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7: 3-D Seismic Exploration

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Published:
January 01, 2001

7.0 INTRODUCTION

Subsurface geological features of interest in hydrocarbon exploration are three dimensional in nature. Examples include salt diapirs, overthrust and folded belts, major unconformities, reefs, and deltaic sands. A two-dimensional (2-D) seismic section is a cross-section of a three-dimensional (3-D) seismic response. Despite the fact that a 2-D section contains signal from all directions, including out-of-plane of the profile, 2-D migration normally assumes that all the signal comes from the plane of the profile itself. Although out-of-plane reflections (sideswipes) usually are recognizable by the experienced seismic interpreter, the out-of-plane signal often causes 2-D migrated sections to mistie. These misties are caused by inadequate imaging of the subsurface resulting from the use of 2-D rather than 3-D migration. On the other hand, 3-D migration of 3-D data provides an adequate and detailed 3-D image of the subsurface, leading to a more reliable interpretation.

A typical marine 3-D survey is carried out by shooting closely spaced parallel lines (line shooting). A typical land or shallow water 3-D survey is done by laying out a number of receiver lines parallel to each other and placing the shotpoints in the perpendicular direction (swath shooting).

In marine 3-D surveys, the shooting direction (boat track) is called the inline direction; for land 3-D surveys, the receiver cable is along the inline direction. The direction that is perpendicular to the inline direction in a 3-D survey is called the crossline direction. In contrast to 2-D surveys in which line spacing can be as much as 1 km, the line spacing in 3-D surveys can be as small as 25 m. This dense coverage requires an accurate knowledge of shot and receiver locations.

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Investigations in Geophysics

Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data

Öz Yilmaz
Öz Yilmaz
Search for other works by this author on:
Society of Exploration Geophysicists
Volume
10
ISBN electronic:
9781560801580
Publication date:
January 01, 2001

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