Layered earth model estimation
Start the North-Sea style analysis (subsalt imaging in the North Sea) by interpreting a set of time horizons from the time-migrated CMP stack (Figure 10.2-3a). These time horizons are then used to obtain the unmigrated time horizons (Figure 10.2-3b) by way of a ray-theoretical forward modeling scheme, sometimes called demigration. Horizon A in Figure 10.2-3a represents the top-salt boundary. The modeled time horizons are assumed to be equivalent to zero-offset reflection times, which are used in coherency inversion. Alternatively, time horizons could have been interpreted from the unmigrated stacked section, directly.
To estimate a layered earth model, the following procedure was implemented:
- Starting from the top, for each layer within the suprasalt region, perform coherency inversion to estimate the layer velocity, and
- 2-D poststack depth migration to delineate the reflector geometry associated with the layer boundary.
- Repeat steps (a) and (b) for all the layers within the suprasalt region, and thus establish the velocity-depth model for that region.
- Assign the salt velocity into the half-space below the top-salt boundary, and
- perform 2-D prestack depth migration to delineate the base-salt boundary.
- Then, assign a vertically varying velocity function into the subsalt region (sometimes referred to as salt flooding), and
- perform 2-D prestack depth migration to obtain and verify the final earth image in depth.
- Convert the image-gather stack from step (g) from depth to time using the velocity-depth model from step (d), apply poststack spiking deconvolution, band-pass filtering, and AGC scaling, then convert back to depth.
Figure 10.2-4 shows the velocity-depth model based on the procedure outlined above. Note that there are three detached salt bodies. Also note the lateral velocity variations, detected by coherency inversion, within each layer in the suprasalt region. The velocity field in the subsalt region is referenced to the water bottom, since the salt bodies do not have much influence on the vertical variations in the background velocity field. To appreciate the raypath distortions caused by the salt bodies, examine the image rays down to the base-salt boundary in Figure 10.2-5. The image rays clearly indicate the need for prestack depth migration for accurate imaging of the subsalt region.
Figure 10.2-6 shows the depth image from prestack depth migration using the velocity-depth model in Figure 10.2-4. Note the rugose top-salt boundary, and relatively smoother base-salt boundary. The accompanying image gathers are shown in Figure 10.2-7. Note the high-amplitude events (A and B) associated with the top- and base-salt boundaries. Flatness of events both in the suprasalt and subsalt regions provides the evidence that the model in Figure 10.2-4 is geologically acceptable. Note the presence of intrasalt events (C) with moveout; they often are associated with shale intrusions. These events could also be associated with multiples or converted waves (4-C seismic method).
As part of a verification procedure, the estimated earth model (Figure 10.2-4) needs to be tested for consistency with the input seismic data. Figure 10.2-8a shows the modeled zero-offset traveltimes superimposed on the unmigrated stacked section as in Figure 10.2-1. These traveltimes are associated with the layer boundaries included in the velocity-depth model of Figure 10.2-4. They should be compared with the traveltimes derived from the forward modeling of the time horizons picked from the time-migrated section (Figure 10.2-8b). Note that the two sets of traveltimes are fairly consistent within the suprasalt region. However, some discrepancy exists for the top-salt event (red horizon) and the base-salt event (yellow horizon).
Figure 10.2-4 The Gulf of Mexico line: layered earth model in depth estimated by a combination of coherency inversion to estimate layer velocities and poststack depth migration to delineate reflector geometries within the suprasalt region, and prestack depth migration to delineate the base-salt boundary.
Figure 10.2-6 The Gulf of Mexico line: depth image from prestack depth migration using the model shown in Figure 10.2-4.
Figure 10.2-7 Part 1: The Gulf of Mexico line: selected image gathers from prestack depth migration using the model shown in Figure 10.2-4. The depth image derived from stacking of the image gathers is shown in Figure 10.2-6.
Figure 10.2-7 Part 2: The Gulf of Mexico line: selected image gathers from prestack depth migration using the model shown in Figure 10.2-4. The depth image derived from stacking of the image gathers is shown in Figure 10.2-6. A and B are events associated with top- and base-salt boundaries, and C is an event within the salt body.
Figure 10.2-7 Part 3: The Gulf of Mexico line: selected image gathers from prestack depth migration using the model shown in Figure 10.2-4. The depth image derived from stacking of the image gathers is shown in Figure 10.2-6.
Figure 10.2-8 The Gulf of Mexico line: (a) modeled zero-offset traveltimes associated with the layer boundaries included in the earth model of Figure 10.2-4. (b) the zero-offset traveltimes of (a) superimposed on the zero-offset traveltimes as in Figure 10.2-3b. Both sets of traveltimes are superimposed on the stacked section as in Figure 10.2-1.
Figure 10.2-9 The Gulf of Mexico line: depth image from poststack depth migration using the model shown in Figure 10.2-4.
Compare the depth images derived from prestack depth migration (Figure 10.2-6) and poststack depth migration (Figure 10.2-9) using the same velocity-depth model (Figure 10.2-4). While the images within the suprasalt region are comparable, prestack depth migration certainly yields a superior image of the base-salt boundary and the subsalt region.