Minimum-phase signal

(a) The minimum-phase signal, (b) the mixed-phase signal, (c) the maximum-phase signal, and (d) the zero-phase signal.

It is a common practice in petroleum seismology to classify signals in rather broad terms according to their frequency content. We here present a classification of seismic signals as a function of their phase spectrum. A classification as a function of bandwidth will be introduced in the next chapter.

The term phase in seismic signals is often referred to as minimum phase, maximum phase, mixed phase, or zero phase. Each of these terms refers to the characteristics of the signal shape and starting location in the data. This figure shows four signals, each of a different phase. Each type of phase is described below:

(i) Minimum phase: The minimum-phase signal, shown in (a), is described as a front-loaded signal. This means that the energy in the signal is concentrated in the front of the pulse. The signal is not symmetrical. The phase of this signal will vary for each frequency component of the signal. Note that the convolution between two minimum-phase signals is always minimum phase. However, convolution between one minimum-phase and one nonminimum-phase signal does not produce a minimum phase signal.

For a group of signals with the same amplitude spectra, the minimum-phase signal will have the smallest phase shift at all frequencies, cause the least time delay, have the most front-loaded energy distribution, and have the largest time-zero sample value.

(ii) Mixed phase: The mixed-phase signal, shown in (b), is described as a signal with its energy concentrated in the center of the pulse. It can be divided into minimum-phase and maximum-phase signals. The signal is usually not symmetrical. The phase of this signal will vary for each frequency component of the signal.

(iii) Maximum phase: The maximum-phase signal, shown in Figure (c), is described as an end-loaded signal. This means that the energy in the signal is concentrated toward the end of the pulse. The signal is not symmetrical. The phase of this signal will vary for each frequency component of the signal. The characteristics of the maximum-phase signal are the opposite of the minimum-phase signal.

(iv) Zero phase: The zero-phase signal, shown in (d), is symmetrical and centered on zero time. The zero-phase signal has the shortest duration and largest peak amplitude of any

signal with the same amplitude spectrum. These characteristics make it the most desirable of all the signals because of its resolution capability. The phase of the zero-phase signal is zero for all frequency components contained within the signal.

Other Images in Chapter 4

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.