Reconstructing the background velocity

(a) CMP shot gather containing a single event with a moveout velocity of 2264 m/s. (b) NMO-correlated gather using the appropriate moveout velocity. (c) Overcorrection because too low a velocity (2000 m/s) was used. (d) Undercorrection because too high a velocity (2500 m/s) was used.

The imaging of the reflector associated with these events is quite sensitive to velocity. We need to correct for the traveltime variations with offsets (moveout) and perform an ``intelligent'' stack. As illustrated in the next figure, with a dataset containing three events, the correct background velocity corresponds to the maximum amplitude moveout correction and stack. This idea essentially amounts to ``focusing'' the seismic traces so that a large response is obtained. When the traces are properly lined up (i.e., properly moveout corrected), then the sum of traces will be maximized. This idea is similar to the focusing actions of a lens or a parabolic reflector for plane waves.

So the basic idea for reconstructing the background velocity is to image our data with various velocity models and to select the model which produces focused images of the subsurface. The two basic components of this approach to estimating the background are (i) the tool used for imaging the data and (ii) the criteria for determining the best velocity model. In the examples in this figure and in the next figure, due to the simplicity of the problem, the imaging tool was NMO-plus-stack, and the criterion for selecting the correct velocity was the amplitude of stack results. Another quantity used is semblance.

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.