gas. Random noise includes natural noise (e.g. wind
and wave motion), incoherent seismic interface and
imperfect static corrections. Without a sound understanding of these factors as well as knowledge of the
limitation of seismic resolution, there is a danger of
misinterpreting noise as real features.
In terms of the parameters that are analysed and the
interpretation that may be drawn from the analyses, a
fourfold hierarchy of seismic interpretation can be
achieved. These are seismic facies analysis, seismic
structural analysis, seismic attribute analysis and seismic sequence analysis. The most important parameters
used for interpretation of seismic data are:
Reflection amplitudes: The strength of the reflections.
As we discussed above, the proportion of the
energy reflected at the boundary between two
beds is a function of the difference in the acoustic
impedances. If we have an alternating series of
different beds, the distance between the bed
boundaries in relation to the wavelength of the
transmitted seismic signals will play a major part.
Reflector spacing: The distance between the reflectors
will indicate the thickness of the bed, but there will
be a lower limit to the thickness that can be
detected, which will depend on the wavelength.
Interval velocity: The interval velocity of a sequence
can provide information about lithology and porosity but this will depend on the stacking velocity and
will not be very accurate.
Reflector continuity: The continuity of reflectors will
be a function of how continuous the sediment beds
are, information which is essential for
reconstructing the environment.
Reflector configuration: If we take the compaction
effect into account, the shape of the reflecting
beds gives us a picture of the sedimentation surface
as it was during deposition. The slope of the
reflectors, for example, represents the slope of
prograding beds in a delta sequence with later differential compaction and tilting superimposed. Erosion boundaries with unconformities will in the
same way show the palaeo-topography during
erosion.
Instantaneous phase: A seismic trace can be considered an analytical signal where the real part is the
recorded seismic signal itself. Mathematically we
can compute the complex seismic trace (imaginary
parts of the signal) and the instantaneous attributes.
The Instantaneous phase is a measure of the
continuity of the events on a seismic section. The
Instantaneous phase is on a scale of +180
to
À180
. The temporal rate of change of the instantaneous phase is the instantaneous frequency.
17.7.1 Seismic Facies Analysis
A seismic profile provides information about the
properties of sedimentary rocks. Seismic facies analysis is the description and geological interpretation of
seismic reflectors representing sequence boundaries. It
includes the analysis of such parameters as the configuration, continuity, amplitude, phase, frequency and
interval velocity. These variables give an indication of
the lithology and sedimentary environment of the
facies (Selley 1998) (Fig. 17.29). Because seismic
reflections mainly represent time lines, i.e. sedimentary beds which were deposited contemporaneously, it
is also possible to a certain extent to interpret the
depositional environment. Large-scale sedimentary
features that may be recognised by the configuration
of seismic reflectors include prograding deltas, carbonate shelf margins and submarine fans (Sherif
1976).
17.7.2 Structural Analysis
Interpreters can draw horizons and faults on in-lines,
cross-lines and arbitrary lines, as well as slices.
Horizons can be automatically tracked on vertical
seismic displays and horizontal slice displays.
Improved tracking algorithms for horizon interpretation, combined with user-interpreted faults and fault
polygons, can produce seismic-based interpretation
maps (Fig. 17.30). Faults on seismic sections are typically expressed by a loss of reflection amplitude. It
must be remembered that the principle we use for
calculating the depth to a reflecting boundary assumes
that the layering is not too far from horizontal. Highangle faults do not reflect the sound wave back to the
receivers to give a signal and faults can therefore not
be seen directly on seismic sections. It is the
truncations and offset of good reflectors which usually
allows us to identify faults. Due to special “edge
effects” that we observe near a fault, the reflector
terminations which should define the fault are not
448
N.H. Mondol
and wave motion), incoherent seismic interface and
imperfect static corrections. Without a sound understanding of these factors as well as knowledge of the
limitation of seismic resolution, there is a danger of
misinterpreting noise as real features.
In terms of the parameters that are analysed and the
interpretation that may be drawn from the analyses, a
fourfold hierarchy of seismic interpretation can be
achieved. These are seismic facies analysis, seismic
structural analysis, seismic attribute analysis and seismic sequence analysis. The most important parameters
used for interpretation of seismic data are:
Reflection amplitudes: The strength of the reflections.
As we discussed above, the proportion of the
energy reflected at the boundary between two
beds is a function of the difference in the acoustic
impedances. If we have an alternating series of
different beds, the distance between the bed
boundaries in relation to the wavelength of the
transmitted seismic signals will play a major part.
Reflector spacing: The distance between the reflectors
will indicate the thickness of the bed, but there will
be a lower limit to the thickness that can be
detected, which will depend on the wavelength.
Interval velocity: The interval velocity of a sequence
can provide information about lithology and porosity but this will depend on the stacking velocity and
will not be very accurate.
Reflector continuity: The continuity of reflectors will
be a function of how continuous the sediment beds
are, information which is essential for
reconstructing the environment.
Reflector configuration: If we take the compaction
effect into account, the shape of the reflecting
beds gives us a picture of the sedimentation surface
as it was during deposition. The slope of the
reflectors, for example, represents the slope of
prograding beds in a delta sequence with later differential compaction and tilting superimposed. Erosion boundaries with unconformities will in the
same way show the palaeo-topography during
erosion.
Instantaneous phase: A seismic trace can be considered an analytical signal where the real part is the
recorded seismic signal itself. Mathematically we
can compute the complex seismic trace (imaginary
parts of the signal) and the instantaneous attributes.
The Instantaneous phase is a measure of the
continuity of the events on a seismic section. The
Instantaneous phase is on a scale of +180
to
À180
. The temporal rate of change of the instantaneous phase is the instantaneous frequency.
17.7.1 Seismic Facies Analysis
A seismic profile provides information about the
properties of sedimentary rocks. Seismic facies analysis is the description and geological interpretation of
seismic reflectors representing sequence boundaries. It
includes the analysis of such parameters as the configuration, continuity, amplitude, phase, frequency and
interval velocity. These variables give an indication of
the lithology and sedimentary environment of the
facies (Selley 1998) (Fig. 17.29). Because seismic
reflections mainly represent time lines, i.e. sedimentary beds which were deposited contemporaneously, it
is also possible to a certain extent to interpret the
depositional environment. Large-scale sedimentary
features that may be recognised by the configuration
of seismic reflectors include prograding deltas, carbonate shelf margins and submarine fans (Sherif
1976).
17.7.2 Structural Analysis
Interpreters can draw horizons and faults on in-lines,
cross-lines and arbitrary lines, as well as slices.
Horizons can be automatically tracked on vertical
seismic displays and horizontal slice displays.
Improved tracking algorithms for horizon interpretation, combined with user-interpreted faults and fault
polygons, can produce seismic-based interpretation
maps (Fig. 17.30). Faults on seismic sections are typically expressed by a loss of reflection amplitude. It
must be remembered that the principle we use for
calculating the depth to a reflecting boundary assumes
that the layering is not too far from horizontal. Highangle faults do not reflect the sound wave back to the
receivers to give a signal and faults can therefore not
be seen directly on seismic sections. It is the
truncations and offset of good reflectors which usually
allows us to identify faults. Due to special “edge
effects” that we observe near a fault, the reflector
terminations which should define the fault are not
448
N.H. Mondol