interpretation is fundamentally based on interpreting changes in
amplitude values that define the seismic trace are typically explained
using the convolutional model. This model states that trace amplitudes
have three controlling factors:
The reflection coefficient (RC) series (geology).
wavelet's interactions through convolution.
(velocity x density) contrasts at geologic boundaries will generally
have higher amplitudes on the seismic trace.
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.
of this paper 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. Part 2 will discuss the factors effecting amplitudes
in seismic processing and interpreter controls on the workstation
(loading, processing and display).
these factors have been considered, then the changes in amplitudes
can be more reliably related to changes in geology.
factors that affect amplitudes, before seismic processing, will
be presented. The magnitude of most of these can only be estimated,
and removing their effects to obtain absolutely true amplitudes
is impossible. Fortunately, relative changes in amplitude have been
shown to be adequate, and have been successfully applied for reducing
risk, such as direct hydrocarbon indicators, estimating lithofacies,
affecting amplitudes are illustrated here in figures 1
and 2. A checklist with brief comments
describing the factors, along with an estimated magnitude of the
effect is provided in table 1.
only the moderate and major effects are discussed below, 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.
factors that have a big effect on amplitudes during the acquisition
of field data are shown in black lettering on figures 1
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.
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.
cases (e.g. salt, volcanics) the amplitudes of underlying reflectors
can be reduced to below the noise level and disappear from the data.
(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 does not exceed a factor of 1.5 as determined
by the size of the side lobes.
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.
on the final stack, AVA effects are in the range of a factor of
2-5 compared to no AVA.
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
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. Of these 21 factors,
five are more important.
for curved ray spherical divergence is one of the primary goals
of seismic processing. The other four factors mentioned above remain
in the seismic data, as they are not normally corrected in seismic
processing -- which will be discussed next month.