Fault planes

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Seismic Data Analysis
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

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).

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Fault planes
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