SYNTHETIC SEISMOGRAM STUDIES OF LATERALLY INHOMOGENEOUS EARTH STRUCTURE
Abstract
Deep seismic reflection and modern crustal seismic refraction and reflection studies require laterally inhomogeneous, synthetic seismogram modeling techniques for detailed interpretation. A method has been developed for calculating synthetic seismograms using the disk ray theory (DRT) technique for two-dimensional, laterally inhomogeneous velocity and Q structures. Two steps are included in the DRT amplitude and waveform synthesis--first, ray tracing for travel-time and amplitude factor calculations, and second, synthesizing of the waveform. The amplitude factors include the effect of geometrical spreading, attenuation due to Q('-1), reflection and transmission coefficients, and free surface conversion coefficients. DRT synthetic seismograms are compared under various conditions to the modified reflectivity and asymptotic ray theory methods for a layer over a half-space model. In addition, DRT synthetic seismograms are compared to the explicit finite difference and Gaussian beam techniques for a block fault model. The comparisons show excellent agreement in both amplitude and waveform character of all arrivals including primary waves, selected multiples, P to S conversions, head waves, and diffraction wave phenomena. Synthetic seismograms were calculated for the crustal model in the eastern Snake River plain, Idaho and compared to the observed seismic data. Seismograms calculated for the CCSS (Commission on Controlled Source Seismology) two-dimensional, complex, geologic model demonstrate constructive and destructive wave interference due to a laterally variable velocity structure. Furthermore, the DRT method used along with the reflection series and hyperbolic superposition methods to investigate the nature of the continental Moho. The Moho models which were studied are based on the observations of refraction and deep seismic reflection profiles. The model studies suggest that, in gross structure the continental Moho is in general laterally continuous and in fine structure the Moho is discontinuous. The thickness of the Moho transition zone is investigated using reflection and P(,m)P and P(,n) amplitude-distance data. The interval thickness of the Moho transition zone is generally 5 km or less.
Degree
Ph.D.
Subject Area
Geophysics
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