B.D. McClellan, J.A. Storrusten, 1983. "Utah-Wyoming Overthrust Line", 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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This seismic line was acquired over a major thrust fault in the Utah-Wyoming Overthrust belt. See the index map for location of the Overthrust belt. Older, high-velocity material has been thrust up over younger, slower material, causing a subthrust velocity anomaly (a pull-up) near the center of the unmigrated section, Figure 1. This pull-up is labeled on Figure 2, which is an interpreted version of Figure 1.
On the interpreted Figure 2, notice also the classic bow-tie pattern in the upper right-hand corner. This bow-tie is indicative of a syncline out in front of the major west-to-east thrust fault.
Figure 3 shows a depth section obtained using the stacking velocities for seismic migration. It has not done a very believable job, since the fairly wild undulations on the left-hand side are not compatible with what we see on the unmigrated data.
A better way is to create a depth model using all available data (sonic logs or generalized interval velocity information, depths at known borehole locations) and to generate synthetic seismic data using this model. The synthetic data should match the real data at the borehole; the trick is to make it match away from the borehole. If the match is good, the depth model can be used as a velocity guide for seismic migration.
In an attempt to improve the understanding (and migration) of this line, forward modeling is used. The depth model is shown in Figure 4. Notice the absence of a subthrust anticline, and notice also the syncline in the upper right-hand corner.
The synthetic seismic traces, shown in Figure 5, are a good match with the real data in Figure 1. In general, such modeling allows seismically compatible depth interpretations to be made, and velocity pull-ups and pull-downs can be quantitatively accounted for in the interpretation process. If there are enough constraints to allow confidence in the model, it can be used as a velocity distribution for seismic migration.
An example of this is shown in Figure 6. The migration obtained using the depth model's velocity distribution is far superior to the migration performed using the stacking velocities alone.
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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.