Resampling steps

Resampling steps.

The principle for selecting a temporal sampling interval is exactly the same as that for selecting the spacing interval between receivers and/or the spacing interval between shot points. The usual temporal sampling interval used today is 2 ms. For a typical seismic record of 75 Hz, the sampling theorem requires that the temporal sampling interval be less than 6 ms to ensure that the recorded wavefield can be reconstructed at any time with maximum fidelity. With a 2-ms sampling interval, we are well within the requirement of the sampling theorem.

Moreover, the data at a 2-ms sampling interval can be resampled to the commonly used 4-ms sampling interval after an antialiasing filter has been applied, as illustrated in this figure. Note that by collecting data at a 2-ms sampling interval, we avoid possible distorting of the desired spectrum with noise which may contain frequencies higher than 125 Hz [i.e., 1/(2Δt), with Δt = 4 ms].

The situation is quite different regarding our usual selection of the spacing interval for receivers, for instance. One typical spacing between receivers used in seismic acquisition today is 25 m. Based on the sampling criterion, all seismic events with an absolute value of the apparent velocity of less than 2750 m/s (in the frequency ranges of 0 to 75 Hz) will be aliased; in other words, the recorded wavefield alone is not enough to properly reconstruct these events at any point in the space other than the point where the measurement is recorded. Seismic events with an absolute value of the apparent velocity of less than 2750 m/s include direct waves, groundroll, air waves, and even some desired reflection primaries. All these events will be aliased. As described above, the spacing interval between receivers must ideally be of the order of 1 m to avoid aliasing. With about 25-m spacing between receivers, a typical land-seismic acquisition requires picking up, putting down, and maintaining 150,000 geophones. Multiplying this number of geophones by a factor of 25 to achieve a 1-m spacing is not yet economically viable. On the other hand, we cannot rely on seismic processing to solve this problem, either. As we will see in the next figure, sometimes the aliased energy overlaps so much with the desired signal that f-k dip filtering becomes impractical. The solution to this aliasing problem, adopted by the oil and gas industry in the 1930s, when reflected seismic experiments were first used for petroleum exploration on land, is known as hard-wired array recording (sometimes called array recording).

Other Images in Chapter 8

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.