A laminated solid

White and Angona computed various velocities in a laminated solid as a function of the proportion of two materials (A and B) making up horizontal layers.

This figure shows compressional velocities for horizontal travel (dashed curve) and vertical travel (solid curve), as a function of the fractional amount of the higher-speed component (material A); V_A is the P-wave velocity of material A, and V_B is the P-wave velocity of material B.

Early contributions to this topic go back to the 19th century. Laboratory and field experiments in the 1950s detected velocity anisotropy when vertically and horizontally traveling waves were found to have different velocities. However, for most of the 70 or so years of petroleum exploration in the 20th century, petroleum seismologists have ignored anisotropy in their models of the earth, and their theoretical and practical developments assumed that waves propagate equally fast in all directions. There are good reasons for this omission. Seismic data through the 20th century were essentially dominated by the P-wave, for which the anisotropic effect is often small, with directional velocity differences of only 3 to 5 percent. When compared to errors due to the assumptions which were included in our models through the 20th century, such as the plane-layer approximation imposed by normal moveout (NMO) and stack processing or the two-dimensional (2D) earth model approximation imposed by acquisition geometry and limited computational resources, 5 percent anisotropy effect is in this context negligible. However, with recent advances in seismic acquisition and processing that we discussed in previous chapters, the reasons for ignoring anisotropy are no longer valid. Moreover, some petroleum seismologists now believe that getting a grip on anisotropic behavior of rock formations can mean the difference between success and failure in reservoir evaluation and development. For instance, if an amplitude versus offset (AVO) study does not take into account the anisotropic behavior of a shale cap rock, the underlying gas-bearing sandstone may be overlooked because the predicted AVO curve (for an oil sand overlain by isotropic shale) would not fit the observed AVO response from the actual survey.

Other Images in Chapter 12

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.