Reverberations and multiples
In this chapter, we shall discuss multiple attenuation techniques based on velocity discrimination between multiples and primaries, and periodicity of multiples. While these techniques seem to have a good conceptual basis, their performance on field data can be disappointing. There are several possible explanations for this.
First, for velocity discrimination techniques to be effective, significant moveout differences must exist between primaries and multiples. However, the inability to exploit the large moveout differences between primaries and multiples in the mute zone works against the methods based on velocity discrimination.
Periodicity of multiples is preserved for the ideal case of zero offset and horizontally layered earth. At nonzero offsets, periodicity often is destroyed even for the horizontally layered case. Nevertheless, periodicity is better preserved for the horizontally layered earth in the slant-stack domain. Multiple attenuation in this domain is discussed in the slant-stack transform.
There is also a problem caused by the application of geometric spreading correction (gain applications), when it is applied using the primary velocity function. This type of correction usually results in enhancement of the amplitudes of multiple reflections. The slant-stack approach (the slant-stack transform) is implemented before the geometric spreading correction, so there is no danger of amplifying the multiple energy.
In Sections F.4 and F.5, we shall review the theory of multiple attenuation using wave extrapolation techniques. These are targeted for a specific class of multiples — those which are either associated with the free surface or water bottom.
We now examine the various types of multiples in recorded marine data in different domains — shot records, common-offset sections and CMP gathers. Most multiple reflections arise from an interface with a strong impedance contrast such as the free surface and water bottom. Figure 6.0-18 shows raypath diagrams for:
- water-bottom multiples of first- and second-order,
- free-surface multiples of first- and second-order,
- peg-leg multiples of first- and second-order,
- intrabed multiples of first- and second-order, and
- interbed multiples of first- and second-order.
These are but a few of the numerous configurations of raypaths associated with multiple reflections encountered in recorded data. Regardless of the type of multiples, they all have two common properties that can be exploited to attenuate them with varying degree of success — periodicity and moveout that is different from primaries.
Figure 6.0-19 shows selected marine shot records which exhibit a broad range of multiples. The shot records over the deep water contain long-period water-bottom multiples and peg-leg multiples associated with reflectors just below the water bottom. Whereas the shot records over the shallow water contain short-period multiples and reverberations. Note the guided waves in the shallow-water records that also contain multiples which have raypaths within the water layer.
Figure 6.0-20 and 6.0-21 show selected CMP gathers and segments of near-offset sections associated with the data as in Figure 6.0-19. The near-offset sections have been moveout-corrected to zero offset — thus the small differences in the arrival times between those on the near-offset trace in the CMP gathers and traces in these moveout-corrected near-offset sections. Observe the existence of a broad range of multiple types in these pairs of CMP gathers and near-offset sections. The velocity spectra computed from the CMP gathers in Figures 6.0-20 and 6.0-21 are displayed in Figure 6.0-22.
- Coherent linear noise
- Treatment of coherent linear noise by conventional processing
- Treatment of reverberations and multiples by conventional processing
- Spatially random noise