Geoscience Reference
In-Depth Information
orientations of subsurface objects, or to measure more than one component of the transmitted lin-
early polarized signal. Examples of the effect of polarization are presented in the following section
on interpretation of GPR data.
7.2.4 v
e l of c i t y
Electromagnetic waves travel at a specific velocity determined primarily by the permittivity of the
material. The relationship between the velocity of the wave and material properties is the fundamen-
tal basis for using GPR to investigate the subsurface. To state this fundamental physical principle in
a different way: the velocity is different between materials with different electrical properties, and a
signal passed through two materials with different electrical properties over the same distance will
arrive at different times. The interval of time that it takes for the wave to travel from the transmit
antenna to the receive antenna is simply called the travel time. The basic unit of electromagnetic
wave travel time is the nanosecond (ns), where 1 ns = 10
−9
s.
The time it takes an electromagnetic wave to travel from one point to another is called the travel
time, which is measured as an inverse function of velocity. Because the velocity of an electromag-
netic wave in air is 3 × 10
8
m/s (0.3 m/ns), then the travel time for an electromagnetic wave in air
is approximately 3.3333 ns per m traveled. The velocity is proportional to the inverse square root
of the permittivity of the material, and because the permittivity of earth materials is always greater
than the permittivity of the air, the travel time of a wave in a material other than air is always greater
than 3.3333 ns/m.
The permittivity is also commonly referred to as the dielectric permittivity, but for simplic-
ity it will be referred to as the permittivity in this topic. The permittivity has been explained as a
measure of polarizability of a material, which causes displacement currents to flow, which in turn
affects the propagation of an electromagnetic wave. The effect of the permittivity on attenuation
was previously discussed.
The permittivity is also directly related to the velocity of propagation of an electromagnetic
wave, which is a very important property for analyzing and processing GPR data. The permittivity
is related to the velocity by the following relationship:
v
v
=
0
(7.1)
m
ε
ε
m
0
where
v
m
is the velocity of the wave through any material,
v
0
is the speed of light in air (3 × 10
8
m/s),
ε
m
is the permittivity of the material, and ε
0
is the permittivity of free space (air in a vacuum), with
a value of 8.85 × 10
−12
Farads/m.
The ratio of the permittivity of the material to the permittivity of air (ε
r
= ε
m
/ε
0
) is called the rela-
tive permittivity. The range of values of relative permittivity is from 1 for air to ~81 for water. The
high permittivity of water is caused by the rotational polarization of the water molecule. Not surpris-
ingly, the quantity of water tends to dominate the relative permittivity of porous rocks and minerals.
7.3
eQUIpMent
7.3.1
s
y s t e M
o
v e R v i e w
GPR equipment consists of antennas, electronics, and a recording device, as shown in Figure 7.5.
The transmitter and receiver electronics are always separate, but in a fixed-mode configuration, they
are often contained in different boxes, and in some systems designed for moving-mode operation, all
