Stacked seismic section

Stacked seismic section after free-surface multiple removal.

In the stacked sections in the previous figure and here, one can notice several locations where multiples interfere with primary events. We have indicated three examples of primary-multiple interference by arrows. The first example (arrow A) is a multiple which lies at 1.83 seconds on CMP 800 and rises to 1.72 seconds at CMP 100. At CMP 580 a slight discontinuity in the event suggests that in this region the multiple overlies a primary event. After the demultiple, the multiple event is attenuated along the length of the seismic section, where there is no primary interference, but leaves a primary event between CMP 300 and CMP 700, which form the top of a mound structure. The second example is illustrated by arrow B. Before the demultiple, we can observe a double set of strong free-surface multiples dipping from right to left, starting at 2.2 seconds on CMP 100. After the demultiple, the free-surface multiples are well attenuated, with a primary event revealed below the lower free-surface multiple between CMP 200 and CMP 350. Arrow C marks the last example of the structure emerging after the demultiple, where a small mound structure at 1.15 seconds emerges from the demultiple seismic section. The interpretation of these three events was supported by other independent analyses. Also, in the case of the example marked by arrow B, we would expect that the estimated source wavelet would allow either the attenuation of both free-surface multiples or fail for both. Hence we concluded that the event between CMP 200 and CMP 350 on the demultipled seismic section is a primary.

The above examples show that the demultiple process has suppressed multiple reflected energy if it is present but has not significantly attenuated the primary energy along with it.

Other Images in Chapter 10

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.