Line-length and area-balance
work with DepthCon2000
Let's look first at line-length checks for
compressional structures. This of course is the standard method of drawing pins
either side of the zone of interest and simply measuring the lengths of beds
between those pins.
Here's a depth-converted seismic section across the
Burnt Timber Thrust, also in Alberta, with some of the eroded geology re-drawn,
and a line-length balance has been made. (The Burnt Timber Thrust is the pale
blue surface in centre of the image). Unfortunately the profile is not long
enough to show the cut-off of the Turner Valley Formation (blue) in the footwall
of the Burnt Timber Thrust, so we don't see the ramp which climbs through the
carbonates and without that information we can't restore the fault at dark blue
level. In the Mesozoics there has been substantial erosion to remove yellow and
green line length and likewise restoration on those markers is not possible
either. Nevertheless, to show how line-length balance might be attempted with
this software, here is a template drawn between the two pins marked in red:
This example is one of the download demo models, and
is fully described in the program help and FAQ notes. We hope you'll be
interested to try working with it. For line-length balance checks between pins,
this is an excellent tool. Line segments are drawn in order from the fixed pin
and define the pre-deformation fault pattern. If the line lengths are more or
less correct, the fault trajectories will look sensible whereas if bed length is
locally wrong, the sketched faults will assume impossible shapes. There is a
large literature on this topic, see for example Boyer and Elliott (1982), Thrust
Systems, AAPG 66 (9), 1196-1230, their Fig. 17 shows a template restoration of
the Mount Crandell Thrust. DepthCon2000 is well able to handle that kind of job.
Shear
(Area-balanced) Restoration
Here
are
two
examples
of
simple
shear
restorations
using
seismic:
Turner
Valley
Returning
to
the
Turner
Valley
model
which
we
have
depth
converted
we
should
need
to
see
a
much
longer
line
than
we
have
here,
to
comment
usefully
on
plausible
restoration.
Looking
at
the
footwall,
clearly
there
are
a
number
of
important
ramps
on
which
the
structure
has
been
displaced
eastwards
by
significant
amounts,
and
to
build
the
fold
we
should
have
to
go
looking
for
a
big
ramp
off-section
well
to
the
west.
But
we
can
do
some
local
modelling,
even
though
its
out
of
sequence,
to
show
that
the
pick
of
the
pale
blue
backthrust
of
the
pop-up
is
probably
on
the
right
lines:
Here
is
what
happens
when
we
draw
it
and
slip
the
hangingwall
rightwards
some
400
metres:
And
then
a
600
metres
leftwards
move
on
the
red
fault,
puts
the
pop-up
back
into
its
back
limb
position,
with
some
local
distortions
of
course
which
we
could
try
to
resolve
if
it
was
important
to
do
so.
To
restore
the
right
side
of
the
pop-up,
rejoin
the
blue
to
blue
on
this
red
fault:
Interestingly,
the
displacements
on
the
pop-up
margins
are
about
the
same
order
of
magnitude,
in
a
sense
we
can
get
rid
of
it
by
pulling
a
wedge
of
rock
sideways
out
of
the
profile.
Its
reasonable
to
draw
the
red
fault
in
the
way
we
have,
through
the
leftward
part
of
the
image:
although
the
fault
isn't
clearly
imaged
on
seismic
the
limb
of
the
fold
is
pervasively
cut
by
left-vergent
slices;
and
the
traces
found
by
TraceInterpreter
are
consistent
with
what
we've
done.
And
that
leaves
a
relatively
simple
thrust
fold
with
three
or
four
major
ramps
in
its
footwall,
on
which
the
slips
are
probably
a
kilometre
or
so
apiece,
and
the
compensating
backthrusts
will
be
found
climbing
westwards
above
the
structure,
in
the
Cretaceous
sequence.
So
I
would
interpret
Turner
Valley
as
being
built
in
an
intercutaneous
wedge,
and
there's
no
need
to
model
it
as
a
fault-propagation
fold.
A
complex
gas
field:
Here's
a
restoration
of
a
North
Sea
sequence
including
several
salt/anhydrite
horizons,
extended
by
Cretaceous
faulting
and
inverted
during
regional
Alpine
compression.
The
strong
event
is
a
carbonate,
folded
and
then
axially
ruptured
so
that
the
limbs
are
overlapping.
Between
the
two
"T"
shaped
rafts
of
dolomite
the
Rotliegendes
reservoir
is
thrusted
rightwards
onto
the
carbonates,
the
normal
stratigraphic
sequence
is
seen
just
to
the
left
of
this
thrust
juxtaposition.
We
can
restore
this,
firstly
by
rejoining
the
severed
ends
of
the
carbonate
using
the
pale
blue
fault,
and
at
the
same
time
removing
the
Rotliegendes
from
the
end
of
the
raft;
then
slipping
back
to
the
left
on
the
dark
blue
fault
to
unfold
the
right-hand
end
of
the
profile:
and
then
we
can
see
the
old
extensional
surface
(orange)
which
separates
the
Rotliegendes
blocks,
and
on
restoring
this
we
can
sketch
the
top
of
the
Rotliegendes
in
yellow,
solving
the
correlation
problem.
This
is
a
difficult
license
to
review,
without
restoration
of
seismic.
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