THE CRUST AS A HETEROGENEOUS OPTICAL MEDIUM, OR CROCODILES IN THE MIST

Based on petrophysical data, geologic maps, and a well log, we present statistical descriptions of likely upper-, middle-, and lower-crustal rocks to characterize the fine-scale heterogeneity observed in crusta l exposures and inferred from deep-crustal seismic data. The statistic al models, developed for granitic and metamorphic upper crust, and for an extended metamorphic lower crust, are used to construct whole-crus tal models of seismic velocity heterogenity. We present finite-differe nce synthetic CMP data from several models which compare favorably wit h field data. The statistical models also permit classification of the seismic reflection experiment and the crustal heterogeneity according to scattering regime. The ''optical'', or scattering properties of im portance for classification are the velocity fluctuation intensity, th e horizontal and vertical correlation lengths of the medium, the corre lation function of the medium, and the velocity population function. F or the crustal properties we measured, the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering re gime, with implications for crustal seismic data processing and imagin g. Notably, deep-crustal signals are likely to have experienced multip le scattering, making common seismic imaging techniques of questionabl e value. Moreover, the details of the unmigrated CMP stacked section b ears little resemblance to the underlying medium.