Fault planes

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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


Dip-moveout correction in practice

Figure 5.2-15 shows selected CMP gathers along a marine line over a structure with fault blocks. CMP gather at midpoint location 1688 (at 2.5 s) exhibits a clear case of conflicting dips associated with two events with significantly different moveouts.

The DMO processing sequence includes the following steps.

  1. Perform velocity analysis sparsely along the line at locations with prominently flat events and create an initial velocity field.
  2. Apply normal-moveout correction using flat-event velocities. Note the event at 2.5 s on CMP gather 1688 associated with steep fault-plane reflections has been overcorrected as demonstrated in Figure 5.2-16, whereas reflections with no dip or negligibly small dip have been flattened.
  3. Apply partial stacking to CMP gathers to reduce the fold from 60 to 30, and sort the moveout-corrected gathers (Figure 5.2-16) to common-offset sections and perform dip-moveout correction. Then, sort back to CMP gathers and compare the selected gathers after DMO correction (Figure 5.2-17) with the same gathers without DMO correction (Figure 5.2-16). Note that the overcorrected event at 2.5 s on CMP gather 1688 has been removed. Again, this is a direct result of the partial migration effect of the DMO correction.
  4. Apply inverse moveout correction (Figure 5.2-18) with the same velocity field that was used for the NMO correction prior to DMO correction (Figure 5.2-16).
  5. Perform velocity analysis at frequent intervals along the line and pick velocity functions which now are supposed to have been corrected for the dip effect. Refer to the velocity analysis at midpoint 1688 shown in Figure 5.2-19. Refer to the velocity spectrum (Figure 5.2-19b) associated with the gather without DMO correction (Figure 5.2-19a) and note the two semblance peaks at 2.5 s — one at 2500 m/s and the other at 2750 m/s. The gather was moveout corrected using the denoted velocity function that includes the 2500-m/s peak, rather than the 2750-m/s peak. DMO correction has partially migrated the steeply dipping event to another midpoint location, and as a direct consequence, has removed the duality in the velocity spectrum at 2.5 s and yielded a more distinctive trend (Figure 5.2-19d) compared to the spectrum derived from the gather with no DMO correction (Figure 5.2-19b).
  6. Create a velocity field using the velocity functions picked from the velocity spectra computed from the DMO-corrected gathers.
  7. Apply moveout correction to DMO-corrected gathers using this velocity field. Selected CMP gathers are shown in Figure 5.2-20 and the corresponding CMP stack is shown in Figure 5.2-21. As a result of DMO correction, the steeply dipping fault-plane reflections have been preserved during stacking. Since a DMO stack is a closer approximation to a zero-offset section in comparison with a CMP stack, time migration of the DMO stack yields an image which shows clearly delineated fault blocks in the vicinity of CMP 1688 (Figure 5.2-22). For comparison, conventional CMP stack and its migration are shown in Figures 5.2-23 and 5.2-24, respectively. Because the fault-plane reflections have not been preserved with adequate strength on the CMP stack (Figure 5.2-23), time migration yields a blurred image of the fault blocks (Figure 5.2-24).

Prestack time migration

Figure 5.3-14 shows a portion of a conventional CMP stack without DMO correction. Steeply dipping events represent diffractions and fault-plane reflections. Because no DMO correction has been applied, these events have not been preserved with as much strength as the gently dipping reflections. As a result, migration of this conventional CMP stack yields an inadequate definition of the intensive fault pattern (Figure 5.3-15).

We shall use the data set shown in Figure 5.3-14 to bring together and review all we learned in this chapter about DMO correction and prestack time migration. To begin with, examine the dip effect on stacking velocities. Based on velocity analysis at CMP location A, a velocity function appropriate for gently dipping reflections was picked. By setting this as the center function and using a range of lower and higher percents of this function, a set of multivelocity-function stack panels shown in Figures 5.3-16 and 5.3-17 was created over a CMP range that includes a zone with gently dipping reflections and steeply dipping events. Note that the gently dipping reflections stack best with 100 percent of the center function (Figure 5.3-16) and the steeply dipping events stack best with 120 percent of the center function (Figure 5.3-17). This 20 percent difference in stacking velocities between the gently dipping and steeply dipping reflections is primarily stems from the dip effect. After DMO correction, both gently dipping and steeply dipping events stack best with 100 percent of the center function (Figures 5.3-18 and 5.3-19).

