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:
-
The reflection coefficient (RC) series (geology).
- The
seismic wavelet.
- 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.
Part 1
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).
When all
these factors have been considered, then the changes in amplitudes
can be more reliably related to changes in geology.
Over 20
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,
etc.
The factors
affecting amplitudes are illustrated 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.
Although
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.
The five
factors that have a big effect on amplitudes during the acquisition
of field data are shown in black lettering on 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 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.
This paper
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.
Compensation
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.
Table 1.
This table provides a checklist of factors that affect amplitudes. Factors
in bold are more important.
Seismic Acquisition (Source)
|
Factors
|
Comments
|
Magnitude
|
| F1) Source Strength
|
Size of dynamite, number of working vibrators, number of sweeps,
etc.
|
Moderate
|
| F2) Source Coupling
|
Source in dry sand, weathering (poor coupling), bedrock, wet soil
(good)
|
Moderate
|
| F3) Source Arrays
|
Designed to attenuate noise but also attenuates dipping primaries
|
Minor
|
| F4) Source Ghost
|
Reflected signature with opposite sign at free surface
|
Minor
|
The Earth
|
Factors
|
Comments
|
Magnitude
|
| F5) Discontinuous Trans
|
When RC large enough (Volcanics, Salt) underlying events not visible
|
Mod-Major
|
| F6) Curved Ray
|
Spherical spreading of energy is the MAJOR effect, factor
of 10 or more
|
MAJOR
|
| F7) Tuning
|
Tuning can be major, up to factor of 2, at pinchouts
can be down to zero
|
Mod-Major
|
| F8) AVA
|
AVA gas effects can be up to a factor of 5
|
Mod-Major
|
| F9) Curved Reflectors
|
Focusing and defocused - minor, exceptions include salt lens -
mod-major
|
Minor
|
| F10) Rugosity
|
At seismic wavelengths most geologic surfaces are "mirror"
smooth
|
Minor
|
| F11) Interbed Multiples
|
If RC contrasts are high then can be moderate problem, generally
minor
|
Minor
|
| F12) Absorption
|
Loss of energy to heat, weighted towards high frequencies
|
Minor
|
| F13) Scattering
|
Loss of energy due to specular reflections, weighted towards high
frequency
|
Minor
|
Seismic Acquisition (Receiver)
|
Factors
|
Comments
|
Magnitude
|
| F14) Receiver Coupling
|
Geophones dampened on dry soil, buried or wet soil couples well
|
Moderate
|
| F15) Surface Multiples
|
Negative RC at surface, then positive at base weathering
|
Minor
|
| F16) Receiver Ghost
|
Reflected RC with opposite sign at free surface, changing surface
RC
|
Minor
|
| F17) Receiver Strength
|
Poorly placed geophones, partial loss of array
|
Minor
|
| F18) Receiver Arrays
|
Designed to attenuate noise but will also attenuate dipping primaries
|
Minor
|
| F19) Geophone
|
Response is a filter that reduces amplitudes
|
Minor
|
| F20) Dynamic Range
|
Pre-1990 with limited 12-15 bit recorders / filters Moderate, now
Minor
|
Minor
|
| F21) S&R "Skips"
|
Missing ground positions / offsets, effects # traces and frequency
content
|
Mod-Major
|