Migration velocity

Results of prestack migration using a homogeneous background velocity: (a) V=2050 m/s, (b) V=2250 m/s, (c) V = 2450 m/s, (d) V = 2650 m/s, (e) V = 2850 m/s, (f) V=3050 m/s, and (g) V = 3350 m/s.

The method of estimating rms velocity described in Figures 11-49 and 11-49 is based on NMO-plus-stack. This method makes sense only if the reflectors are horizontally flat and if traveltime variations with offsets follow a hyperbolic moveout. For large dips, the error in the velocity model can be large.

An alternative solution is to replace NMO-plus-stack with a prestack time migration algorithm. Many constant-velocity migrations are performed, for a number of velocities between V_min and V_max, with a step of ΔV. In the example displayed here, we take V_min = 2050 m/s and V_max = 3350 m/s. ΔV is taken to be equal to 50 m/s. Due to limited space, migration results in this figure are shown only every 200 m/s. Yet we can clearly see the events coming in focusing as we reach the correct velocity and then defocusing as soon as we move away from the correct velocity. For instance, events A and B are best migrated with a 2050 m/s velocity. However, for the event C is best migrated with a 3050 m/s velocity. This example confirms that any imaging algorithms can be used for velocity estimation.

The velocity estimation based on prestack time migration is known as velocity-migration analysis.

Note that the final migration image can be formed from scans in this figure by merging parts of each constant-velocity migration so that every part of the final image section has the right effective velocity.

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.