Seismic imaging of the upper crust is strongly affected by the interactions
of the seismic wavefield with the shallow subsurface. Tn 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.