Velocity semblance plot

Velocity semblance plot. Velocity analysis of synthetic data corresponding to a model of the subsurface with one high-velocity basalt layer. The model is superimposed on the CMP gather. Notice the significant misfit between theoretically computed RMS velocities (black dots superimposed on a white curve) and the semblance peaks (red). This misfit is due to the presence of the basalt layer.

There are various scenarios in which picking the correct velocity in the semblance can be a daunting task. Here, corresponding to a simple model of the subsurface, shows one such example. This model includes one high-velocity basalt layer. In contrast to the previous figure, there is now a large misfit between the theoretically computed rms velocity and the red peaks of the semblance plot at around 3s. This misfit is due to the fact that traveltime variations are now nonhyperbolic, whereas our velocity spectra are computed-based on NMO-plus-stack, which assumes that traveltime variations with offsets follow a hyperbolic moveout. In other words, this misfit is due to the limitations of our imaging tools and the limitations of the ``focusing'' and ``defocusing'' idea. In the next subsections we discuss an extension of this ``focusing'' and ``defocusing'' idea for sophisticated imaging tools.

The semblance plots in this figure also show high-amplitude events arriving at greater traveltimes than the semblance plot in the previous figure. These are clearly not primary, but various multiples and converted waves. This observation illustrates the difficulty of distinguishing real primaries from multiples generated above the basalt.

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.