3. Internal velocity in the beds. The internal velocity
of the bed can provide information about lithology
and porosity.
4. 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.
5. 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.
Seismic profiles provide information about the filling of the basin in response to the type of subsidence
and sedimentary facies.
When interpreting lithology and depositional environment from seismic profiles, it is important to use
sedimentological models as aids. If there are well data
on lithologies, these must also be integrated. The
information we gain from seismic profiles is often
not sufficient by itself for an unambiguous interpretation. There may be several lithological compositions
and environments which could give similar seismic
signatures. Only by looking at the whole basin in a
sedimentological context do we have a good basis for
selecting the interpretation which seems most
reasonable.
8.4
Seismic Interpretation of
Sedimentary Basins
Seismic profiles present a picture of the way the basin
has been filled in. This is a result of an interaction
between the rate of subsidence, rate of deposition and
the energy of the depositional environment. The seismic signatures can be used to interpret facies and basin
infilling.
8.5
Faults and Tectonic Boundaries
It must be remembered that the principle we use for
calculating the depth to a reflecting boundary assumes
that the layering is relatively horizontal.
Primary seismic reflections will be deformed
through tectonic deformation so that they become
tilted or folded. Folded beds will only be realistically
depicted if the folds are sufficiently gentle that the
beds have a low angle of dip.
We can distinguish faults where good reflections
suddenly stop, suggesting an abrupt lateral change in
lithology. Faults are generally too steep to reflect the
sound wave straight back again, and the fault plane
itself will not appear as a reflector on the seismic
profile. Because of the special “edge” effects near
faults, the ends of the reflecting layers which should
define faults will not be quite correctly located on the
seismic profile, and it may therefore be difficult to
trace the fault entirely accurately. The termination of
beds against faults may produce diffraction from a
point source, giving a curved alignment. Special treatment of seismic data (migration) will rectify a fair
number of these errors and give a more correct picture.
In recent years seismic lines have been shot with
smaller and smaller grid spacings to obtain a better
map of the reservoir structure. A three-dimensional
seismic data set is then produced and seismic sections
can be constructed at any angle relative to the grid.
This method also allows us to construct horizontal
time-slices through the structure. This is almost like
a topographic or geological map which is a horizontal
projection of the geology. It is also a very powerful
method of delineating faults and other important structural elements.
Another relatively new development is borehole
seismics, particularly vertical seismic profiles
(VSPs). This method involves firing shots near the
seafloor close to a well and recording signals at regular
depth intervals in the well. The main advantage of
VSPs is that they produce a very good profile of the
seismic velocity as a function of depth, better than a
synthetic seismic log.
8.6
Changes in Sea Level
It has been clear for a long time that there are
unconformities in sedimentary sequences which can
be correlated over long distances, and that there were
periods in geological history with a high sea level and
others when it was low.
Proximal onlaps are due to sedimentation moving
landwards over an unconformity surface. If we are
260
K. Bjørlykke
of the bed can provide information about lithology
and porosity.
4. 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.
5. 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.
Seismic profiles provide information about the filling of the basin in response to the type of subsidence
and sedimentary facies.
When interpreting lithology and depositional environment from seismic profiles, it is important to use
sedimentological models as aids. If there are well data
on lithologies, these must also be integrated. The
information we gain from seismic profiles is often
not sufficient by itself for an unambiguous interpretation. There may be several lithological compositions
and environments which could give similar seismic
signatures. Only by looking at the whole basin in a
sedimentological context do we have a good basis for
selecting the interpretation which seems most
reasonable.
8.4
Seismic Interpretation of
Sedimentary Basins
Seismic profiles present a picture of the way the basin
has been filled in. This is a result of an interaction
between the rate of subsidence, rate of deposition and
the energy of the depositional environment. The seismic signatures can be used to interpret facies and basin
infilling.
8.5
Faults and Tectonic Boundaries
It must be remembered that the principle we use for
calculating the depth to a reflecting boundary assumes
that the layering is relatively horizontal.
Primary seismic reflections will be deformed
through tectonic deformation so that they become
tilted or folded. Folded beds will only be realistically
depicted if the folds are sufficiently gentle that the
beds have a low angle of dip.
We can distinguish faults where good reflections
suddenly stop, suggesting an abrupt lateral change in
lithology. Faults are generally too steep to reflect the
sound wave straight back again, and the fault plane
itself will not appear as a reflector on the seismic
profile. Because of the special “edge” effects near
faults, the ends of the reflecting layers which should
define faults will not be quite correctly located on the
seismic profile, and it may therefore be difficult to
trace the fault entirely accurately. The termination of
beds against faults may produce diffraction from a
point source, giving a curved alignment. Special treatment of seismic data (migration) will rectify a fair
number of these errors and give a more correct picture.
In recent years seismic lines have been shot with
smaller and smaller grid spacings to obtain a better
map of the reservoir structure. A three-dimensional
seismic data set is then produced and seismic sections
can be constructed at any angle relative to the grid.
This method also allows us to construct horizontal
time-slices through the structure. This is almost like
a topographic or geological map which is a horizontal
projection of the geology. It is also a very powerful
method of delineating faults and other important structural elements.
Another relatively new development is borehole
seismics, particularly vertical seismic profiles
(VSPs). This method involves firing shots near the
seafloor close to a well and recording signals at regular
depth intervals in the well. The main advantage of
VSPs is that they produce a very good profile of the
seismic velocity as a function of depth, better than a
synthetic seismic log.
8.6
Changes in Sea Level
It has been clear for a long time that there are
unconformities in sedimentary sequences which can
be correlated over long distances, and that there were
periods in geological history with a high sea level and
others when it was low.
Proximal onlaps are due to sedimentation moving
landwards over an unconformity surface. If we are
260
K. Bjørlykke