Geology Reference
In-Depth Information
Synthetic seismogram
Geological
section
Acoustic
impedance
Reflection
coefficient
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1
2
2
3
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4
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5
5
Fig. 4.52
The synthetic seismogram.
waveforms of seismic traces are filled in black. This has
the desirable effect of merging the shaded areas from
trace to trace to form continuous black lines across the
section.These black lines guide the eye of the interpreter
to correlate features across the section, and hence make a
structural interpretation. The undesirable effect of this
display is that the precise amplitude and shape of the
waveform, which has been the subject of so much effort
during data acquisition and processing, is lost. The
amplitude of a normally reflected wave is directly related
to the reflection coefficient at the interface, and hence
the physical properties (density and velocity) of the for-
mations. Thus, variations in amplitude along a reflector
should indicate changes in the properties of the
formations.
These properties can be viewed by presenting an
image of the seismic section where the amplitude of
the seismic wave is displayed as a colour scale. Changes of
amplitude along a continuous reflector will then be em-
phasized by the colour change, rather than hidden in
a broad black line. Such amplitude changes may be
related to changes in the pore fluid in the rocks, and in
favourable circumstances can be
direct hydrocarbon
indicators
(DHIs). Amplitude is merely the simplest ex-
ample of a property (
attribute
) of the seismic wave which
can be examined for its geological significance. Others
include the seismic wave phase and the frequency con-
tent. From the waveform amplitudes the acoustic
impedance of each formation can be estimated, and if
S-wave data are available Poisson's ratio can be found.
On a yet more detailed level, the amplitude variation of
reflected wavelets with source-receiver offset (AVO)
within each CMP gather can be analysed. This
AVO
effect
can be particularly diagnostic in distinguishing be-
tween amplitude effects due to rock matrix variation
and those due to pore fluids. An excellent review of this
complex subject is given in Castagna and Bachus (1993).
4.15 Single-channel marine
reflection profiling
Single-channel reflection profiling is a simple but highly
effective method of seismic surveying at sea that finds
wide use in a variety of offshore applications. It repre-
sents reflection surveying reduced to its bare essentials: a
marine seismic/acoustic source is towed behind a survey
vessel and triggered at a fixed firing rate, and signals re-
flected from the sea bed and from sub-bottom reflectors
are detected by a hydrophone streamer towed in the
vicinity of the source (Fig. 4.54). The outputs of the
individual hydrophone elements are summed and fed to
a single-channel amplifier/processor unit and thence to
a chart recorder.This survey procedure is not possible on
land because only at sea can the source and detectors
be moved forward continuously, and a sufficiently high
