Stacked section

Stacked section before and after free-surface multiple attenuation.

The Troll dataset considered here is also a 2D line. It was acquired in very bad weather in 1994 by Western-Geco, which was then Geco-Prakla. So this dataset contains a significant swell noise due to bad weather. We have applied a low-cut filter up to 12 Hz to reduce this noise to an acceptable level. The other preprocessing steps were the mute of the direct wave and the √{t} amplitude scaling for the 3D-to-2D amplitude correction.

The Kirchhoff series requires that the input data contain near offsets, including the zero-offset. In the Troll dataset, the nearest offset is 37.5 m, with 18.75 m spacing between offsets. To fill up the two missing offsets (0 m and 18.75 m), we decided to duplicate the nearest offset, thus avoiding extrapolation for the missing near offsets.

As in the Barent Sea example, we use the Kirchhoff series and numerically predict the vertical component of the particle velocity. We have also assumed in our application of this series that pressure does not contain ghosts by treating any ghost effects as part of the source signature. The direct wave was also muted.

To analyze the results of the Kirchhoff series on the Troll data, we performed an NMO stack of data before and after the demultiple. This figure shows NMO-stacked sections before and after the demultiple. The only processing difference between the two seismic sections is the Kirchhoff demultiple process. We have used the first five terms of the Kirchhoff series.

The Troll area is fairly horizontally flat. So, with a careful velocity analysis we expect the NMO stack to reduce a significant amount of multiple energy by the differential moveout. Yet by comparing the sections before and after the demultiple, we can still see a significant improvement after the application of the Kirchhoff series.

We have highlighted three examples of primary-multiple interferences in the next two figures.

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.