Geoscience Reference
In-Depth Information
in which particle motion is perpendicular to the propagation direction. Both types have velocities that
depend on the elastic moduli and density of the rock:
]
1
/
2
V
p
=
[(
k
+
4
µ/
3)
/ρ
V
s
=
[
µ/ρ
]
1
/
2
where
k
is the bulk modulus,
µ
is the shear modulus, and
ρ
is the density. Other types of seismic
wave exist, confined to the vicinity of layer boundaries, but they are not important to the interpreter
of seismic data.
Shear modulus
Shear deformation of a material involves change in shape without change in volume. The shear
modulus of a material is a measure of its resistance to shear stress. For liquids, which will flow freely
to accommodate any change in shape of their containing vesssel that does not involve change in
volume, the shear modulus is zero.
Stack
In general, the adding of a number of traces together to improve signal to noise ratio. Most often
used to refer to the adding of traces with different source-receiver offsets but a common midpoint
(fig. 3.7
)
. The traces are first corrected for the increased travel-time at the longer offsets due to the
oblique travel path; this is the Normal Moveout (NMO) correction. They may also be corrected for
effects of subsurface dip and lateral velocity variation, by some form of migration process.
Stacking velocity
The velocity field that, when used to calculate NMO correction, gives the best alignment of the traces
across the CMP gather and therefore the highest amplitude in the stacked trace. It is only loosely
connected to the actual velocities of seismic waves in the earth, owing to effects of dip and curvature
of the reflector and the impact of lateral variations in the overburden.
Static correction
A static correction is a time shift that is applied uniformly across a particular trace. For example,
the effect might be to shift an entire trace downwards by 8 ms. A neighbouring trace might have a
different time shift applied. Such shifts are typically needed when processing land data, to remove
near-surface time delays particular to each shot and receiver location, as a result of changes in elevation
and thickness changes in a near-surface low-velocity layer.
Stratigraphic trap
The simplest sort of oil or gas trap is a domal anticline, or four-way dip closure. Variants on this may
require an element of sealing along a fault, but the possible existence of such a
structural trap
can be
inferred from the top reservoir map alone. Sometimes, however, a trap requires an element of lateral
lithological change to work. This might, for example, be lateral transition from sand to shale within
the reservoir formation. Such a
stratigraphic trap
cannot be found from the structural map alone; the
lateral change in rock properties needs to be present. It may be inferred from geological argument
alone, but seismic evidence for the required transition will reduce the (normally high) risk associated
with such a prospect.
Strike section
Seismic section shot perpendicular to the dip direction of the main reflectors, often sub-parallel to the
main faults and therefore hard to interpret on 2-D seismic sections because sub-horizontal reflections
at nearly the same travel-time may relate to different fault blocks on either side of the line or directly
below it.
