Application of a Statics Solution, Wyoming Overthrust
J.H. Johnson, M.S. Yancey, P.S. D‘Onfro, 1983. "Application of a Statics Solution, Wyoming Overthrust", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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Lateral changes in seismic velocity are common in structurally complex areas. Because these changes can create false structures on seismic time sections, one must interpret with extreme care. Where a lateral change in velocity occurs near the surface, it may cause a "statics" problem. An idealized example of this is illustrated in Figure 1. Areal data example of this problem is shown in Figure 2; a seismic section from the Wyoming Overthrust belt. The dip reversal marked at 1 sec at the east end of Figure 3 (an interpreted version of Figure 2), looks like an excellent structure to drill, however a detailed study of this line eliminated this prospect.
In Figure 3, the long thin white line indicates a fault that can be seen in outcrop (Fault A). Two prominent reflectors are highlighted by thick gray lines. A clue that something is wrong with the section is the signal deterioration or "data bust" slightly east of center. The vertical alignment of this data bust and the fact that the reflectors are offset by 400 msec across the bust, suggest that there is either a statics problem or a vertical fault. A good interpreter should suspect a static problem. Displaying two stacked sections, one made from near offset traces and the other made from far offset traces, verified that there was a statics problem and helped to solve it.
This particular statics problem is caused by the abrupt velocity change from the low-velocity fill west of the fault to high-velocity rocks immediately east of the fault. The low-velocity fill is shaded in gray in Figure 3. Reflections that travel through the low-velocity fill will be delayed relative to those that travel through the high-velocity rocks. A "velocity sag" results below the fill, that is, reflectors below the fill appear on a seismic section to be deeper than those below the high-velocity rocks. The data bust results from the CDP stacking process. Half of the traces that will be summed to form the stacked trace resemble the stacked traces east of the data bust and the other half resembles the delayed traces west of the bust. When the halves are summed in the stacking process the result is a loss of reflection continuity.
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.