Constant half-space velocity analysis

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Seismic Data Analysis
Series Investigations in Geophysics
Author Öz Yilmaz
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store

Image gathers from prestack depth migration are used to estimate layer velocities in two ways — constant half-space velocity analysis and residual moveout analysis. Suppose that the velocity-depth model has been established for the first n − 1 layers, and that we want to estimate the layer velocity for the nth layer. Using the known overburden velocity-depth model for the first n − 1 layers, assign a constant velocity to the half-space below that includes the nth layer. Perform prestack depth migration and output image gathers at some appropriate interval along the line. The image gathers should exhibit flat character for the events associated with the n − 1 layers, but show a residual moveout for the event associated with the nth layer. Repeat the analysis using the same overburden model and a range of constant velocities assigned to the half-space.

Consider the velocity-depth model in Figure 9.3-5a. The substratum below the base-carbonate layer boundary is the half-space that contains the flat reflector at 2.5 km. The half-space velocity is 5000 m/s. Selected image gathers from prestack depth migration using this model are shown in Figure 9.3-6c, and the depth image is shown in Figure 9.3-5b. Keep the same overburden model as in Figure 9.3-5a, but assign a constant velocity of 4500 m/s to the half-space as shown in Figure 9.3-7a. The resulting image gathers are shown in Figure 9.3-9a and the depth image in Figure 9.3-7b. The event on the image gathers associated with the flat reflector exhibits a small, but still detectable residual moveout that suggests erroneously low velocity for the layer above the flat reflector. Repeat the analysis for a half-space velocity of 5500 m/s (Figure 9.3-8a). Selected image gathers are shown in Figure 9.3-9b and the stack of image gathers in Figure 9.3-8b.

The velocity-depth models (Figure 9.3-5a, 9.3-7a, 9.3-8a) corresponding to the image gathers in Figures 9.3-6c, 9.3-9a, and 9.3-9b, respectively, have the same overburden, but with three different constant half-space velocities — 5000, 4500, and 5500 m/s, assigned to the substratum below the carbonate sequence. First, examine the residual moveout on the image gathers for the event associated with the flat reflector F. Although the moveout differences are subtle, the flatness is achieved with the 5000-m/s half-space velocity. By analyzing the image gathers from constant half-space velocity scans, we can make optimum velocity picks at analysis locations that best satisfy the flatness criterion for the layer under consideration.

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Constant half-space velocity analysis
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