seismic data; seismic imaging; Kirchhoff Prestack Depth Migration; Fresnel Volume Migration; San Andreas fault system
The present thesis uses advanced imaging techniques to determine new structural seismic depth images from the old industry reflection seismic data set SJ-6 across the San Andreas fault system in south central California.
Three different migration techniques, Kirchhoff Prestack Depth migration, Fresnel Volume migration and Reflection Image Spectroscopy, respectively, are used to determine the reflectivity structure of the whole crust beneath the SJ-6 profile line. The imaging results are compared and evaluated with respect to the image quality and the observable subsurface structures. Kirchhoff Prestack Depth migration is a standard imaging technique that is suitable to image complex geological structures. Compared to the other applied methods it is less time consuming but produces considerable migration artefacts especially in cases of low data coverage.
The advanced Fresnel Volume migration uses the principle of Fresnel Volumes to restrict the migration operator to the region around the actual reflection point. In this way, migration artefacts are significantly reduced and far more distinct structures can be observed in the seismic images.
Reflection Image Spectroscopy considers the relation between the signals wavelength and the size of the heterogeneities in the subsurface by individually migrating discrete frequency bands. Additional small- as well as large-scale subsurface structures are imaged.
Each of the three migration techniques is performed by using the same local 3D background velocity model derived from earthquake tomography.
The imaged structures are finally interpreted by comparing the imaging results with other recent geophysical studies.
It is shown that the individual terranes across the San Andreas fault system significantly differ in their reflectivity structure.
Sequentially layered strong reflectors are identified in the lower crust of the Salinian Block southwest towards the San Andreas fault.
The San Andreas fault zone itself appears as a near vertical zone that is 4 km in width and lacks of distinct reflectors. It successively broadens with increasing depth below the brittle-ductile transition zone where non-volcanic tremor locations are mainly located southwest to the San Andreas fault surface trace but inside the non-reflective zone. For that reason, these specific seismic signals are attributed to the deep extensions of the San Andreas fault zone that can be traced through the entire crust according to its distinctive reflectivity structure.
So called Great Valley ophiolithes are imaged for the first time in this particular region beneath the San Joaquin Valley east to the San Andreas fault zone.
This work demonstrates that old seismic data sets contain a multitude of subsurface information that traditional processing techniques are not able to extract.
This way, previous interpretations can be complemented and reevaluated in consideration of recent investigation results.
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