Introduction to faults
Looking at a fault on seismic data
1 km
This seismic profile has one interpreted
horizon marked as a yellow dashed line
and a fault marked on with a black line.
Fault
Interpreted horizon
Interpreted horizon
1 km
A fault is defined by describing the
direction and amount of movement.
1 km
Downthrow
Upthrow
The only type of movement that can be
seen on a seismic profile is a vertical
displacement and so this is a dip-slip
fault.
The movement can be marked on with
arrows and is called the throw.
The south side of this fault is called the
downthrow side.
The north side is called the upthrow side.
1 km
Downthrow
Upthrow
Why is there no strike-slip movement
seen?
Because the seismic is a 2 dimensional
image of the sub-surface looking only
from the side.
1 km
Downthrow
Upthrow
The upthrow and downthrow are used as
descriptors of the fault, they do not
indicate the actual nature of the
movement.
For example the south side may have
stayed still and the tectonic forces moved
only the north side up, there are several
other possibilities:
Only the south side moved down, the
north stayed still.
OR
Both sides have moved a bit, the
movement on either side may not have
been equal.
OR EVEN
Both sides have moved many times, both
up and down in successive periods of
tectonic activity. Each time this happens
the fault is re-activated.
1 km
Downthrow
Upthrow
The amount of displacement can be
measured. In the field this would be in
metres but here the seismic data is in
TWT and so must be measured in msec.
The next slide shows how the throw is
measured on an enlarged view.
The throw is measured
as a vertical distance.
It is taken as the place
where the horizon and
the fault intersect.
The throw is 100msec.
100msec
Enlarged view of the fault
1 km
Downthrow
Upthrow
Notice the scale bar at the top of the
data. This is a far larger area than you
would normally see in the field.
How many metres do you think 100msec
represents on this data?
On this seismic profile the seismic
velocity at TWT 1000msec is 4200m/s.
100msec
Calculate the throw in metres using this
seismic velocity and the distance, speed,
time formula triangle.
Converting the throw to metres
The seismic velocity is 4200m/s.
100msec
The throw is 100msec TWT.
1. The units need to match. Convert 100msec TWT to one way time.
100 ÷ 2 = 50msec
2. Next convert 50msec to seconds
1 second = 1000msec
1 ÷ 1000 = 0.001 therefore 1msec = 0.001s
0.001 × 50 = 0.05 and so 50msec = 0.05s
3. Use the distance, speed, time formula triangle:
Put your finger over the number you want to calculate, which is distance:
speed = 4200m/s
time = 0.05s
4200 × 0.05 = 210m
At the yellow horizon the throw of the fault is 210m to the south.
Summary
1 km
Downthrow
Upthrow
This seismic profile has one interpreted horizon
marked as a yellow dashed line and a fault
marked on with a black line.
A fault is defined by describing the direction and
amount of movement.
The only type of movement that can be seen on
a seismic profile is a vertical displacement and
so this is a dip-slip fault. The movement is
called the throw and is marked on with arrows.
The south side of this fault is called the
downthrow side.
100msec
The north side is called the upthrow side.
These names are used as descriptors of the
fault, they do not indicate the actual nature of
the movement, for example the south side may
have stayed still and the tectonic forces moved
only the north side up, there are several other
possibilities.
The amount of displacement can be measured.
In the field this would be in metres but here the
seismic data is in TWT and so must be
measured in msec.
Using seismic velocity the throw is calculated as
210m to the south at the interpreted horizon.
1 km
Downthrow
Upthrow
This seismic profile has one interpreted horizon
marked with a dashed white line and a fault
marked on with a black line.
A fault is defined by describing the direction and
amount of movement.
The only type of movement that can be seen on
a seismic profile is a vertical displacement and
so this is a dip-slip fault. The movement is
called the throw and is marked on with arrows.
The south side of this fault is called the
downthrow side.
100msec
The north side is called the upthrow side.
These names are used as descriptors of the
fault, they do not indicate the actual nature of
the movement, for example the south side may
have stayed still and the tectonic forces moved
only the north side up, there are several other
possibilities.
The amount of displacement can be measured.
In the field this would be in metres but here the
seismic data is in TWT and so must be
measured in msec.
Using seismic velocity the throw is calculated as
210m to the south at the interpreted horizon.