A Wide Angle Seismic Profile
V.J. Hughes, R.S. White, E.J.W. Jones, D.H. Matthews, J.A. Brewer, D.K. Smythe, 1983. "A Wide Angle Seismic Profile", 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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Standard marine seismic reflection profiles are recorded by towing the acoustic source and hydrophone receivers from a single ship. The maximum offset is governed by the length of the streamer, but the ultimate aim is to produce a seismic cross section at normal incidence beneath the shooting line, and all the processing, from common depth point stacking through to migration, is directed to this end. Indeed, if arrivals such as refractions occur on the far traces they are generally muted early in the processing sequence. In our contribution, we compare a conventional multichannel reflection profile with a wide-aperture profile (source-receiver separation 10 km; 6mi) in the same region, and show that both data sets contain complementary and useful information on basin and basement structure.
On wide angle profiles we can map reflections and refractions that may not be visible on conventional seismic reflection sections. For example, if the impedance contrast across an interface is very small it may not generate large enough reflections to be observed at normal incidence, but still produce high amplitude refractions and supercritical reflections at wide angles. Another arrival that can often be seen on wide angle but not on normal incidence profiles are doubly mode-converted shear waves. On marine profiles the source can only generate, and the receivers can only detect, compressional waves. Mode conversion to or from shear waves in the crust can only occur at nonnormal incidence, so wide angle profiles are clearly well suited to mapping converted shear waves. Wide angle profiles also provide measurements of lateral velocity changes of crustal refractors by examining the apparent phase velocity of the seismic arrivals across the array.
The wide-aperture profile illustrated here was generated using a standard multichannel seismic streamer and digital recorder on one ship, the RRS CHALLENGER, which sailed in line astern at a constant offset of 10 km (6.2 mi) behind a second ship, the MV STARELLA. The seismic source on twelve channel Geomechanique hydrophone streamer with a 100 m (328 ft) group spacing. ATexas Instruments DFS III with a 4 msec sampling rate was used to record the data. On the shooting ship, the airgun was fired once every 40 seconds, giving a shot spacing of 100 m (328 ft) at the profiling speed of 5 knots. The offset between the ships was continuously monitored with an accuracy of about 1 m using a Decca radar trisponder. Small corrections were made to the speed of the receiving ship so as to maintain a constant offset and variations in range were for the most part kept to less than 30 m (98 ft). Timing control on the two ships was by separate crystal-controlled clocks calibrated against a radio Universal Time signal. Navigation was primarily by Mainchain Decca and Satellite Navigator fixes, supplemented by Doppler logs.
The traveltimes of the wide aperture profile illustrated have been corrected for clock drift on the shooting and receiving ships and for the small time differences caused by range variations about the mean of 10km (6 mi).
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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.