statics in the presence of laterally varying structure (i.e., obtaining the long-wavelength static component) can be challenging. In this paper, we demonstrate that Rayleigh waves can image the laterally varying near-surface shear-wave velocity at the Coronation Field, Canada. We find that Rayleigh-wave group velocities exhibit a first-order dependence on the local topography and offer a simple explanation for this behavior. We refer to this behavior as “the topography effect” because we expect it holds for surface waves in a wide variety of settings. The topography effect offers a rare data-driven test for the accuracy of a tomogram. We find that fine-scale features observed in Rayleigh-wave tomography, which appear to be close to or below the resolution limit, do in fact correlate with the local topography and are resolved. A notable deviation from the topography effect exists at the Coronation Field, and we offer further explanations for why it may not be observed in particular cases. In areas where we observe the topography effect we obtain relatively good static corrections from the inverted shear-wave velocity depth model. The area where the topography effect breaks down at the Coronation Field poses a challenge for surface-wave imaging and subsequent statics determination. For such areas we likely need more advanced methods of surface-wave inversion.