Snapshots of wave propagation

Snapshots of wave propagation through (a) a heterogeneous material and (b) a homogeneous material. The source is an explosive one which generates only P-waves, and the quantity recorded here is the divergence of the particle velocity. Note that the direct-pulse P-wave remains fairly coherent, despite the fact that significant energy is being shifted from the direct pulse back to the coda.

Scatterings also decrease the amplitude of the first-arriving (direct arrivals) seismic pulse by shifting energy from the direct arrival back into the coda. This apparent attenuation is called scattering attenuation. Unlike the intrinsic attenuation, which is related to anelastic processes we will discuss later, the scattering attenuation is not a measure of energy loss but a measure of energy redistribution, just as the geometrical spreading losses described in Chapters 2 and 7. The scattering attenuation can depend very strongly on frequency and is very path-dependent, because it depends on the particular power spectrum of the distribution of heterogeneity encountered by a wavefield propagating through the earth. It is usually modeled by using random media. (a) shows snapshots of a wavefield at different times as it propagates through material that has a 10-percent variance for its distribution of velocity heterogeneities. By comparing the response of wave propagation through a random medium in (a) and that of wave propagation through a homogeneous medium in (b), we can notice the amplitude of the direct pulse in the case in which the response to the random medium decays more rapidly than the direct pulse in the homogeneous medium case. The amplitude decay of the direct pulse due to scattering is characterized as apparent attenuation. Another illustration of this apparent attenuation is shown in the next figure. Note that the direct P wave remains fairly coherent, but a complex suite of later arrivals is generated by the heterogeneity. In Section 8 of this chapter, we discuss in more detail the various mechanical models used in seismology to describe scattering and intrinsic attenuations.

Other Images in Chapter 12

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.