Seismic data

Examples of primaries, receiver ghosts (receiver-side reverberations), free-surface multiples (source-side reverberations), and internal multiples in OBS data. These events can be decomposed into (a) upgoing and downgoing events (U/P), or (b) into P-wave and S-wave arrivals (P/S).

Seismic data recorded on geophones deployed on land, on the sea floor, or in boreholes contain upgoing and downgoing P- and S-wave arrivals. Some events are primaries, whereas other events are multiples. Most current imaging tools in seismic data processing require that multiples or at least downgoing components of the recorded data must be attenuated. The imaging tools also require that P-wave and S-wave arrivals be separated and imaged separately. To fulfill these requirements, petroleum seismologists have developed various techniques for splitting recorded seismic data into upgoing and downgoing P- and S-wave arrivals, into total upgoing and downgoing wavefields, and into primaries and multiples.

Wave-equation-based wavefield decomposition plays an important role in the preprocessing of seismic data. Wavefield decomposition by means of the wave equation can be roughly divided into two categories. In the first are methods that separate the elastic wavefield into upgoing and downgoing P- and S-waves (for short, denoted by P/S decomposition), or separate the wavefield into total upgoing and downgoing waves (for short, denoted by U/D decomposition). In the second are methods that attempt to eliminate free-surface related multiples (and possibly internal multiples). Whereas P/S and U/D decomposition typically act on one shot gather at a time, full surface-related multiple removal requires a considerably larger data volume. The two categories of preprocessing can be run independently of each other.

Other Images in Chapter 9

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.