f-k dip filtering

f-k dip filtering for a VC shot close to the cable. (a) Raw shot gather for a VC cable located at 362.5 m from the shot point. (b) The f-k spectrum of the raw shot gather. (c) and (d) Separation of the f-k spectrum of the raw shot gather into negative and positive wavenumber. (e) and (f) Separation of the raw shot gather into upgoing wavefield and downgoing wavefield.

Again, this figure shows an example of a vertical cable shot gather for one cable using a receiver spacing of 6.25 m. The model used to generate these data is shown in the previous figure. We can see that most of the events are linear and have opposite gradients, just as in VSP data. In the f-k spectrum, the events in the vertical cable data are arranged as a function of the gradients only. The downgoing wavefield is located in the positive wavenumbers, and the upgoing wavefield is located in the negative wavenumbers. As we can see in this figure, this separation is very clear. The up/down separation based on $f-k$ filtering consists of zeroing energy in the $f-k$ domain corresponding to the negative wavenumbers to extract the downgoing field and zeroing energy in the f-k domain corresponding to the positive wavenumbers to find the upgoing field, as illustrated in this figure.

One may expect that the f-k dip-filtering method is more difficult to apply for shot points located far away from the cable because the arrivals of seismic events is no longer linear function of receiver positions. It turns out that this is not actually the case. For a shot located at 1 km from the cable, except for the direct wave, all the rest of the events remain linear, with a positive gradient for downgoing waves and a negative gradient for upgoing waves. We can see that the up/down separation is quite effective, even in this case.

Note that this procedure is generally applied on shot gather domain, in which the dip separation of events is more effective. However, the effectiveness analysis of the up/down separation can also be carried out in the receiver gathers, as illustrated in the next figure. Although the downgoing wavefields contain only multiples, the up/down separation is not totally equivalent to multiple attenuation because the upgoing wavefield also includes also multiple events, as illustrated in the next figure.

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.