Abstract

Before the Consortium for Continental Reflection Profiling (COCORP) deep crustal reflection profile across the Wind River range, Wyoming, can be understood, problems involving velocity, multiple reflections, and structural style associated with thrusting must be resolved. Measurements from boreholes to maximum depths of 7.8 km show that a strong velocity inversion is associated with overpressured zones, primarily in Cretaceous shales. One-dimensional (1-D) synthetic seismograms generated using the detailed velocity distribution closely duplicate the seismic trace on line 1 and produce multiple reflections of significant amplitude to record times of 10 sec. Other data including auto-correlograms demonstrate presence of abundant multiple reflection energy to times of 10 to 12 sec on lines 1 and 2 and suggest that most of the deep events on these lines are multiple reflections. Because this area has not been known as a problem area for multiples in shallow (industry) reflection surveys, we conclude that multiples are a greater impediment for deep crustal reflection studies than has previously been recognized and that the sedimentary section must be treated much like the weathered zone in shallow seismic studies. Two-dimensional (2-D) modeling and hand migration are used to determine structure in sedimentary rocks beneath the thrust Precambrian wedge. They suggest that structural as well as velocity uplift is found under the thrust, that a wedge of overturned sedimentary rocks parallels the thrust, and that the deeper sedimentary rocks bend down into the thrust.

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