Removal of the dip effect on stacking velocities by DMO correction is further demonstrated by velocity analysis before and after DMO correction. Figure 5.3-20 shows the CMP gather and its velocity spectrum at location A as in Figure 5.3-14. At first impression, the velocity spectrum may suggest the presence of multiples. Nevertheless, there actually exist two sets of picks — the low-velocity picks as shown in Figure 5.3-21 associated with the gently dipping reflections, and the high-velocity picks as shown in Figure 5.3-22 associated with the steeply dipping fault-plane reflections and possibly diffractions. By applying NMO correction to the CMP gather using the low-velocity function in Figure 5.3-21, events that correspond to the gently dipping reflections are flattened whereas events that correspond to the steeply dipping reflections and diffractions are overcorrected. By applying NMO correction to the CMP gather using the high-velocity function in Figure 5.3-22, events that correspond to the steeply dipping reflections are flattened whereas events that correspond to the gently dipping reflections and diffractions are undercorrected.

The velocity spectrum at the same analysis location as for Figure 5.3-20 after the application of DMO correction shows a single, unambiguous velocity trend as shown in Figure 5.3-23. We have resolved the dip effect on stacking velocities and thus eliminated the multiple number of velocity picks as in Figure 5.3-20. As a result of the partial migration effect of DMO correction, energy is moved from one CMP location to another. Consequently, reflection-point smearing is removed and events on CMP gathers improvise a horizontally layered earth model.

For comparison, the low-velocity function associated with the gently dipping reflections in Figure 5.3-21 is superimposed on the velocity spectrum after DMO correction in Figure 5.3-24. As anticipated, the picks implied by the velocity spectrum after DMO correction are fairly close to the velocity function associated with the gently dipping reflections. Similarly, the high-velocity function associated with the steeply dipping reflections and diffractions in Figure 5.3-22 is superimposed on the velocity spectrum after DMO correction in Figure 5.3-25. In this case, the picks implied by the velocity spectrum after DMO correction are significantly lower than the velocity picks associated with the steeply dipping reflections.

The velocity analysis is repeated after DMO correction as shown in Figure 5.3-26. The picked velocities are then used to stack the data (Figure 5.3-27). Compare with the conventional CMP stack in Figure 5.3-14 and note that the DMO stack shown in Figure 5.3-27 has preserved the events with conflicting dips with different stacking velocities — the gently dipping reflections with the velocity function posted in Figure 5.3-21 and the steeply dipping fault-plane reflections with the velocity function posted in Figure 5.3-22. The resulting migrated section in Figure 5.3-28 shows a superior image of the fault blocks as compared to the migrated section without DMO correction (Figure 5.3-15).

For prestack time migration, we shall follow the same sequence as for the salt-flank data set.

  1. Sort the moveout-corrected CMP gathers to common-offset sections. Figure 5.3-29 shows portions of three selected common-offset sections. Note the presence of steeply dipping fault-plane reflections. Without DMO correction, CMP stacking fails to preserve these reflections as shown in Figure 5.3-14.
  2. Apply DMO correction to each common-offset section. Figure 5.3-30 shows portions of the three selected common-offset sections as in Figure 5.3-29 after DMO correction. Note the steeply dipping fault-plane reflections. With DMO correction, CMP stacking preserves these reflections as shown in Figure 5.3-27.
  3. Following DMO correction, each common-offset section is assumed to be a replica of a zero-offset section, and thus, can be migrated using a zero-offset migration algorithm. In this case, a single, vertically varying velocity function was used to migrate the NMO- and DMO-corrected common-offset sections. Figure 5.3-31 shows portions of the three selected common-offset sections as in Figure 5.3-30 after common-offset migration.
  4. Sort the migrated common-offset sections into CMP gathers. Events on these gathers are now assumingly close to correct subsurface positions after migration. Figure 5.3-32 shows a CMP gather after common-offset migration.
  5. Apply inverse NMO correction using the velocities picked before DMO correction and perform velocity analysis. The velocity spectrum computed from migrated data is shown in Figure 5.3-32 with the DMO velocities posted on it for comparison. The CMP gather after inverse NMO correction using the velocity function shown in Figure 5.3-32 is shown in Figure 5.3-33.
  6. Apply NMO correction using the velocity picks after migration (Figure 5.3-34). The velocity field derived from the post-migration velocity picks as in Figure 5.3-34 is shown in Figure 5.3-35, and the stack based on this velocity field is shown in Figure 5.3-36. This stack indeed is equivalent to prestack time-migrated section. Note that, however, common-offset migration actually was done using a single, vertically varying velocity function as in step (c).
  7. To obtain the migrated section with the velocity field (Figure 5.3-35) derived after common-offset migration, first perform inverse migration of the resulting stack from step (f) using the same velocity function as in step (c) to obtain a zero-offset section equivalent to an unmigrated stack as shown in Figure 5.3-37. Then, migrate this zero-offset section using the post-migration velocity field shown in Figure 5.3-35 to obtain the final result from prestack time migration sequence described here (Figure 5.3-38).

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