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GCPitfalls
in 3-D Seismic
Interpretation*
By
Alistair R. Brown1
Search and Discovery Article #40146 (2005)
Posted March 14, 2005
*Adapted from the
Geophysical Corner column in AAPG Explorer, March, 2005, prepared by the author,
who presented this article as the keynote address at the RMAG/DGS 11th
annual 3-D
Seismic
Symposium March 11, 2005, in Denver, Colorado.. Appreciation
is expressed to him, as author and editor of Geophysical Corner, and to Larry
Nation, AAPG Communications Director, for their support of this online version.
1Consulting reservoir geophysicist, Dallas, Texas ([email protected])
As a consultant I am often in a position to review
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I was invited to listen to a presentation on Workstations are magnificent tools, but the answers are still to be found in “the minds of men." How many of us realize that the precision of machine autotrackers is typically around one-quarter of a millisecond? In good data this precision represents geology and must be exploited. Thus, autotrackers are indispensable tools of modern interpretation. Derivatives of autotracked time maps, such as residual, dip, and azimuth can yield vital structural detail not visible in any other way.
Overdependence on Windowed Amplitude Horizon amplitude versus windowed amplitude is another common pitfall. Windowed amplitude is more modern, but this doesn't mean that we use it to the exclusion of horizon amplitude that has been available for 20 years. RMS (root mean square) amplitude seems to be the most popular type of windowed amplitude. This has splendid application for various reconnaissance endeavors. Figure 1 shows RMS amplitude over a 500 ms window revealing many small bright spots in the Frio Formation of South Texas. The squaring of the amplitude values within the window gives the high amplitudes maximum opportunity to stand out above the background contamination. Horizon amplitude (Figure 2), extracted along the high precision autotrack, is much better for studying a single reservoir. Horizon amplitude suffers no contamination but requires that the horizon has been correctly identified and tracked. Horizon slices thus remain the best amplitude displays for selecting the optimum drilling location or measuring the area of a reservoir.
Data phase
and polarity critically determine
I believe
that every Figure 3 shows a good well and a dry hole both penetrating high amplitudes. This data is American polarity; so red-over-blue (trough-over-peak) is the character of low impedance prospective sand. We should have been able to recognize that the blue-over-red amplitude was a poor prospect.
Acquisition and Processing Defects
The
presence of surface obstacles or the lack of access (no permit) causes
reduced and variable
Recommendations to help today's interpreter get more geology out of 3-D
· Expect detailed subsurface information. · Do not rely on the workstation to find the answer. · Use all the data. · Understand the data and appreciate its defects. · Use time (or depth) slices/horizontal sections. · Visualize subsurface structure. · Use machine autotracking and snapping. · Select the color scheme with care. · Question data phase and polarity.
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Tie
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Try to believe
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Understand the · Prefer horizon attributes to windowed attributes. · Use techniques that maximize signal-to-noise ratio. |