Geoscience Reference
In-Depth Information
descriptions of seismic methods (Coffeen, 1978; Dobrin and Savit, 1988; Lines and Newrick, 2004;
Pelton, 2005a, 2005b; Telford et al., 1976), and readers are referred to these for more information.
However, some basic attributes of the seismic waves and their propagation through Earth materials
along with general data collection and analysis considerations and potential seismic method agricul-
tural applications are discussed as follows.
8.3.2 s
e i s M i c
w
a v e
t
y P e s
There are two categories of seismic waves: “surface waves” and “body waves.” Surface waves, as
the name implies, are seismic waves that travel only along Earth's surface. Seismic body waves,
although capable of traveling along the surface directly from source to sensor, can also travel with
a vertical component through soil and rock well below ground. P-waves (also called primary waves,
compressional waves, and longitudinal waves) are a type of seismic body wave having an elastic
back-and-forth particle motion orientation that coincides with the direction of wave propagation.
P-waves can be transmitted through solid, liquid, and gas materials. P-waves are the fastest seismic
waves, and their velocity,
V
P
, within a soil or rock material is given by the following equation:
4
3
µ
k
+
V
P
=
(8.1)
ρ
where
k
is the bulk modulus, μ is the rigidity modulus (or shear modulus), and ρ is density. As
indicated by Equation (8.1), the P-wave velocity in soil or rock depends only on elastic moduli and
density of the soil or rock.
S-waves (also called secondary waves, shear waves, and traverse waves) are the second seismic
body wave type and have an elastic particle motion that is perpendicular to the direction of wave
propagation. There are two kinds of S-waves: the SV-wave and the SH-wave. The particle motion
for an SV-wave has a vertical component. SH-waves, on the other hand, have a particle motion that
is completely horizontal. S-waves are only capable of traveling through solid material, not liquids or
gases. S-waves are slower than P-waves and have a velocity,
V
S
, given by
V
S
=
µ
ρ
(8.2)
where all quantities have been previously defined. The S-wave velocity, as indicated by Equation
(8.2), is governed strictly by shear stress elastic behavior and density of the soil or rock through
which the S-wave travels.
There are two types of surface waves: Rayleigh waves and Love waves. Rayleigh wave particle
motion is elliptical retrograde in a vertical plane oriented coincident with the direction of wave
propagation. The Rayleigh wave amplitude decreases exponentially with depth. For a given soil or
rock material, the Rayleigh wave velocity is approximately nine-tenths of the S-wave velocity for
the same material (= 0.9
V
S
). Love waves occur only where there is a low S-wave velocity layer at the
surface that is underlain by a layer with a much higher S-wave velocity. Love waves are essentially
SH-waves transmitted via multiple reflections between the top and bottom of the low seismic veloc-
ity surface layer. Accordingly, the Love wave particle motion is horizontal and perpendicular to the
direction of wave propagation. The overall Love wave velocity for a particular soil or rock material
is less than the S-wave velocity for the same material. Neither surface wave is capable of being
transmitted through liquids, such as water.
