A Three-Dimensional Study of Seismic Diffraction Patterns from Deep Basement Sources
F. Steve Schilt, Sidney Kaufman, George H. Long, 2016. "A Three-Dimensional Study of Seismic Diffraction Patterns from Deep Basement Sources", Seismic Diffraction, Kamil Klem-Musatov, Henning Hoeber, Michael Pelissier, Tijmen Jan Moser
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In 1975 the Consortium for Continental Reflection Profiling (COCORP) acquired 37 km of 12-fold common-midpoint (CMP) stacked seismic reflection profiles in Hardeman County, Texas (Oliver et al. 1976). The entire crustal section was the object of study, so a record length of 15 sec (two-way traveltime) was recorded. Three separate profiles were arranged in an H-shaped pattern to provide two-dimensional (2-D) surface coverage.
In addition to numerous reflections, the seismic sections exhibit pronounced apparent diffractions. These deep diffractions are more numerous and pronounced here than on any other CO-CORP survey to date. In many cases diffractions on different profiles intersect at the profile junctions. If these intersecting diffractions represent the same structural feature observed along different profiles, the diffraction time-surface provides an estimate of the location of the diffractor and an effective velocity (analogous to a stacking velocity) to the depth of the diffractor. Arrival times of these diffraction events were read, and a least-squares technique was developed to model intersecting diffractions simultaneously and derive location and velocity parameters, assuming point or line diffractors. Nine apparent diffraction events were identified, occurring mostly in the 5-9 sec range (~15–30 km). Point diffractor models generally give a closer fit than line models, with most of the resulting velocities between 6.0 and 7.5 km/sec. However, the introduction of a radial dimension (such as a dome instead of a point) significantly lowers these velocities without affecting the quality of fit.
The locations of the point models, when projected to the surface, are distributed roughly along a northwest-southeast trend. The trend is subparallel to the Wichita Mountains and may indicate deep intrusive and/or tectonic structures related to their evolution. These results are important evidence that strong heterogeneities on the scale of a few kilometers are present at midcrustal depths.
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The use of diffraction imaging to complement the seismic reflection method is rapidly gaining momentum in the oil and gas industry. As the industry moves toward exploiting smaller and more complex conventional reservoirs and extensive new unconventional resource plays, the application of the seismic diffraction method to image sub-wavelength features such as small-scale faults, fractures and stratigraphic pinchouts is expected to increase dramatically over the next few years. “Seismic Diffraction” covers seismic diffraction theory, modeling, observation, and imaging. Papers and discussion include an overview of seismic diffractions, including classic papers which introduced the potential of diffraction phenomena in seismic processing; papers on the forward modeling of seismic diffractions, with an emphasis on the theoretical principles; papers which describe techniques for diffraction mathematical modeling as well as laboratory experiments for the physical modeling of diffractions; key papers dealing with the observation of seismic diffractions, in near-surface-, reservoir-, as well as crustal studies; and key papers on diffraction imaging.