The three basic measurements of strain

Illustration of (a) linear strain, (b) volumetric strain, and (c) shear strain.

In petroleum seismology, the deformability of materials is an important characteristic of wave propagation, because it is needed to properly parametrize materials. Unfortunately, particle displacement is not a sufficient measurement of deformation, especially for particles inside a material. The classic example used to illustrate this point is a stone being kicked around; the stone will rotate and move. Hence the displacement is nonzero, and all particles of the stone have maintained their relative positions during the rotations. In this case, the displacement can be an indicator of the stone's mass but not its deformability. Strains, which are a set of relative displacements, are a more appropriate means of describing deformability (i.e., change of shape). We will see that strain, as a tensor, is defined in terms of the spatial derivatives of the components of the displacement field.

Except in the immediate vicinity where seismic waves are generated, the shape changes of rock formations are generally very small, about 0.001 percent of length changes and about 0.01 radian rotation. Hence we do not discuss in this book the complications related to large strains, we limit ourselves in to infinitesimal strain (i.e., small strain).

The three basic measurements of strain are length change (or longitudinal strain), volume change (or volumetric strain), and angular change (or shear strain).

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.