Geology Reference
In-Depth Information
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9.12 Ground-penetrating radar
Ground-penetrating radar (GPR) (Davis & Annan
1989) is a technique of imaging the subsurface at high
resolution.Although analogous in some ways to the seis-
mic methods, it is included in this chapter as the propa-
gation of radar waves through a medium is controlled by
its electrical properties at high frequencies. A compre-
hensive account of modern advances in GPR is given by
Reynolds (1997).
GPR is a non-destructive technique and can conse-
quently be applied in urban and sensitive environments.
GPR has many geological applications, such as imaging
shallow soil and rock structure at high resolution, locat-
ing buried channels and mapping the water table. It also
has several non-geological uses such as in archaeology,
for the location of buried walls or cavities, and in foren-
sic investigations, for the location of recently-disturbed
ground where a burial has taken place.
GPR is similar in its principles to seismic reflection
profiling (see Chapter 4) and sonar (see Section 4.15)
surveying. A short radar pulse in the frequency band
10-1000 MHz is introduced into the ground. Radar
velocities are controlled by the
dielectric constant
(
relative
permittivity
) and conductivity of the subsurface.
The velocity of a radar wave (
V
) is given by:
K
=
r
2
r
1
=
2
1
(9.10)
)
(
+
+
)
e
e
(
r
2
r
1
2
1
where
e
r1
and
e
r2
are the relative permittivities of
the two media separated by the interface and
V
1
and
V
2
the radar velocities within them. Velocities of geo-
logical materials generally lie within the range
0.06-0.175 m ns
-1
.
Dielectric permittivity does not usually vary by more
than a factor of 10 in most natural materials, so it is the
more highly variable resistivity that controls the depth of
penetration of a radar pulse. Generally, depth of penetra-
tion increases with increasing resistivity. Penetration is
of the order of 20 m, although this may increase to 50 m
under optimal conditions of low conductivity. As with
seismic waves, there is a trade-off between depth of
penetration and resolution, with the greater penetration
achieved with the lower frequencies.
A transmitting antenna generates a wavetrain which
comprises a pulse of radio waves with a frequency of
about 50 kHz. This is transmitted into the subsurface.
The arriving pulse is scanned at a fixed rate for a time ad-
justed to be of the order of the two-way travel time of the
pulse.The pulse received by the receiving antenna is sim-
ilarly a wavetrain, but differs from the transmitted wave-
train because of the modifications caused to it during its
passage through the subsurface. The fact that the wave-
train comprises more than one wavelet complicates the
subsequent interpretation. Since velocities of radar
waves can be of the order of 0.3 m ns
-1
, accurate timing
instrumentation is essential. The returned radar signals
are amplified, digitized and recorded; the resulting data
can be displayed on a radargram, which is very similar to
a seismogram.
The depth of penetration of radio waves depends on
their frequency and the nature of the material being
surveyed. Figure 9.23 shows how the penetration varies
in different materials over the frequency range 1-
500 MHz.The permittivity of water is high compared to
dry materials, so the water content and porosity are
important controls on penetration.
There are three basic modes of deployment in GPR
surveys, all of which have their seismic counterparts
(Fig. 9.24):
1.
Reflection profiling (Fig. 9.24(a)), in which the
transmitter and antenna are kept at a small, fixed separa-
tion; this is often achieved by using the same antenna for
transmission and reception.
c
V
=
(9.9)
(
m
rr
)
where
c
is the velocity of light
in vacuo
(3 ¥ 10
8
ms
-1
),
m
r
the relative magnetic permeability (Section 7.2), which
is close to unity for non-magnetic rocks, and e
r
the
relative dielectric permittivity.
In high resistivity rocks (>10
2
ohm m) the propaga-
tion velocity of the pulse is mainly controlled by e
r
.
Dielectric conduction takes place in such poor conduc-
tors and insulators, which have no free carriers, by the
slight displacement of electrons with respect to their nu-
clei. Water has a dielectric constant of 80, whereas in
most dry geological materials the dielectric constant is in
the range 4-8. Consequently, the water content of mate-
rials exerts a strong influence on the propagation of a
radar pulse.
A contrast in dielectric properties across an interface
causes reflection of part of a radar pulse with a diminu-
tion of energy according to the reflection coefficient
K
,
which is analogous to the seismic case (see Section
3.6.1),
