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 crustal exposures and inferred from deep-crustal seismic data. The statistical models, developed for granitic and metamorphic upper crust, and for an extended metamorphic lower crust, are used to construct whole-crustal models of seismic velocity heterogenity. We present finite-difference synthetic CMP data from several models which compare favorably with 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 importance for classification are the velocity fluctuation intensity, the horizontal and vertical correlation lengths of the medium, the correlation function of the medium, and the velocity population function. For the crustal properties we measured, the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering regime, with implications for crustal seismic data processing and imaging. Notably, deep-crustal signals are likely to have experienced multiple scattering, making common seismic imaging techniques of questionable value. Moreover, the details of the unmigrated CMP stacked section bears little resemblance to the underlying medium.