EFFECTS OF THE SHALLOW SUBSURFACE ON UPPER CRUSTAL SEISMIC-REFLECTIONIMAGES

Seismic imaging of the upper crust is strongly affected by the interac tions of the seismic wavefield with the shallow subsurface. Tn order t o better understand these effects, we have evaluated the seismic respo nses of a suite of canonical models of the upper crystalline crust and the near-surface region, Our modelling is restricted to two dimension s, but otherwise includes the full seismic wavefield, notably the effe cts of attenuation and topographic variations along the free-surface, We find that S-wave scattering is important in the upper crust and con tributes significantly to the vertical component of the seismic reflec tion response. The backscattered wavefield undergoes mode conversions when it interacts with the free-surface. Even moderate topographic var iations or velocity variations in the near-surface region enhance thes e mode conversions and cause additional scattering. Much of the Earth' s surface is covered by thin layers of unconsolidated material or weat hered bedrock with high attenuation and high velocity contrasts at the layer boundaries. Scattered and mode-converted seismic energy gets tr apped in these layers and thus interacts repeatedly with the free-surf ace and its topography. In analogy to large-scale seismic resonance ef fects of sedimentary valley fills, the level and duration of source-ge nerated noise depends heavily on the degree of attenuation in the shal low layers: low attenuation causes high noise levels and vice versa. I n contrast, the level of source-generated noise is less sensitive to n ear-surface layer thicknesses and velocity contrasts. (C) 1998 Elsevie r Science B.V. All rights reserved.