Digital processing

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Digital Imaging and Deconvolution: The ABCs of Seismic Exploration and Processing
Series Geophysical References Series
Title Digital Imaging and Deconvolution: The ABCs of Seismic Exploration and Processing
Author Enders A. Robinson and Sven Treitel
Chapter 2
ISBN 9781560801481
Store SEG Online Store

With the digital computer, a whole new step in seismic exploration was added: digital processing. Reflection seismology now involves three steps: acquisition, processing, and interpretation. Acquisition refers to the generation and recording of seismic data. Sources and receivers are laid out on the surface of the earth in various types of grid patterns. The objective is to probe the unknown geologic structure below the surface (Figure 3). A source can be made up of an array of vibrators (called vibroseis) or of dynamite shots or of air guns. A receiver is an array of geophones on land or an array of hydrophones at sea. The sources are activated one at a time rather than all together.

Figure 3.  Two examples of subsurface geology.

Suppose a single source is activated. The resulting seismic waves travel from the source point into the earth. The waves pass down through sedimentary rock strata from which the waves are reflected upward. A reflector is a contrast in physical properties (elasticity and/or density) that gives rise to a seismic reflection. The reflected energy returns to the surface, where it is recorded. For each source activated, many receivers surround the source point. Each recorded signal, called a seismic trace, is associated with a particular source point and a particular receiver point. The signals as recorded are referred to as raw traces. The events on the raw trace are produced by the subsurface structure of the earth. Events that result from primary reflections are wanted; all other events are unwanted.

In areas of intensive structural change, the ensemble of raw traces is unable to show the true geometry of subsurface structures. In some cases, it is possible to identify an isolated structure, such as a fault or a syncline, on the basis of its characteristic reflection pattern. However, in NR regions, the raw record section does not give a usable image of the subsurface at all. The objective of digital signal processing is to filter and rearrange the raw data to form a digital image of the subsurface structure of the earth. Seismic processing makes it possible to produce accurate and useful 3D digital seismic images. Today, except in special situations, an interpreter never has to look at raw traces.

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