Wave propagation in the heterogeneous lower crust – Finite Difference calculations

Wave propagation in heterogeneous media is not only characterized by reflection, transmission and conversion of seismic energy but also by effects such as scattering and tunneling and can be observed on many scales. We investigate elastic wave propagation in the lower crust of the earth. It is remarkable that distance and time scales in a deep crustal reflection problem can be easily transformed into an exploration/production oriented problem. In that analog, the lower crust corresponds to some fractured medium or a medium with laminated inter bedding of source rocks, such as, sand and shale. We model surface seismic reflection data by positioning the source close to the surface. Wide-angle refraction data are simulated by placing the source into the lower crust. Teleseismic data are generated by having a plane or point source beneath the target zone. On that scale, a source with a frequency of 1Hz essentially sees an equivalent homogeneous medium, while a source with a dominant frequency of 5Hz, sees fine scale discontinuities as observed in various real data. Using a finite-difference technique, we employ models with spatially varying subsurface parameters. The fine scale heterogeneities are thin reflector segments, whose length and distance from each other are governed by a Poisson’s probability distribution. Wave type conversions are surprisingly well confined and can be easily identified in seismograms as on snapshots. The ultimate goal of this investigation is to determine whether we can image those reflector segments and determine their Vp/Vs ratio.