Rays in a complex laterally heterogeneous medium

Rays in a complex laterally heterogeneous medium. (a) The background model without the rays and (b) the background model overlaid by the rays.

The imaging algorithms which utilize the rms velocity as the background velocity model are known as time imaging because the velocity model is used only to recover the geometry through moveout correction and stack without changing the time position. An example of time migration. Strictly, these imaging algorithms must be limited only to cases in which the background is depth-dependent, because the concept of rms velocity is not applicable for laterally heterogeneous media. In fact, in laterally heterogeneous media, each ray connecting the source to the image point leads to a different rms velocity, and similarly, each ray connecting a receiver to the image point can yield a different rms velocity. Thus the equivalent rms velocity for laterally heterogeneous background media can be even more complex than the actual velocity model. Moreover, the idea of approximating bending rays by straight rays can easily become unworkable, as the example in this figure illustrates. Except in cases like the one illustrated here, time imaging can lead to wrong geometries of reflectors in addition to a wrong depth. So when interpreting time-imaged data, it is important to examine the velocity model used in the imaging process.

So if time imaging is likely to lead to a wrong image and to a wrong position, why is it being used at all in the industry? The main attraction of time imaging is that it is computationally very effective compared to depth imaging, which we will discuss next. However, with recent technological advances in computing speed, data storage, and software ``intelligence,'' the usefulness of time imaging for oil exploring will continue to diminish. However, for educational purposes, it will still be very useful because the derivations of time imaging are quite similar to those of depth imaging, with the advantage that the derivations of time imaging are analytic, thus providing more insight into imaging problems than insights provided by depth imaging.

Other Images in Chapter 11

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.