Evidence from sonic logs indicates that velocity fluctuations in the
upper crystalline crust are remarkably uniform. This motivates a
generic approach to classifying upper-crustal 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 Kármán autocovariance function with
a 100 m, ? ? 0.15, and ? ? 300 m s?1. 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 (? = 0.50), the high fractal dimension
(i.e. small values of ?) 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