SEISMIC SCATTERING IN THE UPPER CRYSTALLINE CRUST BASED ON EVIDENCE FROM SONIC LOGS

Evidence from sonic logs indicates that velocity fluctuations in the u pper crystalline crust are remarkably uniform. This motivates a generi c approach to classifying upper-crustal seismic heterogeneity and to s tudying implications for seismic wave propagation. The resulting canon ical model of upper-crustal seismic structure is characterized by a sp atially isotropic von Karman autocovariance function with a approximat e to 100 m, v approximate to 0.15, and sigma approximate to 300 m s(-1 ). Small-angle scattering theory is used to predict the transition fro m weak to strong scattering as well as phase fluctuations and scatteri ng attenuation. Compared with 'exponential' random media (v=0.50), the high fractal dimension (i.e. small Values of v) of upper-crustal hete rogeneity causes smaller phase fluctuations, and transition from weak to strong scattering at lower frequencies and shorter path lengths. Ac oustic finite-difference modelling shows that seismic reflections from deterministic features surrounded by heterogeneities are severely deg raded when they fall into the strong scattering regime. Conversely, tr aveltime fluctuations of transmitted waves are found to be relatively insensitive to the transition from weak to strong scattering. Upper-cr ustal scattering Q is predicted to lie between 600 and 1500, which is one to two orders of magnitude higher than e-values inferred from seis mic data. This suggests that seismic attenuation in the upper crystall ine crust is dominated by anelastic effects rather than by scattering