Velocity analysis using common-reflection-point gathers

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Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store


The next field data example demonstrates the use of constant-velocity prestack time migration to generate selected CMP gathers in their migrated positions to determine migration velocities. Figure 5.4-12 shows a CMP stack without DMO correction; the exploration objective requires delineation of fault blocks which manifest themselves with phantom diffractions. These diffractions conflict with the gently dipping reflections associated with the surrounding strata. Poststack time migration of the CMP stack without DMO correction falls short of providing a crisp image of the fault blocks (Figure 5.4-13).

Dip-moveout correction preserves the phantom diffractions and fault-plane reflections on stacked data as shown in Figure 5.4-14. As a result, migration of the DMO stack (Figure 5.4-15) enables a better delineation of the fault planes as compared with the migration of the conventional CMP stack without DMO correction (Figure 5.4-13).

Table 5-4. Velocity function derived from the constant-velocity migration panel of Figure 5.4-16.
Two-way Zero-Offset time, ms RMS Velocity Picked, m/s
0 1500
450 1500
700 1600
900 1750
1050 1900
1200 2200
1500 2500
2000 2800
2350 3000
2500 3000
Table 5-5. Velocity function derived from the constant-velocity migration panel of Figure 5.4-17.
Two-way Zero-Offset time, ms RMS Velocity Picked, m/s
0 1500
400 1500
750 1700
900 1800
1050 2000
1350 2300
1500 2600
2050 2900
2500 3000

To derive a migration velocity field, we perform prestack time migration using a range of constant velocities and generate selected gathers at their migrated positions. The interval for output gathers is specified in accordance with the degree of lateral velocity variations that can be accommodated by time migration. An interval equal to a cable length is a reasonable rule to follow. Figures 5.4-16 through 5.4-19 show migration velocity panels at four midpoint locations. The way to use these panels to pick rms velocity functions at analysis locations is similar to the use of a CMP gather that has been normal-moveout corrected using a range of constant velocities (Figure 3.2-11). In both cases, a velocity function is picked based on the criterion of flatness of events. The flatness of an event on a moveout-corrected gather means that the moveout velocity associated with that event is optimum. Similarly, the flatness of an event on a CMP gather created by prestack time migration means that the image below the midpoint location associated with that gather is the same irrespective of offset, and thus migration velocity is correct. Erroneously too low velocity yields a moveout on a gather from prestack time migration similar to the moveout of an event on a CMP gather which has been overcorrected. Similarly, erroneously too high velocity yields a moveout on a gather from prestack time migration much like to the moveout of an event on a CMP gather which has been undercorrected. Observe flat, over- and undercorrected events in the velocity panels shown in Figures 5.4-16 through 5.4-19, and pick for each CMP location rms velocity functions. Tables 5-4 through 5-7 list the picked rms velocity functions.

Table 5-6. Velocity function derived from the constant-velocity migration panel of Figure 5.4-18.
Two-way Zero-Offset time, ms RMS Velocity Picked, m/s
0 1500
375 1500
700 1700
950 2000
1100 2200
1350 2350
1500 2450
2500 3000
Table 5-7. Velocity function derived from the constant-velocity migration panel of Figure 5.4-19.
Two-way Zero-Offset time, ms RMS Velocity Picked, m/s
0 1500
350 1500
650 1750
750 1800
950 2000
1250 2500
1750 2700
2500 3000

There is one very important aspect of the constant-velocity migration panel as in Figure 5.4-19 and the constant-velocity moveout panel as in Figure 3.2-11. When picking a moveout velocity function from the latter, you are tracking the same event at the same reflection point from gather to gather in the same panel — overcorrected at low velocities, flat at optimum velocity value and undercorrected at high velocities. Whereas, when picking an rms velocity function from the former, you are not tracking the event at the same reflection point because of the lateral positioning effect of migration.

By interpolating between the picked rms velocity functions as listed in Tables 5-4 through 5-7, a velocity field for time migration can be created (Figure 5.4-20). By using this velocity field, data are migrated before stack. The common-reflection-point (CRP) gathers in Figure 5.4-21 exhibit flatness of events — a way to check the accuracy of the migration velocity field. Stacking the CRP gathers yields the image from prestack time migration as shown in Figure 5.4-22. Admittedly, there is some undermigration caused by the 3-D geometry of the fault planes which can be adequately imaged only by 3-D migration. Nevertheless, compare with migrations of the conventional stack (Figure 5.4-13) and DMO stack (Figure 5.4-15), and note that the faults have been delineated much more distinctively. Such differences — conflicting dips with different stacking velocities associated with fault blocks and salt diapirs where vertical velocity variations may be beyond the accuracy of DMO correction, are one motivation for doing time migration before stack.

Limitations in picking reliable velocity functions are the same as those associated with constant-velocity moveout panels (Figure 3.2-11). Shortening of effective cable length at shallow times because of muting, interference of linear noise and multiples all impose a limit on the picking accuracy (noise and multiple attenuation).

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Velocity analysis using common-reflection-point gathers
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