Skip to Main Content
Book Chapter

A Wide Angle Seismic Profile

By
V.J. Hughes
V.J. Hughes
Bullard Laboratories, Department of Earth Sciences, University of Cambridge
Search for other works by this author on:
R.S. White
R.S. White
Bullard Laboratories, Department of Earth Sciences, University of Cambridge
Search for other works by this author on:
E.J.W. Jones
E.J.W. Jones
Bullard Laboratories, Department of Earth Sciences, University of Cambridge
Search for other works by this author on:
D.H. Matthews
D.H. Matthews
Bullard Laboratories, Department of Earth Sciences, University of Cambridge
Search for other works by this author on:
J.A. Brewer
J.A. Brewer
Bullard Laboratories, Department of Earth Sciences, University of Cambridge
Search for other works by this author on:
D.K. Smythe
D.K. Smythe
Bullard Laboratories, Department of Earth Sciences, University of Cambridge
Search for other works by this author on:
Published:
January 01, 1983

Abstract

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).

You do not currently have access to this article.

Figures & Tables

Contents

AAPG Studies in Geology

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
A. W. Bally
Search for other works by this author on:
American Association of Petroleum Geologists
Volume
15
ISBN electronic:
9781629810188
Publication date:
January 01, 1983

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal