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GCRecognizing Faults in Seismic Data*
By
Alistair Brown1
Search and Discovery article #40073
*Adapted for online presentation from the Geophysical Corner column in AAPG Explorer June, 2001, entitled “Is It a Subtle Fault, or Just Noise?,” and prepared by the author. Appreciation is expressed to the author, to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Consulting reservoir geophysicist, Dallas, Texas.
Long gone are the days when faults appeared only as steps on vertical seismic sections. If we use today's better data and exploit modern workstation tools available, we should do a much better job of recognizing and understanding faults in 3-D seismic data. Faults cause breaks in continuity of seismic horizons. These discontinuities generate diffraction patterns and, before the days of seismic migration, diffraction patterns were what the seismic interpreter sought as an indication of faulting.
Migration in 2-D will collapse
diffractions to some extent, whereas migration in 3-D should do much better.
Major faults are still recognized on vertical sections and their throw estimated
by offset in character correlation. For this, double-gradational color is the
best mode of display. Spatial patterns of faulting are revealed on time
slices
(or depth
slices
). These horizontal sections must be used in conjunction with
vertical sections to establish sensible fault geometries.
Composite and chair displays are established ways of combining these orthogonal sections together. In a chair display, one looks at a horizontal slice where it intersects a vertical section. You are able to see the map pattern of a fault along with its offset in a cross-section view. Various other kinds of volumetric display also help to study and visualize faults.
Much of the science of fault detection concerns the recognition of subtle faults. On a normal vertical section a single-gradational color scheme, such as gradational gray (Figure 1), is usually best, as this type of display enhances the terminations of low amplitude events. The detection of subtle faults, however, is highly dependent on good data quality and high signal-to-noise ratio. Some extra care and attention in data collection and processing is always beneficial.
uCoherence & fault recognition
uCoherence & fault recognition
uCoherence & fault recognition
|
Coherence and Fault Recognition
Coherence is an invention of five years
ago that has had a beneficial impact on fault recognition. The coherence
transformation suppresses the continuity of seismic reflections and
emphasizes discontinuities such as faults. Coherence data are best
viewed as
Figure 2 is a
Once the major faults have been
recognized and the tectonic framework established, machine autotracking
should be used to complete horizon surfaces. The autotracker follows the
crest of an identified peak or trough with very high precision. The
resultant
The edge map of Figure 4 clearly distinguishes the short north-south faults from the long arcuate one. The arcuate fault is about seven kilometers long and looks impressive on the edge map -- however, it has negligible throw and is barely visible on any vertical seismic section. It was first recognized on this edge display and appears to be caused by an igneous intrusion.
As shown, the use of Distinguishing subtle faults from
various kinds of noise is always a value judgment, so experience is
useful. Interpreters tend to look at more than one type of The two panels of Figure 3 show the graben on both the dip and residual displays; some of the minor wiggly features, probably noise, occur on only one. Three-dimensional seismic data today typically contains an enormous amount of geologic detail. Faults are clearly an important part of this information. The modern interpreter must use all the interpretation tools available to find and understand the faults affecting the reservoir. With practice and experience, one can extract the subtle but valuable details inherent in the data. |