Image 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


Suppose that we already have estimated the velocity-depth model in Figure 9.3-1 down to 1-km depth. We shall construct the remaining part of the model layer-by-layer — the shale-marl sequence, the imbricate structures and the substratum, by prestack depth migration. Our goal here is to examine image gathers from prestack depth migration for layer velocity estimation and the stack of the image gathers for reflector geometry delineation. The results of the layer-by-layer velocity-depth model estimation based on image-gather analysis are shown in Figures 9.3-3, 9.3-4, and 9.3-5.

Figure 9.3-3a shows the velocity-depth model with the known shallow sequence down to 1 km, and the unknown part that is represented by the half-space below. The velocity assigned to the half-space is that of the shale-marl sequence (Figure 9.3-1). Note that this layer has a vertical velocity gradient. Figure 9.3-3b shows the depth image from prestack depth migration using the velocity-depth model in Figure 9.3-3a. Superimposed on this section are the layer boundaries in the velocity-depth model.

Figure 9.3-6a shows selected image gathers from prestack depth migration. The event T associated with the top of the carbonate sequence exhibits a flat character on the image gathers. This indicates that the velocity field for the layer above is correct. Consequently, the depth image in Figure 9.3-3b, which was obtained by stacking the image gathers as in Figure 9.3-6a, yields the correct reflector geometry for the top of the carbonate sequence. Interpret the top of the carbonate sequence from this section and insert it as a layer boundary into the velocity-depth model (Figure 9.3-4a).

Next, assign the velocity for the carbonate sequence (5700 m/s) to the half-space below the top-carbonate boundary as shown in Figure 9.3-4a. Using this model, perform prestack depth migration and obtain the depth image shown in Figure 9.3-4b. The image gathers indicate flat events for the top T and base B of the carbonate sequence (Figure 9.3-6b), thus confirming that the velocity of the carbonate sequence is correct. Additionally, the depth image in Figure 9.3-4b, which was obtained by stacking the image gathers as in Figure 9.3-6b, yields the correct reflector geometry for the base of the carbonate sequence. Interpret the base of the carbonate sequence from this section and insert it as a layer boundary into the velocity-depth model (Figure 9.3-5a).

Finally, assign the substratum velocity (5000 m/s) to the half-space below the base-carbonate boundary as shown in Figure 9.3-5a. Using this model, perform prestack depth migration and obtain the depth image shown in Figure 9.3-5b. The image gathers indicate flat events for the top T and base B of the carbonate sequence, and the flat reflector F within the substratum (Figure 9.3-6c). The velocity-depth model in Figure 9.3-5a is the same as the true velocity-depth model in Figure 9.3-1. Additionally, the section in Figure 9.3-5b, which was obtained by stacking the image gathers as in Figure 9.3-6c, represents the correct image in depth. Interpret the flat reflector at 2.5 km from this section and insert it as a layer boundary into the velocity-depth model (Figure 9.3-5a).

Now examine further the image gathers in Figure 9.3-6. The flatness of an event on image gathers is an indication of the accuracy of the velocity field associated with the layer above the layer boundary that is represented by that event. Actually, to declare the layer velocity as correct and accurate, not only the event associated with the base-layer, but also all events above that event should be flat. Note that in Figure 9.3-6a all events down to and including the top-carbonate are fairly flat.

The "nonflatness" of an event on image gathers is detectable only if there is sufficient cable length. For instance, the shallow events on image gathers in Figure 9.3-6a extend to a narrow range of offsets as a result of muting. As a result, it is difficult, if not impossible, to infer how flat these events are.

The events associated with the base-carbonate and the underlying flat reflector exhibit residual moveout on image gathers in Figure 9.3-6a. The event curvature for both layer boundaries is upward; this suggests that the velocity field assigned to the half-space which includes these two layer boundaries (Figure 9.3-3a) is erroneously low.

The detectability of residual moveout on image gathers is possible, again, only if there is sufficient cable length. The smaller the effective cable length for an event, the less detectable is the residual moveout, thus the poorer the velocity resolution. In the limit of zero offset, velocity resolution becomes nill. The residual moveout also is influenced by the magnitude of the layer velocity and the depth of the layer boundary.

Using the correct velocity for the carbonate sequence (Figure 9.3-4a), the event associated with the base-carbonate becomes flat on image gathers (Figure 9.3-6b). Nevertheless, the event associated with the flat reflector now exhibits a downward curvature, suggesting that the half-space velocity (5700 m/s) in Figure 9.3-4a is erroneously low for the substratum.

Finally, using the correct velocity for the substratum (Figure 9.3-5a), the event associated with the flat reflector F exhibits a flat character on image gathers (Figure 9.3-6c). Not only is this event flat in Figure 9.3-6c, but so are all the other events above. Suppose that an image gather contains 10 events, and that all are flat except the sixth from the top. This does not imply that the velocity-depth model is correct for all the layers except for the sixth layer. Instead, it implies that the velocity-depth model is correct down to and including the fifth layer, and the deeper part of the model is incorrect.

Flatness of all the events on image gathers is a means of verifying the accuracy of the velocity-depth model used in prestack depth migration. This is a necessary but not sufficient condition for verifying the accuracy of a model. As stated earlier in introduction to earth modeling in depth, by using nonzero-offset data, we can hope to resolve the velocity-depth ambiguity for a reflector if the data used in inversion have been recorded with offsets greater than the reflector depth. The additional limitations in model verification based on image-gather analysis are the reflector depths and the magnitude of the layer velocities. Specifically, the deeper the reflector or the higher the velocity of the layer above, the less the detectability of a residual moveout on image gathers (Figure 9.3-6).

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