Raypaths

Diagram illustrating symbols used in the derivation of the traveltime equation for a reflected ray.

This figure illustrates raypaths for forwards and reverses over a vertical step and resulting traveltime curves. The area of interest is a region surrounding the vertical step. We are essentially dealing with two half-spaces at significant distances from the step in either direction. First, we concentrate on the forward traverse. Early arrivals consist of direct and head waves in a classic pattern, producing a two-segment traveltime curve. The last raypath to follow the normal state of affairs is QF. The position of F on the surface is controlled by both z₁ and the critical angle; however, it is extremely unlikely that a receiver will be located at this exact spot. The last receiver to record head-wave energy from the shallower portion of the interface is immediately to the right of F.

Some wave energy follows the path BC, encounters the bottom of the step, produces diffractions, and travels along CD. Then energy traveling along the deeper portion of the interface generates waves following such paths as CG and DN, which also will return at the critical angle. Once again, the position of G on the surface is controlled by the critical angle and z₁ + h. At some point to the left of G, this energy arrives first. These arrival times plot along a straight line with a slope of 1/V_P2. Note that this line must be displaced later in time because raypaths such as DN are longer than ones such as QF, which has an intercept time of t_i2, whereas DN has an intercept time of t_i1.

Other Images in Chapter 3

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.