Towed-streamer acquisition

An illustration of towed-streamer acquisition. The vessel tows an array of airguns and streamers of hydrophones behind the boat while traveling at a roughly constant speed (it takes about 15 seconds for a typical seismic boat to move 50 m).

Again this figure illustrates a towed-streamer acquisition, in which a ship is towing a set of cables containing receivers to record signals generated by seismic sources as the vessel maneuvers across potential petroleum reservoirs. These cables of receivers, generally called streamers, are towed at a depth of between 5 to 10 m below the sea surface.

A typical streamer today is 5,000 m to 10,000 m long. It carries several hundred sensors, known as hydrophones, which record pressure changes. In conventional acquisition, each seismic receiver is composed of 12 to 24 hydrophones which are summed before or after recording, depending on the processing objectives (we will discuss some of these processing objectives in Chapter 8). The spacing between receivers (i.e., the center of a group of hydrophones) is generally 12.5 m.

Typical acquisition vessels today can tow 12 to 16 streamers spaced 50 to 100 m apart. One of the major challenges with towed streamers is maintaining constant streamer spacing.\footnote{Present seismic data processing requires that the data be uniformly sampled in space. That is why the distance between streamers must be constant.} Currents, tides, and other forces can cause streamers to feather, or drift laterally, from programmed positions, and in extreme cases they get tangled. Tangled streamers have to be reeled back to the vessels and untangled manually, resulting in nonproductive time for a detailed discussion of these technological challenges and current solutions).

Other Images in Chapter 7

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.