Structural interpretation

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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

A 3-D structural interpretation session may begin with viewing selected inline and crossline sections to acquire a regional understanding of the subsurface geology. Other orientations, such as vertical sections along a dominant dip direction, also may be needed to determine the structural pattern. Time slices then are studied to check the structural pattern. These previews may be made dynamic in an interactive environment; vertical or horizontal sections can be viewed in rapid succession as one would a film strip from a motion picture. Any change in structure in space and time can thus be grasped with ease.

Figure 7.5-14 shows an image volume derived from 3-D prestack time migration of data from a marine 3-D survey. There are 296 inlines and 1300 crosslines in the data volume. Figures 7.5-15 and 7.5-16 show snapshots of the image volume as it is being visualized in the inline and crossline directions, respectively. The time slices shown in Figure 7.5-17 also represent snapshots from the 3-D visualization session. Note the complex structural pattern represented by a principal fault across the survey area from left to right and a series of fault blocks on both sides of this fault.

The water bottom and six deeper time horizons were picked using a combination of seed detection and line-based interpretation.

  1. First, the seismic event for each horizon was identified within the image volume. Then, at locations with good continuity and signal-to-noise ratio, seed points were placed at the seismic event. For consistency, all seed points for a given horizon were assigned identically — either at trough or peak. By using the seed detection method, the event was tracked away from each seed point as far as possible laterally in all directions. The result of seed detection is a series of surface patches associated with the horizon under consideration (Figure 7.5-18). Figure 7.5-19 shows the surface patches for the five horizons for which seed detection was applied. The water-bottom horizon is not shown since it is almost flat. The seed detection was not suitable for the deepest horizon H6. Note that for horizons H1-H5, seed detection was largely successful away from the fault zones.
  2. Where seed detection failed, control points were picked along a grid of selected inlines and crosslines (Figure 7.5-20). In fact, the deepest horizon H6 was picked entirely using the line-based interpretation. Figure 7.5-21 shows the surface patches derived from seed detection and line-based picks for all six horizons.

The combined result of seed-based and line-based interpretation of all six horizons is shown in Figure 7.5-22. Horizons H1-H6 of Figure 7.5-22 are ordered from top to bottom. Specifically, interpretation has produced a set of picks for each horizon. These picks are used in grid calculations to create complete surfaces for all the horizons over the entire survey area. Figure 7.5-23 shows the gridded surface associated with horizon H3 using the picks from seed-based and line-based interpretations shown in Figure 7.5-20. The gridded surfaces for all six horizons based on the picks shown in Figure 7.5-21 are shown individually in Figure 7.5-24. The 3-D view that exhibits the final result of picking shown in Figure 7.5-22 is repeated for the final result of gridding as shown in Figure 7.5-25.

During and after an interpretation session, it is imperative to check the consistency of the picked horizons with the seismic data volume that was used in the interpretation. Figure 7.5-26 shows combined views of the time horizons and seismic cross-sections from the image volume as in Figure 7.5-18. The final step in structural interpretation is time-to-depth conversion of time horizons interpreted from the time-migrated volume of data. This step requires knowledge of interval velocities for each layer, and as such, it implies earth modeling in depth. We shall therefore refer to the discussion on time-to-depth conversion and calibration of the results of depth conversion to well data in earth modeling in depth.

A structural interpretation session normally would include explicit delineation of fault framework. Nevertheless, the objective in the present case study is the investigation of stratigraphic features within the reservoir zone, and as such, the time horizons shown in Figure 7.5-22 were interpreted without the explicit definition of the fault patterns. Instead, the fault surfaces were interpreted as part of the horizons themselves.

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