Effects of the shallow subsurface on upper crustal seismic reflection images

Abstract

Seismic imaging of the upper crust is strongly affected by the interactions of the seismic wavefield with the shallow subsurface. In order to better understand these effects, we have evaluated the seismic responses of a suite of canonical models of the upper crystalline crust and the near-surface region. Our modelling is restricted to two dimensions, but otherwise includes the full seismic wavefield, notably the effects of attenuation and topographic variations along the free-surface. We find that S-wave scattering is important in the upper crust and contributes significantly to the vertical component of the seismic reflection response. The backscattered wavefield undergoes mode conversions when it interacts with the free-surface. Even moderate topographic variations or velocity variations in the near-surface region enhance these mode conversions and cause additional scattering. Much of the Earth's surface is covered by thin layers of unconsolidated material or weathered bedrock with high attenuation and high velocity contrasts at the layer boundaries. Scattered and mode-converted seismic energy gets trapped in these layers and thus interacts repeatedly with the free-surface and its topography. In analogy to large-scale seismic resonance effects of sedimentary valley fills, the level and duration of source-generated noise depends heavily on the degree of attenuation in the shallow layers: low attenuation causes high noise levels and vice versa. In contrast, the level of source-generated noise is less sensitive to near-surface layer thicknesses and velocity contrasts.