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Amplitude Versus Offset (AVO) The reflection seismology method used in the oil exploration industry produces images of the subsurface geology using acoustic energy. Pressure waves are emitted into the earth by a source at the surface and energy is reflected back up from geological layers to be detected by receivers at the surface. Signals are recorded over a range of source and receiver offsets. The detected signals are processed to form a geological image. The reflectivity of each geological interface depends on the acoustic properties of the layers above and below. The reflectivity can also vary significantly with reflection angle or offset. This reflectivity variation, called amplitude versus offset, or AVO, can be used to make inferences about the properties of the rock layers and the fluids contained within the pore spaces. In particular AVO variations can be used to indicate the presence of gas in a structure. An acoustic wave encountering an interface will divide into four components: Usually most of the energy is transmitted as a P wave through the layer with a small percentage reflected as a P wave and even smaller percentages transmitted and reflected as S waves. (When P (or S) wave energy is converted into S (or P) wave energy at an interface, it is termed "mode conversion".) Ideally, the sum of the energy in the reflected and transmitted waves will equal the energy of the incident wave. The signal recorded at the surface usually consists predominantly of reflected P wave energy. The reflection energy from a particular point on a layer can be observed over a range of offsets on a common depth point (CDP) gather.
The AVO Tools Modelling System The AVO Tools modeling system shows the amplitudes of the reflected and transmitted waves over a range of incidence angles for a specified interface. Input parameters include: Curves can be plotted for: Amplitudes can be calculated for:
The Main Menu form (Figure 1) is used to set some of the modeling parameters and access the application modules. Note: the algorithm used to calculate the
amplitudes gives the magnitude only. In some cases, the wave polarity reverses.
If anyone has an algorithm that produces negative numbers for reversed polarity,
please let me know.
The Layer Properties form (Figure 2) is activated by selecting Edit \ Layer Properties from the Main Menu. The acoustic behaviour of an interface is controlled by the properties of the top and bottom layers. The following properties can be defined for
each layer: The reflection and transmission curves can be automatically updated after each parameter change. The curves can be cleared after each change so that only the current curves are shown.
Figure 3 shows the AVO curve and raypath angle plots for a particular set of layer properties. The slider control on the raypath angle form is used to change the angle of the incidence P wave represented by the black line. As this is moved, the angles of the corresponding reflected and transmitted P and S waves are also shown. The vertical black line on the amplitude curve form shows the corresponding angle selected.
The numerical values of the calculated reflected and transmitted P and S waves can be displayed in a grid view (Figure 4).
Installation Instructions: Please Note: Download AVO Tools.ZIP (4,282,834 bytes)
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