Time migration and depth migration in the Green Canyon

Comparison of time migration and depth migration in the Green Canyon area of the Gulf of Mexico. (a) and (c) show the time-migration section, and (b) and (d) show the depth migration. Notice that (a) and (c) are identical sections; we have only added to (c) an interpretation of the salt bodies. Similarly, (b) and (d) are identical sections; we have only added to (d) an interpretation of the salt bodies. So the time-migrated section in (c) shows two distorted salt bodies, whereas the depth-migrated section in (d) has retained the general shape of the salt bodies, including the flanks of the salt body on the right. A indicates two anticlinals in the time-migrated section.

(a) shows time imaging and depth imaging for a vertical section from the Green Canyon area of the Gulf of Mexico. The time-imaged section shows two anticlinal structures created by the salt intrusion. A more realistic interpretation will obviously include the pull up due to the high salt velocity but it is still clearly as good as that from depth migration in (b). The salt body in depth migration image has a domed top, a flat base, and a shadow beneath, obscuring deeper reflections. The salt intrusion on the right appears to have pierced through the top of the anticline and left a dome of salt behind.

With depth imaging, the picture significantly changes and often easier to interpret than the time migration image. The salt body on the left is still domed, but thicker, with a sloping base. Layers can now be seen below the salt. The salt feature on the right looks entirely different. Instead of two disconnected salt bodies, the new image shows a single hourglass-shaped body with clearly delineated sides and base. Instead of rising in an anticlinal structure, sediments are truncated along the flanks of the salt hourglass because the velocity pull up of the top salt over hang is not compensated for in time migration.

Other Images in Chapter 11

An example of the sailing path of a marine vessel in a towed-streamer survey

Images - Chapter 8 An example of the sailing path of a marine vessel in a towed-streamer survey. Note that the time for turning from one sailing line to another is about nine hours for vessels carrying streamers that are 10 km long. The dotted line indicates the turning legs of the sailing path.

This figure illustrates a typical sailing path of a 3D survey; the vessel travels back and forth, shooting and collecting data along many parallel lines, resulting in seismic data generated along lines 25 to 50 meters apart. Note that it takes about nine hours to turn from one sailing line to another for a vessel carrying 10-km-long streamers. Today, data are recorded even when the vessel is turning.

A shot diagram

Images - Chapter 7 A display of source and receiver distribution of a 2D seismic line in the so-called shot diagram. The rows corresponding to common-shot gathers and columns to common-receiver gathers. The diagonal is the zero-offset section, and all the other lines parallel to the diagonal are common-offset gathers (also known as common-offset sections). The lines perpendicular to the diagonal are the CMP gathers.

Another illustration of towed streamer acquisition

Images - Chapter 7 Another illustration of towed streamer acquisition

Interference noise

Images - Chapter 7 An illustration of interference noise in seismic data before and after. This figure shows the stack of seismic interference noise contaminated shots from another line in the Gulf of Mexico. Interference noise is clearly visible. Attenuation of seismic interference noise can be achieved by the use f-x prediction filters. Courtesy of Western Geco.

Other examples of structural traps

Images - Chapter 1 Structural traps. (A) Tilted fault blocks in an extensional regime. The seals are overlying mudstones and cross-fault juxtaposition against mudstones. (B) Rollover anticline on thrust. Petroleum accumulations may occur on both the hanging wall and the footwall. The hanging wall accumulation is dependent on a subthrust fault seal, whereas at least part of the hanging wall trap is likely to be a simple, four-way, dip-closed structure. (C) Lateral seal of a trap against a salt diapir and compactional drape trap over the diapir crest. (D) Diapiric mudstone associated trap with lateral seal against mud wall. Diapiric mud associated traps share many common features with that of salt. In this diagram, the diapiric mud wall developed at the core of a compressional fold. (E) Compactional drape over a basement block commonly creates enormous low-relief traps. (F) Gravity-generated trapping commonly occurs in deltaic sequences. Sediment loading causes gravity-driven failure and produces convex-down (listric) faults. The hanging wall of the fault rotates, creating space for sediment accumulation adjacent to the fault planes. The marker beds (grey) illustrate the form of the structure that has many favourable sites for petroleum accumulation. Adapted from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience. Oxford: Blackwell Science.