Seismic attenuation and the nature of reflecting horizons within the crust

Near-vertical incidence deep-reflection seismograms recorded in southern Alberta, are used as a basis for investigating crustal characteristics. Synthetic seismograms with frequency- and depth-dependent attenuation are generated to aid in the study. Comparison of the autopower spectra, of selected time intervals on field records with similar spectra computed from theoretical seismograms is an effective means of inverting the seismic wave data into models representing the properties of the lower crust. Such studies can provide estimates of the variation with depth of the specific attenuation factor Q and information concerning the nature of the deep transition zones that reflect seismic energy. Our model for Q in the crust in southern Alberta is not unique, but an acceptable structure requires considerable variation in the upper 2 km with an average near 300. The magnitude of Q in the lower 40 km is approximately 1500 and probably increases with depth. A comparison of observed and theoretical data has enabled velocity-depth models of the reflecting horizons in the basal 15 km of crust to be constructed. Models consisting of first-order discontinuities, linear gradients in velocity over depth intervals of less than one wavelength, or step-like velocity increases with depth are generally unacceptable. Only the transition-zone model comprised of thin sills of alternating high- and low-velocity material satisfies most of the observational data. In this model individual layers are less than 0.2 km thick, and the total extent of the transition zone is less than 1 km.