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GCUnderstanding Seismic Amplitudes*
By
Steve Henry1
Search and Discovery Article #40135 (2004)
*Adapted from the Geophysical Corner columns in AAPG Explorer, July and August, 2004, entitled, respectively, “Understanding Seismic Amplitudes” and “More Amplitude Understanding” and prepared by the author. Appreciation is expressed to the author, to Alistar R. Brown, editor of Geophysical Corner, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1GeoLearn, Houston, Texas ([email protected])
Seismic interpretation is fundamentally based on interpreting changes in amplitude. The changing amplitude values that define the seismic trace are typically explained using the convolutional model. This model states that trace amplitudes have three controlling factors:
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The reflection coefficient (RC) series (geology).
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The seismic wavelet.
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The wavelet's interactions through convolution.
Large impedance (velocity x density) contrasts at geologic boundaries will generally have higher amplitudes on the seismic trace. Interpreters associate changes in seismic amplitudes with changes in the geology; this is a good assumption only if all of the factors that affect trace amplitudes have been considered.
This article is intended to provide the interpreter with a checklist of the factors that should be considered when associating amplitude changes on the seismic trace with changes in geology. First, it presents the major effects that interpreters need to understand about seismic acquisition, where the wavelet is generated and the field trace recorded, and the interaction of the wavelet with the geology. Of 21-listed factors that affect seismic amplitudes through seismic acquisition and the earth, five are most important. One of the primary goals of seismic processing is to compensate for curved ray spherical divergence, which is one of the five factors. The other four factors before processing remain in the seismic data, as they are not normally corrected in seismic processing.
Second, this article discusses the factors affecting amplitudes in seismic processing and interpreter controls on the workstation (loading, processing, and display). When all these factors have been considered, then the changes in amplitudes can be more reliably related to changes in geology.
uFactors in processing & interpretation
uFactors in processing & interpretation
uFactors in processing & interpretation
uFactors in processing & interpretation
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Factors Affecting Amplitude Before Seismic Processing
Factors
that affect amplitudes, before seismic processing, are illustrated in
Figures 1 and 2.
A checklist of 21 factors, along with brief comments describing the
factors and an estimated magnitude of the
Although
only the moderate and major effects are discussed here, it is important
to keep in mind how the amplitudes are being used in the interpretation.
If a well is being proposed based solely on an amplitude anomaly, even a
minor to moderate
The five
factors that have a big
Laterally
discontinuous (F5) high impedance geologic features can greatly
reduce the amount of energy transmitted to the underlying geology. This
reduces the amplitude of otherwise high amplitude reflectors beneath and
over a lateral distance of half a spread length off the sides of the
anomaly. In extreme cases (e.g. salt, volcanics), the amplitudes of
underlying reflectors can be reduced to below the
Tuning (F7)
occurs when the separation between RC creates constructive or
destructive interference of the wavelet's center and side lobes. This
interference can increase or decrease amplitudes and is most evident in
areas of geologic thinning such as angular unconformities or
stratigraphic pinch-outs. The magnitude of this
Amplitude
variations with angle (AVA or AVO) relate relative amplitude changes (F8)
in pre-stacked data to combined rock and pore-space fluid properties.
This
The
placement of sources and receivers on the surface of the earth is not
always uniform, resulting in missing ground positions (F21) that
can have a moderate to major
Factors Affecting Amplitudes Arising in Processing and Interpretation In addition to the 21 factors that could affect seismic amplitudes through seismic acquisition and the earth, 14 other additional factors, listed here (Table 2), arise in seismic processing and interpretation. Again, only the major factors are 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. In order to assist in the understanding of workstation amplitudes, the processing of a pair of seismic traces is reviewed. 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 3). 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 4 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:
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
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 3, 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 alter amplitudes further (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. Factors have been described that affect seismic amplitudes through seismic acquisition, the earth, seismic processing, and seismic interpretation. Seismic interpretation contains most of the major amplitude factors (Table 2), 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.
Sheriff, R. E., 1975, Factors affecting seismic amplitudes: Geophysical Prospecting, v. 23, p. 125-138.
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