Principle of 4C-OBS acquisition

Principle of 4C-OBS acquisition. Both pressure and shear-wave data can be obtained by placing sensors (geophones and hydrophones) on the sea floor. P-P is a convention for pressure waves traveling down to a reflector, where they are reflected upward as pressure waves. P-S refers to pressure waves traveling downward to the reflector upon mode-conversion to upward-reflected shear waves.

The towed-streamer experiment records P-waves, but no S-waves are directly recorded, although the wavepath below the sea floor may include some S-wave paths. The S-waves are not directly recorded because the receivers are in seawater, and water, like all nonviscous fluids, supports only P-waves, not S-waves.

In a marine four-component (4C) ocean-bottom seismic (OBS) experiment, which is also known simply as a marine 4C experiment, the receivers are located at the sea floor. Every receiver station is a four-component sensing system: three components of the particle velocity field are recorded from a three-component geophone, and the pressure field is recorded from a hydrophone. One geophone component is oriented vertically, and two are oriented horizontally, perpendicular to each other. Although the sensing system is stationary on the seabed and usually wired to a recording vessel, a source vessel towing a marine source array shoots on a predetermined grid on the sea surface. One possible acquisition geometry is illustrated in here. It consists of two vessels, one recording vessel and one shooting vessel which tows one or more energy sources. When the shooting is complete, the sensing system is retrieved and redeployed in a nearby location. The shooting campaign continues.

OBS surveys are not limited to two-vessel operations. To speed up the data-acquisition process and thus reduce cost, three-vessel operations are also common. While the source boat is traversing the survey area and the second vessel is recording, the third vessel is involved in retrieving and moving sensor cables and setting up the next receiver lines ahead of the source vessel. This three-vessel configuration allows the source boat to run a nearly continuous operation. Another solution to reduce the cost of acquisition is to operate with one seismic vessel. This vessel then obviously will be a receiver deployment and shooting vessel. For every cable, data are recorded into recording buoys. When the sensors are not in cables, but in ocean-bottom node-type systems with sensors housed in units which are inserted into the sea floor by remotely operated vehicles, recording must take place locally, inside the sensor unit.

One of the major requirements in 4C-OBS experiments is that geophones be well coupled to the sea floor in order to record both high-quality P-waves and S-waves. Because shear waves do not travel through water, the geophones must be in direct contact with the seabed to capture the motion of the seabed and not the change of pressure in seawater. The process of ensuring direct contact between the geophones and the seabed or any other solid material is called coupling. The 4C-OBS experiment requires precise coupling of geophones with the seabed.

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.