Geology Reference
In-Depth Information
3
Elements of seismic surveying
problems involving the detection and mapping of sub-
surface boundaries of, normally, simple geometry.They
also identify significant physical properties of each sub-
surface unit.The methods are particularly well suited to
the mapping of layered sedimentary sequences and are
therefore widely used in the search for oil and gas. The
methods are also used, on a smaller scale, for the mapping
of near-surface sediment layers, the location of the water
table and, in an engineering context, site investigation of
foundation conditions including the determination
of depth to bedrock. Seismic surveying can be carried
out on land or at sea and is used extensively in offshore
geological surveys and the exploration for offshore
resources.
In this chapter the fundamental physical principles on
which seismic methods are based are reviewed, starting
with a discussion of the nature of seismic waves and
going on to consider their mode of propagation through
the ground, with particular reference to reflection and
refraction at interfaces between different rock types. To
understand the different types of seismic wave that
propagate through the ground away from a seismic
source, some elementary concepts of stress and strain
need to be considered.
3.1 Introduction
In seismic surveying, seismic waves are created by a con-
trolled source and propagate through the subsurface.
Some waves will return to the surface after refraction or
reflection at geological boundaries within the subsur-
face. Instruments distributed along the surface detect the
ground motion caused by these returning waves and
hence measure the arrival times of the waves at different
ranges from the source. These travel times may be con-
verted into depth values and, hence, the distribution of
subsurface geological interfaces may be systematically
mapped.
Seismic surveying was first carried out in the early
1920s. It represented a natural development of the
already long-established methods of earthquake seis-
mology in which the travel times of earthquake waves
recorded at seismological observatories are used to de-
rive information on the internal structure of the Earth.
Earthquake seismology provides information on the
gross internal layering of the Earth, and measurement of
the velocity of earthquake waves through the various
Earth layers provides information about their physical
properties and composition. In the same way, but on a
smaller scale, seismic surveying can provide a clear and
detailed picture of subsurface geology. It undoubtedly
represents the single most important geophysical survey-
ing method in terms of the amount of survey activity and
the very wide range of its applications. Many of the
principles of earthquake seismology are applicable to
seismic surveying. However, the latter is concerned
solely with the structure of the Earth down to tens of
kilometres at most and uses artificial seismic sources,
such as explosions, whose location, timing and source
characteristics are, unlike earthquakes, under the direct
control of the geophysicist. Seismic surveying also uses
specialized recording systems and associated data pro-
cessing and interpretation techniques.
Seismic methods are widely applied to exploration
3.2 Stress and strain
When external forces are applied to a body, balanced in-
ternal forces are set up within it.
Stress
is a measure of the
intensity of these balanced internal forces.The stress act-
ing on an area of any surface within the body may be re-
solved into a component of normal stress perpendicular
to the surface and a component of shearing stress in the
plane of the surface.
At any point in a stressed body three orthogonal planes
can be defined on which the components of stress are
wholly normal stresses, that is, no shearing stresses act
along them. These planes define three orthogonal axes
