More Amplitude Understanding — Part II

See Part I in July 2004 Explorer

Last month's article on "Understanding Seismic Amplitudes" listed 21 factors that could affect seismic amplitudes through seismic acquisition and the earth. This article presents 14 additional factors that arise in seismic processing and interpretation.

Again, only the major factors will be discussed; unfortunately, any of the factors could be important in interpreting amplitude changes as changes in geology.

It is good practice for interpreters to inform seismic processors (good processors appreciate this) how they will use the data in interpretation (i.e. structural, stratigraphic, AVA, etc.). Processors will appreciate your insights, because they will be using tens of processing programs containing hundreds of parameters. Many of the programs and parameters alter seismic amplitudes.

Table 2 summarizes the most important of these amplitude-altering processing steps. You can use this table in amplitude discussions with your processor.

This article, to assist understanding of workstation amplitudes, reviews the processing of a pair of seismic traces. Both the wavelet shape and amplitude from the top of high impedance sands are followed from the raw field traces to their final processed form.

Image Caption

Figure 1.
Illustrative earth model. Reflection coefficients for sands are identical. Red dots are where the amplitudes have been calculabed in figure 2. Blue dot is position of amplitude for dipping Sand 3 before seismic migration moved amplitude 1100 m laterally and 275 m (200 ms) vertically updip (Vavg-2860 m/s).

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Last month's article on "Understanding Seismic Amplitudes" listed 21 factors that could affect seismic amplitudes through seismic acquisition and the earth. This article presents 14 additional factors that arise in seismic processing and interpretation.

Again, only the major factors will be discussed; unfortunately, any of the factors could be important in interpreting amplitude changes as changes in geology.

It is good practice for interpreters to inform seismic processors (good processors appreciate this) how they will use the data in interpretation (i.e. structural, stratigraphic, AVA, etc.). Processors will appreciate your insights, because they will be using tens of processing programs containing hundreds of parameters. Many of the programs and parameters alter seismic amplitudes.

Table 2 summarizes the most important of these amplitude-altering processing steps. You can use this table in amplitude discussions with your processor.

This article, to assist understanding of workstation amplitudes, reviews the processing of a pair of seismic traces. Both the wavelet shape and amplitude from the top of high impedance sands are followed from the raw field traces to their final processed form.

To begin this journey, consider a simplified earth in which thick (200m) sands reside in a shale-dominated section (Figure 1). Two of the sand units are flat (sands 1 and 2) and the third is dipping at 15 degrees (sand 3).

The normal incidence reflection coefficients for the top of all of the sand units are identical. Ideally, workstation amplitude for these sands would also be identical.

Figure 2 shows the wavelets from the top of these thick sands as seen through successive stages of seismic processing. Reading from left to right, first observe the zero-offset, raw field trace before processing. Then notice the changes in the seismic amplitudes due to an idealized processing sequence of:

  • Curved-ray, spherical-divergence correction.
  • Minimum-phase deconvolution.
  • Normal moveout (NMO) correction and stack.
  • Migration.

After-migration amplitude values (shown in red) should ideally be identical for all the top sand reflectors. The only amplitudes that are identical are the two from the flat sand 1. The deeper flat sand visible on Trace 1 should have the same amplitude value as the shallower reflector; unfortunately, spherical-divergence correction programs (F23) typically under-correct deep amplitudes.

This correction only accounted for one of the many amplitude-altering factors encountered by the wavelet during its round-trip between the surface and the top of sand 2. The observed amplitude at sand 2 also is a function of the phase and frequency content of the wavelet (F24), which is different from sand 1 due to attenuation and maybe assumptions in deconvolution.

On the migrated version of Trace 2 we view the dipping-reflector from sand 3 at the same time as sand 2 on Trace 1. The dipping sand has lower amplitude than the flat sand, due to NMO (F26) combined with stack (F29).

In our simple model, we assume that a flat-reflector value was used for the NMO correction. Thus, NMO is correct only for flat reflectors so that the stack process attenuates the amplitude of dipping reflectors, due to uncorrected dip contamination of the NMO velocities. In addition, dipping reflectors are displaced from their apparent location on the stacked section. Thus, they must be migrated (F32) to their proper subsurface positions.

As shown in Figure 1, the amplitude that will be displayed on the workstation for Trace 2 at 3.0 sec. was actually from a point 1100 m laterally and 275 m vertically down dip.

The journey of these amplitudes is not yet completed, as we must load the data onto the workstation for our interpretation.

In the loading process, a percentage of the largest peaks and troughs may be clipped (squared off) to improve the visual dynamic range (F34). Only the largest amplitudes are affected, but these are often of the greatest interest as possible direct hydrocarbon indicators.

With the seismic data now loaded, interpreters have many opportunities to further alter amplitudes (F35). For example, 2-D line balancing programs change gains, timing, frequency and phase. In addition, user-applied settings for filtering, phase rotation and even the selection of color bars change how amplitudes are perceived.

Amplitudes are the basic input to seismic attribute analysis calculations.

Taken as a whole, these two articles contain descriptions of the factors that affect seismic amplitudes through seismic acquisition, the earth, seismic processing and seismic interpretation. Seismic interpretation contains most of the major amplitude factors (Table 1), and the interpreter controls these based on knowledge (F36). By not understanding the factors that affect amplitudes, drilling decisions can be in error.

On the other hand, relative amplitudes provided to interpreters are, with care, being successfully used for reducing risk and discovering hydrocarbons. You can improve your amplitude-based interpretations by considering the factors described. Your interpretation is on the firmest foundation by comparing amplitudes that are at approximately the same two-way time and have similar overlying geologic sections.

Relating amplitudes to geology on vertically separated reflectors, or in areas of laterally changing geology, is risky — and a reason for many unsuccessful wells.