Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 353

former constituting the signal in seismic surveying and the
latter being a form of methodological noise (see Section
2.4.2 ) . The most important difference between them is that
surface waves propagate in the vicinity of a boundary of a
volume of material, e.g. the surface of the Earth in a seismic
survey. In contrast, body waves can travel through the
interior of the volume as well as along its surfaces, i.e. they
can penetrate into the subsurface and can provide infor-
mation about its characteristics.

a)

Bulk modulus ( K ) Axial modulus ( Y ) Shear modulus ( m )

b)

Compression Compression
Te nsi on

Te nsi on

Propagation
direction

6.2.1 Elasticity and seismic velocity

Extent of wavelet

As a seismic wave propagates through the subsurface, the
rock is temporarily deformed, or strained. The amount of
strain is small and of very short duration (except in the
immediate vicinity of the seismic source) and under these
conditions the strain is proportional to the applied stress.
After the passage of the wave, the rock regains its original
shape, i.e. it displays elastic behaviour. It can be useful to
think of wavelets as localised packets of elastic strain
energy travelling through the rock.
The response of a material to different kinds of strain is
described using elastic constants. Put simply, high values of
these constants indicate a greater resistance to deform-
ation, i.e. the material is more rigid, whilst ductile materials
have lower values. A set of elastic moduli quantify a mater-
ial

Movement of a
point

P-wave

Dextral
shear

Dextral
shear

Sinistral
shear

Sinistral
shear

Propagation
direction

Extent of wavelet

Movement of a
point

S-wave

Figure 6.2 Elastic deformation of a material. (a) The three elastic
moduli quantifying the response of a cube of material to
compression, uniaxial strain and shear strain. (b) Deformation of
the propagating medium associated with the passage of a P-wave
(uniaxial strain) and an S-wave (shear strain); redrawn, with
permission, from Bolt ( 1982 ) .

s response to different kinds of strains. These include
the bulk modulus (

'

κ ), a measure of the material ' s ability to

resist compression; the shear modulus ( μ ), related to shear

strains; and the axial modulus ( ), describing the response

to a uniaxial stress when there is no strain in other direc-
tions ( Fig. 6.2a ).
Different kinds of seismic wave cause different types of
strains as they propagate. The speed with which a body
wave travels through a material depends on the density (

Ψ

For a more detailed description of elastic properties and
the mechanisms by which seismic waves travel, see Dobrin
and Savit ( 1988 ) and Lowrie ( 2007 ) .

ρ )

and the relevant elastic modulus of the material.

6.2.2 Body waves

s

elastic modulus
ρ

Speed bodywave ¼ ð 6 : 2 Þ There are two types of body wave ( Fig. 6.2b ) . The rst type

is like sound waves. As they propagate, the rocks undergo a
series of uniaxial compressions and tensions causing a
point in the subsurface to oscillate along the direction of
propagation. These waves travel more quickly than other
kinds of seismic wave; they are thus the

Seismic speed is often directionally dependent (seismic
anisotropy) and is usually greatest parallel to any planar
fabric within the rock. Also, noting that velocity is speed in
a speci ed direction, it is correct to de ne a material

rst to be detected
and are called primary (or P-) waves. They are also referred
to as compressional, irrotational or longitudinal waves.
P-waves can travel through solid material such as rock
and also liquids such as pore water. The vast majority of

s
seismic velocity in terms of both speed and direction. In
practice, speed and velocity are used interchangeably, and
to comply with common practice we will use the term
velocity throughout our descriptions of the seismic method.

'