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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.
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Factors
Affecting Amplitude Before Seismic
Factors
that affect amplitudes, before seismic 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 effect would need to be examined, as it could have a significant impact. The five factors that have a big effect on amplitudes during the acquisition of field data are shown in black lettering in Figures 1 and 2. The most important of these factors is the loss of energy due to curved-ray spherical divergence (F6). This effect on amplitudes is often approximated by the inverse square of distance, which for constant velocity is the inverse square of time. This factor is smaller for reflectors that are separated by less time, and minor for most lateral changes. For the real, non-constant velocity earth, this effect is greater (1/v2t) but still inadequate for recovering true amplitudes of deep reflectors. 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 noise level and disappear from the data. 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 effect can be major, but normally it does not exceed a factor of 1.5 as determined by the size of the side lobes. 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 effect can be large for some gas effects. The appearance of this offset-dependent variation will be much less apparent on the stacked trace that contains the summation of all offsets. Overall on the final stack, AVA effects are in the range of a factor of 2-5 compared to no AVA. 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 effect on amplitudes. Often, buildings, platforms, lakes, rivers, etc., must be avoided; stations are skipped; and traces will be missing from the stacking bin. This reduces the ability of stack to reduce random noise -- but the greater effect is a frequency unbalancing.
Factors Affecting Amplitudes Arising in
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
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
Table 2 summarizes the most important of
these amplitude-altering
In order
to assist in the understanding of workstation amplitudes, the
Figure 4 shows the wavelets from the top of
these thick sands as seen through successive stages of seismic
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 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 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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