Geology Reference
In-Depth Information
Radar A-Scan
Radar B-Scan
Direct
Wave
Direct
Wave
1
5
5
Scatterer
1
2
10
10
2
Wavefront
Amplitude
Distance
Figure 10.1
Ground penetrating radar (GPR) measurement and display.
Boundaries 1 and 2 separate layers with different electrical properties, and
signals can also be 'scattered' from isolated objects. A-scans and B-scans
are the radar equivalents of the 'traces' and 'sections' of seismic reflection
work.
ground is unity, and that the reflection, dispersion and attenuation of the
radar signals are therefore caused only by changes in conductivity,
σ
,and
relative electric permittivity,
(which defines the ability of the medium to
transmit or 'permit' an electric field). It is also usually assumed that these are
scalar quantities, independent of the direction of the radiating field. These
assumptions are not strictly true but are adequate for simple treatments.
The velocity of an electromagnetic wave in an insulator is given by:
ε
=
c
/
√
(
µ
·
ε
)
=
c
/
√
ε
V
or, taking
µ
=
1
,
V
Here
c
(
=
3
×
10
8
ms
−
1
, 300 000 km s
−
1
or 0.30 m ns
−
1
) is the velocity of
light in
free
(empty) space. In conducting media there are further compli-
cations, which can be described in terms of a
complex permittivity
(
K
=
=
√
(
ε
+
ω
(
=
2
π
f
)isthe
angular frequency
. Large loss tangents imply high signal
attenuation.
Table 10.1 lists typical radar parameters for some common materi-
als. Velocities are generally well below the free-space value. Electrical
conductivities at radar frequencies differ, sometimes very considerably, from
DC values, often increasing with frequency at roughly log-linear rates (Fig-
ure 10.2).
The radar wavelength in any material is equal to the wave velocity divided
by the frequency, i.e.
j
σ/ω
), where
j
−
1), and a
loss tangent
(tan
α
=
σ/ωε
), where
λ
=
c
/
f
√
ε
