Abstract
Shot gathers from the Parkfield, California, deep crustal seismic
reflection line, recorded in 1977 by COCORP, reveal coherent events
having horizontal to reverse moveouts. These events were migrated
using a multioffset three-dimensional Kirchhoff summation method.
This method is a ray-equation back projection inversion of the acoustic
wave field, which is valid under the Born, WKBJ, and far-field assumptions.
Migration of full-wave acoustic synthetics, having the same limitations
in geometric coverage as the COCORP survey, demonstrates the utility
of the imaging process. The images obtained from back projection
of the survey data suggest that the Gold Hill fault carries ultramafic
rocks from the surface to 3 km depth at a dip greater than 45 degrees,
where it joins the San Andreas fault, which may cut through more
homogeneous materials at shallow depths. To the southwest, a 2 km
Tertiary sedimentary section appears to terminate against a near-vertical
fault. The zone between this fault and the San Andreas may be floored
at 3 km by flat-lying ultramafics. Lateral velocity inhomogeneities
are not accounted for in the migration but, in this case, do not
seriously hinder the reconstruction of reflectors.
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