A "simple" sub-basalt model
Chapter 1 - Introduction to Petroleum SeismologyA very simplified sub-basalt model overlain by a sketch of a raypath which describes the wave propagation from the source to a sub-basalt rock and from this sub-basalt rock to the receiver. Notice that the angle of reflection at the sub-basalt rock is quite small.
The large variations in seismic properties and structure of basalts lead to a wide range of seismic imaging difficulties. Consider, for instance, the "simple" situation of a high-velocity homogeneous and isotropic basalt. Primaries reflected from sub-basalt sedimentary layers will have experienced the transmission effect from four large acoustic impedance contrasts (two on their way down and two on their way up), giving an unusually weak rendering of the primaries. In addition, high acoustic impedance contrast interfaces lead to strong multiple energy, tending to arrive at the same time as sub-basalt primaries, thus limiting the imaging resolution of potential hydrocarbon reserves below basalt. The key challenge of processing data in basaltic rock areas is very likely the poor signal-to-noise ratio of the acquired data.
Other Images in Chapter 1
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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.
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