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Seismic scattering in the upper crystalline crust based on evidence from sonic logs

SUMMARY Evidence from sonic logs indicates that velocity fluctuations in the upper crystalline crust are remarkably uniform. This motivates a generic approach to classifying uppercrustal seismic heterogeneity and to studying implications for seismic wave propagation. The resulting canonical model of upper-crustal seismic structure is characterized by a spatially isotropic von Karman autocovariance function with a z 100 m, ~~0.15, and CJ ~300 m s-’. Small-angle scattering theory is used to predict the transition from weak to strong scattering as well as phase fluctuations and scattering attenuation. Compared with ‘exponential’ random media (v = 0.50), the high fractal dimension (i.e. small values of v) of upper-crustal heterogeneity causes smaller phase fluctuations, and transition from weak to strong scattering at lower frequencies and shorter path lengths. Acoustic finite-difference modelling shows that seismic reflections from deterministic features surrounded by heterogeneities are severely degraded when they fall into the strong scattering regime. Conversely, traveltime fluctuations of transmitted waves are found to be relatively insensitive to the transition from weak to strong scattering. Upper-crustal scattering Q is predicted to lie between 600 and 1500, which is one to two orders of magnitude higher than Q-values inferred from seismic data. This suggests that seismic attenuation in the upper crystalline crust is dominated by anelastic effects rather than by scattering