Snapshots

Snapshots of wave propagation. The jet colorscale displayed here will be used to displayed snapshots throughout this chapter.

Let us drop a stone into a tank containing water. A disturbance of a very short duration occurs at the point of impact. While the deformed area returns to its initial equilibrium, the disturbance gradually expands away from the point of impact. This figure shows three snapshots of this phenomenon, known as wave propagation. The disturbance varies with time and position in space. The speed at which the wave moves from one point to another depends on the physical properties of the medium. Energy decay from similar points also depends on the physical properties of the medium as well as the type of wave (body wave, surface wave, etc.) propagating in the medium. These properties and others that we will introduce later allow us to use wave propagation to probe media, even those as complex as the subsurface.

Unfortunately, we cannot see or directly analyze wave propagation in the subsurface because we are dealing with a dark and compact medium. However, we can put sensors at certain locations at the surface and/or inside the earth, through boreholes, to record the evolution of a disturbance with time.

This narrow view of a disturbance is similar to observing the New York marathon from a specific roadside location instead of from an aerial view in a plane. Although we cannot view all the athletes everywhere at the same time, we will have the opportunity to see and judge the speed of each of them. If more people are positioned at different locations on the route, an even more accurate picture of the race can be formed.

Other Images in Chapter 2

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.