A.C. Evans, D.N. Parkinson, 1983. "A Half-Graben and Tilted Fault Block Structure in the Northern North Sea", 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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Seismic line 74-6 was shot 250 km (155 mi) northeast of Shetland near the boundary between United Kingdom and Norwegian waters in the northern North Sea (Figure 1).
The major features of the section are the Magnus Ridge, in which lies the BP Magnus Field (De'Ath and Schuyleman, 1981) and the North Shetland Trough. These are examples from a series of similar structures which extend northeast from the Shetland Islands basement platform. They result from the reactivation of Caledonide lineaments by Middle Jurassic to Early Cretaceous dilational tectonics which were ultimately to lead to the opening of the Atlantic Ocean (Threlfall, 1981; Hay, 1978). The effect of these movements has been to produce half-grabens where deeply buried Jurassic Kimmeridge Clay formation source rocks alternate with potentially trapping ridges, the whole sealed by onlapping Cretaceous rocks. Such Mesozoic tilted fault blocks provide the trapping structures for most of the major northern North Sea oil fields.
The field data for line 74-6 included strong water bottom multiples, particularly beneath the Base Cretaceous reflection, and the deeper part of the North Shetland Trough was masked by long period multiples generated in the shallower section above. There was also a significant level of low velocity noise.
The processing sequence for the section shown here is given in the "Geophysical Title Block." Array simulation was used early to attenuate low velocity noise. This also improved the performance of some of the later stages by simplifying the identification of reflection events. in addition to use of deconvolution to attenuate multiples, a process was introduced which involved dip filtering of CDP gathers. The CDP tack was weighted to improve the continuity of deep reflections. Finally, thesection was migraed to collapse diffractions and move dipping reflections to their correct position.
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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.