3-D poststack time versus depth migration
Consider a 3-D survey over a hypothetical salt-dome structure. The base map is shown in Figure 8.4-1. The synthetic 3-D survey data consist of 481 inlines and 481 crosslines with 25-m trace spacing in both directions. Selected cross-sections of the 3-D velocity-depth model and the associated zero-offset inline sections are shown in Figure 8.4-2. The salt dome has a circular symmetry with a flat base. The base map shown in Figure 8.4-1 actually is a time slice at 1200 ms to show the circular symmetry.
Keep in mind that the sections in Figure 8.4-2 are the cross-sections of the 3-D zero-offset wavefield along the traverses that coincide with the cross-sections of the 3-D velocity-depth model. In other words, we are not starting with the cross-sections of the velocity-depth model in Figure 8.4-2 and generating a set of 2-D zero-offset wavefields from them. Strictly speaking, the only section that does not contain any sideswipe energy is the center line I-241. Note the energy reflecting off the flank of the salt dome and being recorded on the lines away from the center line.
First, we perform one-pass implicit 3-D time migration (Section G.1) on the entire 3-D zero-offset synthetic data and display the same lines after 3-D time migration (Figure 8.4-3). The migration velocity field is based on the true subsurface velocity-depth model used in computing the 3-D zero-offset wavefield (Figure 8.4-2). Also, for comparison, pretend that two lines — center line I-241 and line I-181 away from the center, are part of a 2-D survey and perform 2-D time migration. Note the following observations:
- The top of the salt, albeit the vertical scale is in time, has been imaged properly with 3-D time migration, while the base of the salt has not. This is because the salt diapir acts as a complex overburden.
- The 2-D time migration produced the correct result for the top of the salt only along the center inline (I-241), because there are no sideswipes on this line; therefore, there is no need for 3-D migration. However, on a line away from the center line, say I-181, even the top of the salt has not been imaged properly by 2-D time migration, let alone the base of the salt. This is because the line contains sideswipes off the flank of the salt dome.
- Now you may have second thoughts about 2-D seismic exploration. Fortunately, most structures have dominant strike and dip directions. Properly oriented 2-D migrated lines often yield an acceptably accurate structural picture. However, one lesson to be learned from comparing the 3-D and 2-D time migrations of line I-181 shown in Figure 8.4-3 is that the migration velocities that yield an acceptable 2-D migrated section may be quite different from the true subsurface velocity model required by 3-D migration.
Figure 8.4-1 A time slice used as a base map for a synthetic 3-D survey. The velocity-depth model and the modeled 3-D zero-offset data for selected inlines are shown in Figure 8.4-2.
We now perform one-pass implicit 3-D depth migration (Section G.1) of the 3-D zero-offset synthetic data shown in Figure 8.4-2. After 3-D depth migration of the entire volume of the 3-D zero-offset wavefield using the true 3-D velocity-depth model, we display the same lines (Figure 8.4-4). Also, for comparison, pretend that the two lines — center line I-241 and line I-181 away from the center, are part of a 2-D survey and perform 2-D depth migration, again using the correct velocity-depth model. Note the following observations:
- The top and base of the salt now have been imaged properly with 3-D depth migration. The complex overburden problem has been solved. (Compare the left column in Figure 8.4-3 with that in Figure 8.4-4.)
- The 2-D depth migration produced the correct subsurface model only along the center inline (I-241) because there are no sideswipes on this line. However, on a line away from the center line, say I-181, neither the top nor the base of the salt have been imaged properly. This is because the line contains sideswipes off the flank of the salt dome.
- By performing 2-D depth migration iteratively to converge to assumingly correct depth model, we may be forcing the model to converge to something very different from the truth. This stems from the treatment of the sideswipes as events that are in the plane of profile.
- 3-D poststack depth migration
- Two-pass versus one-pass 3-D poststack depth migration
- Implicit versus explicit 3-D poststack depth migration
- 3-D poststack datuming