a Discrete-Scatterer Model of the Seismic P Coda
Abstract
This thesis demonstrates that discrete-scatterer models can be used to approximate the seismic response characteristics of the earth's crust. A scattering model can be used to produce more realistic synthetic seismograms than can conventional layered-reflector models. The seismic P coda was modeled as a composite of wavelets scattered from heterogeneities in a geologic medium. Both angle and frequency dependence of the scattering response were considered. The response was calculated individually for each scatterer in the medium, adjusted for path effects and system response, and convolved with a source wavelet. The responses of all scatterers were combined to obtain the complete response of the medium. Effective scattering cross-section was calculated to determine the transfer of energy from the direct wave to scattering and to estimate values of Q for models with a range of scatterer size and spacing. The response of an individual scatterer was approximated as that of an elastic, spherical heterogeneity embedded in an elastic host material. Scattering responses were calculated using Mie theory to solve the elastic wave equation. Synthetic codas were prepared using a P-wave source and models that contained up to a few thousand scatters. Models employed several combinations of scatterer size and spacing, effective Q, and source-receiver geometry. Specific models were developed to simulate codas recorded from blasts and earthquakes in Alabama and Tennessee. Simulations showed that the seismic coda is most sensitive to scatterer size in media that have high absorptive Q. The shift from primarily backscattering, for ka < 1, to forward scattering, for ka > 1, causes the character of the coda to change significantly as a function of scatterer size when ka ~ 1. Synthetic codas also demonstrated the effect of source depth on the ratio of the amplitude of the direct wave to the amplitude of the coda waves.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1990
- Bibcode:
- 1990PhDT.......143C
- Keywords:
-
- SEISMIC P CODA;
- Geophysics; Physics: Acoustics