Geology Reference
In-Depth Information
4
Seismic reflection surveying
mation, the horizontal variations of velocity may be
ignored.
Figure 4.1 shows a simple physical model of
horizontally-layered ground with vertical reflected
ray paths from the various layer boundaries. This model
assumes each layer to be characterized by an
interval veloc-
ity v
i
, which may correspond to the uniform velocity
within a homogeneous geological unit or the average
velocity over a depth interval containing more than one
unit. If
z
i
is the thickness of such an interval and
t
i
is the
one-way travel time of a ray through it, the interval
velocity is given by
4.1 Introduction
Seismic reflection surveying is the most widely used and
well-known geophysical technique.The current state of
sophistication of the technique is largely a result of the
enormous investment in its development made by the
hydrocarbon industry, coupled with the development of
advanced electronic and computing technology. Seismic
sections can now be produced to reveal details of geo-
logical structures on scales from the top tens of metres
of drift to the whole lithosphere. Part of the spectacu-
lar success of the method lies in the fact that the raw data
are processed to produce a seismic section which is an
image of the subsurface structure. This also provides a
trap for the unwary, since the seismic section is similar to,
but fundamentally different from, a depth section of the
geology. Only by understanding how the reflection
method is used and seismic sections are created, can the
geologist make informed interpretations. This chapter
provides the essential knowledge and understanding to
support interpretation of seismic reflection data. It builds
up systematically from the basics of seismic wave re-
flection from rock layers, and refers back to relevant
material in Chapters 2 and 3.
z
i
i
v
=
i
t
The interval velocity may be averaged over several depth
interva
ls
to yield a
time-average velocity
or, simply,
average
velocity
.Thus the average velocity of the top
n
layers in
Fig. 4.1 is given by
V
n
n
Â
Â
z
v
t
i
ii
i
=
1
i
=
1
V
=
=
n
n
Â
Â
t
t
i
i
4.2 Geometry of reflected ray paths
i
=
1
i
=
1
In seismic reflection surveys seismic energy pulses are
reflected from subsurface interfaces and recorded
at near-normal incidence at the surface. The travel
times are measured and can be converted into estimates
of depths to the interfaces. Reflection surveys are most
commonly carried out in areas of shallowly dipping
sedimentary sequences. In such situations, velocity
varies as a function of depth, due to the differing
physical properties of the individual layers. Velocity
may also vary horizontally, due to lateral lithological
changes within the individual layers. As a first approxi-
or, if
Z
n
is the total thickness of the top
n
layers and
T
n
is
the total one-way travel time through the
n
layers,
Z
T
V
=
n
n
4.2.1 Single horizontal reflector
The basic geometry of the reflected ray path is shown in
Fig. 4.2(a) for the simple case of a single horizontal re-
