On the front face of the block diagram in Fig. 1.8
values of the horizontal stress are shown using a
contour plot. This illustrates quantitatively how the
opening of a model dike changes the stress field in
the surrounding rock. To either side of the dike negative (compressive) stresses are induced that push
inward on any small element of rock. These
induced compressive stresses would tend to prevent
normal faults from forming because they increase
the frictional resistance to sliding. However, immediately over the dike, positive (tensile) stresses are
induced. These tensile stresses stretch the rock and
could contribute to the formation of normal faults.
The distribution of horizontal stress at the surface
has two maxima located symmetrically about the
plane of the model dike. These two maxima correlate with the two normal faults that develop to
either side of the dike plane. In this way the model
has successfully addressed the questions we asked
about the relationship between dikes and graben.
Does the success of the model provide all one
needs to know to interpret the radar lineaments
on Venus? Certainly it is supportive of the concept
that dikes can induce slip on normal faults, but it
does not prove that dikes actually exist beneath
the graben. Some of the paired radial bright and
dark lines on the radar image at (15ЊS, 215ЊE) can
be traced down the flanks of the volcano where
they merge into a single bright radar lineament
(labeled “f” on Fig. 1.7). These single lineaments
are interpreted as open fissures that formed as
dikes neared the surface (Koenig and Pollard,
1998). This interpretation is consistent with the
presence of dark lobate regions, interpreted as lava
flows, emerging from near the distal end of some
fissures and extending outward in a radial direction (labeled “tlf” on Fig. 1.7). These flows provide
compelling evidence that dikes underlie the radial
fractures and graben, and are the conduits for the
escape of magma from beneath the volcano.
1.3 Faulting in a North Sea
hydrocarbon reservoir
R. N. Farvolden and J. A. Cherry (1991) wondered if
geology departments are preparing their students
for the twenty-first century:
After many years of observing geology departments,
our experience indicates that they have adjusted to the
scientific revolution of recent decades, but have largely
ignored the technical, economic, and social changes
that influence the practice of geology and thereby
have ignored the professional lives of their students.
Structural geologists can make important contributions to natural resource recovery, including
water, oil, gas, and minerals. They can play key
roles in the management of the environment, for
example in the long-term storage of radioactive
materials and the contamination of fractured
aquifers by hazardous chemicals.
One of the most interesting new areas of
research in structural geology related to natural
resources is the investigation of folds and faults
using data from seismic reflection surveys
(Kattenhorn and Pollard, 2001). These data are
gathered by the petroleum industry to image
hydrocarbon reservoirs (Sheriff and Geldart,
1995). To carry out a reflection survey in a sedimentary basin, seismic waves are generated by
impulsively striking the surface of the Earth.
Depending upon the depth of investigation
required, these waves are generated using explosive charges, mechanically driven vibrators, or
simply a hand-held hammer. Some of the waves
travel down into the Earth, reflect off sedimentary
layers or other structures, and travel back to the
surface where they are recorded by a string of
portable seismographs laid out along the survey
line. The depth to different reflecting horizons
can be computed by identifying the two-way travel
time (down and back) for each reflection, and by
knowing the velocity for acoustic waves in the
rock. A series of impulses is generated along the
survey line and reflections are recorded at each
seismograph. The abundant data help to reduce
the uncertainty in identifying and locating the
reflecting horizons using a variety of data processing techniques.
The output of this processing is a seismic
reflection cross section (seismic section) of the Earth
immediately under the survey line. For example,
consider the northern part of the North Sea
(Fig. 1.9a) on the Norwegian Continental Shelf
(Maerten et al., 2000; Maerten et al., 2002). Figure
1.9b is a seismic section taken from approximately
150 km west of the Norwegian coast (Faerseth
1.3 FAULTING IN A NORTH SEA HYDROCARBON RESERVOIR
11
values of the horizontal stress are shown using a
contour plot. This illustrates quantitatively how the
opening of a model dike changes the stress field in
the surrounding rock. To either side of the dike negative (compressive) stresses are induced that push
inward on any small element of rock. These
induced compressive stresses would tend to prevent
normal faults from forming because they increase
the frictional resistance to sliding. However, immediately over the dike, positive (tensile) stresses are
induced. These tensile stresses stretch the rock and
could contribute to the formation of normal faults.
The distribution of horizontal stress at the surface
has two maxima located symmetrically about the
plane of the model dike. These two maxima correlate with the two normal faults that develop to
either side of the dike plane. In this way the model
has successfully addressed the questions we asked
about the relationship between dikes and graben.
Does the success of the model provide all one
needs to know to interpret the radar lineaments
on Venus? Certainly it is supportive of the concept
that dikes can induce slip on normal faults, but it
does not prove that dikes actually exist beneath
the graben. Some of the paired radial bright and
dark lines on the radar image at (15ЊS, 215ЊE) can
be traced down the flanks of the volcano where
they merge into a single bright radar lineament
(labeled “f” on Fig. 1.7). These single lineaments
are interpreted as open fissures that formed as
dikes neared the surface (Koenig and Pollard,
1998). This interpretation is consistent with the
presence of dark lobate regions, interpreted as lava
flows, emerging from near the distal end of some
fissures and extending outward in a radial direction (labeled “tlf” on Fig. 1.7). These flows provide
compelling evidence that dikes underlie the radial
fractures and graben, and are the conduits for the
escape of magma from beneath the volcano.
1.3 Faulting in a North Sea
hydrocarbon reservoir
R. N. Farvolden and J. A. Cherry (1991) wondered if
geology departments are preparing their students
for the twenty-first century:
After many years of observing geology departments,
our experience indicates that they have adjusted to the
scientific revolution of recent decades, but have largely
ignored the technical, economic, and social changes
that influence the practice of geology and thereby
have ignored the professional lives of their students.
Structural geologists can make important contributions to natural resource recovery, including
water, oil, gas, and minerals. They can play key
roles in the management of the environment, for
example in the long-term storage of radioactive
materials and the contamination of fractured
aquifers by hazardous chemicals.
One of the most interesting new areas of
research in structural geology related to natural
resources is the investigation of folds and faults
using data from seismic reflection surveys
(Kattenhorn and Pollard, 2001). These data are
gathered by the petroleum industry to image
hydrocarbon reservoirs (Sheriff and Geldart,
1995). To carry out a reflection survey in a sedimentary basin, seismic waves are generated by
impulsively striking the surface of the Earth.
Depending upon the depth of investigation
required, these waves are generated using explosive charges, mechanically driven vibrators, or
simply a hand-held hammer. Some of the waves
travel down into the Earth, reflect off sedimentary
layers or other structures, and travel back to the
surface where they are recorded by a string of
portable seismographs laid out along the survey
line. The depth to different reflecting horizons
can be computed by identifying the two-way travel
time (down and back) for each reflection, and by
knowing the velocity for acoustic waves in the
rock. A series of impulses is generated along the
survey line and reflections are recorded at each
seismograph. The abundant data help to reduce
the uncertainty in identifying and locating the
reflecting horizons using a variety of data processing techniques.
The output of this processing is a seismic
reflection cross section (seismic section) of the Earth
immediately under the survey line. For example,
consider the northern part of the North Sea
(Fig. 1.9a) on the Norwegian Continental Shelf
(Maerten et al., 2000; Maerten et al., 2002). Figure
1.9b is a seismic section taken from approximately
150 km west of the Norwegian coast (Faerseth
1.3 FAULTING IN A NORTH SEA HYDROCARBON RESERVOIR
11