VC survey

VC survey: (a) PSDM of four VC positions using only the true receivers. The cable separation was 1250 m. For the Green's function calculation, the maximum amplitude raytracer was used. Note that the steep fault-plane (event B) reflections are quite well imaged. (b) PSDM of 4 VC positions using only the virtual receivers. The cable separation was 1250 m. For the Green's function calculation the maximum amplitude raytracer was used. Note that the steep fault-plane reflections (event B) are not as well imaged as they are by the true receivers, whereas at shallower levels (events A) there are no shadow zones due to insufficient coverage are present.

The present inversion and migration algorithms, like the ones described above, are designed for imaging primary reflections. In coming years, we anticipate that inversion and migration will also be applied to multiples, receiver ghosts of primaries, downgoing wavefields, etc.---in other words, to some of seismic energy which is disregarded by the present processing algorithms. Actually, this type of imaging is already happening in the processing of vertical cable (VC) data, in which receiver ghosts of primaries are imaged in addition to the primaries themselves.

The principle obviously applies to OBS data recorded in deep water.

The imaging results here and in the next two figures were obtained by prestack depth migration (PSDM), described earlier. The exact interval velocities used in the finite-difference modeling, resampled at 50 m, were supplied as the migration velocity model the PSDM application. The method for generating the Green's functions was based on the Eikonal equations.

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.