Correlation procedures in building a depth migration velocity model
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The work flow for building a depth migration velocity model includes several interpretive tasks which are critical to timely execution and successful completion of a depth migration project. These tasks involve correlation of horizons which define intervals and bodies having distinctly different velocity character. Automatic versus manual tracking of these horizons depends on the signal-to-noise ratio (S/N) and image fidelity of the seismic data, and the mode of tracking can be expected to change both vertically and laterally as these elements of data quality vary.
Generic processing sequence
The following generic processing sequence, for marine 3D seismic data in a subsalt exploration project, outlines the four main steps in building a depth migration velocity model and highlights some of the concerns for interpretation at each step:
- Water-flood migration to image the sea floor. In this step, the entire data volume is migrated with the velocity of sea water. Most often tracking of the sea floor reflection is straightforward and can be done automatically. In areas with significant sea floor relief such as along the Florida Escarpment in the eastern Gulf of Mexico and the Sigsbee Escarpment in the central and western Gulf of Mexico, accurate interpretation of the sea floor is crucial to the success of subsequent migration steps.
- Sediment-flood migration to image the top of the salt. Having picked the sea floor horizon on the water-flood migration from step 1, the migration velocity model is updated by replacing the sea water velocities below the sea floor horizon with a sediment-velocity field. After the data volume is migrated a second time the top of salt reflection is correlated, again either automatically or manually according to considerations of data quality as mentioned previously. Correlation of the top of salt is less straightforward than picking the sea floor and can be difficult where the horizon is rugose, steeply dipping or not well imaged.
- Salt-flood migration to image the base of the salt. Having picked the top of salt on the sediment-flood migration from step 2, the sediment-velocity model is updated by replacing the sediment velocities below the top of salt horizon with salt velocity. After the data volume is migrated a third time the base of salt horizon is correlated with the same attention to data quality. Correlation of this horizon is the most difficult and time-consuming of the three horizons picked in the depth migration sequence, and in areas of very poor S/N can be done only in model-guided fashion.
- Final migration to image the subsalt section. Having picked the base of salt on the salt-flood migration from step 3, the migration velocity model is updated by replacing the salt velocity below the base of salt horizon with sediment velocities, and the data volume is migrated one last time. This final migration produces the data volume on which exploration for subsalt targets is based.
Where there are multiple overlying or irregularly-shaped salt bodies, which can be envisioned as at any point within the 3D survey area where a vertical well would penetrate more than one top of salt (and, necessarily, more than one base of salt), steps 2 and 3 in this sequence must be repeated to image successively deeper salt bodies properly. In conjunction with processing geophysicists and project managers, interpreters decide whether or not to repeat these steps based on the extent and degree of salt complexity, the imaging accuracy required for acceptable definition of subsurface targets, and the time and funding available for additional processing and interpretation. This decision whether or not to proceed with extra imaging steps will be project-specific and not always simple or straightforward. Regardless of the number of migrations deemed necessary for building an accurate, fit-for-purpose depth migration velocity model, an interpreter needs to understand that his correlations at each step in the migration sequence must as accurately as possible account for subsurface complexity because they are critical to the accuracy of subsequent steps.
In a depth migration project the horizons which bound bodies with anomalously low or high velocity (such as salt) must be correlated in such a way that these bodies are completely closed spatially before proceeding with the final migration (step 4 in the preceding migration sequence). Figure 1 schematically illustrates a technique for picking top and base of salt horizons that facilitates construction of closed salt bodies on a single-valued system (recall that a single-valued system allows only one Z value for any X-Y position). Figure 1a shows the outline of a salt body on which the top of salt horizon (“top salt 1” in light blue) picked on a sediment-flood migration extends beyond the right-hand edge of the body. This horizon is manually projected as shown to ensure that it will intersect and overlap a similarly projected base of salt horizon picked on a subsequent salt-flood migration (“base salt 1” in red). The base of salt horizon and a deeper top of salt (“top salt 2” also shown in light blue) are picked in similar fashion to ensure that they overlap and hence close the body. This second deeper top of salt or “salt flank” horizon must be named and picked as a separate horizon. When all of the top and base of salt horizons for all salt bodies in a project have been picked, the overlap areas are trimmed away and the salt bodies are then completely defined as shown in Figure 1b. The edges of salt bodies should be picked very carefully, often on every line within several traces (bin widths) of the actual intersection of the top and base of salt horizons, to ensure that the bodies are smoothly and fully closed, because even small errors in picking at these edges can have detrimental effects on the fidelity of the final migration.
A salt suture is defined as the boundary between two salt bodies that have merged or coalesced, is often observed on depth-migrated data. The acoustic impedance contrast between adjoined salt bodies or between salt and sediments entrained in a suture zone can give rise to a reflection marking the position of a suture. In some cases there can be portions of a suture zone across which there is no impedance contrast, e.g., there is “clean” salt against “clean” salt, and so no reflection will be visible; the suture is not detected, but geologically it is still there. In these areas the suture is picked, if picked at all, in model-guided fashion based on the shapes of the individual (unsutured) salt bodies or the trend of the suture where it is visible.
Imaging horizons versus geologic horizons
Figure 3a is a schematic of the correlations of three events which define two merged salt bodies: top of salt, base of salt and salt suture. Figure 3b shows the top and base of salt events picked for building a migration velocity model for depth imaging; note that the salt suture shown in Figure 3a is not needed for imaging unless the two salt bodies are assigned different velocity values. Figures 3c and d show how the two salt bodies are defined separately using the salt suture and appropriate portions of the top and base of salt horizons as originally picked; in particular, note that the salt suture actually is part of the top of salt surface for the body on the left and at the same time part of the base of salt surface for the body on the right. These “geologic” top and base of salt horizons are constructed by merging the suture with edited copies of the original top and base of salt horizons picked for imaging. Editing and merging of horizons in this way requires careful data management, to say nothing of the demands on data management caused by picking salt, especially base of salt, on multiple seismic volumes processed with different migration algorithms, each having its own distinct imaging advantage.
The essential work of picking top and base of salt horizons is unfortunately regarded by some as a bottleneck in building a depth migration velocity model. For reasons such as poor image quality and the need for model-guided picking, these correlations can often be “so difficult as to be almost impossible” (Dix, 1952). They cannot be done quickly and easily in every instance, and many times the places in which the correlations are the most important are where they take the most time. An interpreter should follow his own standards of quality in this work within the greater context of project schedules and costs, and should always strive to maintain a stable balance between these frequently opposing concerns.