Last month's article
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
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.
2 summarizes the most important of these amplitude-altering
processing steps. You can use this table in amplitude discussions
with your processor.
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.
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).
incidence reflection coefficients for the top of all of the sand
units are identical. Ideally, workstation amplitude for these sands
would also be identical.
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:
moveout (NMO) correction and stack.
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.
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
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
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.
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.
of these amplitudes is not yet completed, as we must load the data
onto the workstation for our interpretation.
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.
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.
are the basic input to seismic attribute analysis calculations.
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.
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.
amplitudes to geology on vertically separated reflectors, or in
areas of laterally changing geology, is risky -- and a reason for
many unsuccessful wells.