P-to-P AVA

P-to-P AVA (amplitude variations with angles) response to two different models. For a typical seismic aperture, the two responses are almost identical.

Five fundamental issues are associated with solving inverse problems, especially those related to petroleum seismology: (i) uniqueness, or how to be sure that the model of the subsurface obtained from a given dataset is the only such model which can explain that dataset; (ii) instability, i.e., a ``small'' perturbation of data can lead to a ``large'' perturbation of the inverse problem solution; (iii) convergence when inverse problems are solved iteratively; (iv) uncertainties due to inaccuracies in the physical models which allow us to predict data for a given model parameter or due to the incompleteness of these physical models and uncertainties in the measurements; and (v) the cost of the forward-problem step in the inverse-problem solution. If this list may sound like an old mathematics class, just go through the examples that follow, and you will realize that these issues are not just academic; they are real petroleum exploration and production concerns.

To add some concreteness to the issues related to the inverse problem solutions that we have just raised, let us look at a couple of examples. This figure shows identical pre-critical seismic AVA (amplitude variations with angles) responses to two very different models before the 30-degree incident angle---thus the issue of uniqueness. Notice also that for angles beyond the critical angle (i.e., greater than a 30-degree incident angle), there are enough differences between the two AVA responses. These differences can be used to distinguish these two models. In other words, a substantial amount of nonuniqueness in petroleum seismology problems can be resolved just by improving our theory or eliminating some of our assumptions, and by improving our acquisition geometries to collect, for instance, long offsets so that data corresponding to angles of incidence greater than 30 degrees can be recorded.

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.