Geoscience Reference
In-Depth Information
7.2 bASIC pRInCIpleS
GPR uses the principle of scattering electromagnetic energy in the form of an electromagnetic wave
to locate buried objects. The basic principles and theory of operation for GPR have evolved through
the disciplines of electrical engineering and seismic exploration, and GPR specialists tend to have
backgrounds either in geophysical exploration or electrical engineering. The fundamental principle
of operation is the same as that used to detect aircraft overhead, but with GPR, antennas are moved
over the surface, similar to a sonic fish-finder, rather than rotating about a fixed point. This has led to
the application of field operational principles that are analogous to the seismic reflection method.
It is not necessary to understand electromagnetic theory to use and interpret GPR data, but it
is important to know a few basic principles and have an empirical understanding of how electro-
magnetic energy travels (propagates) in the subsurface. We begin with the most basic description of
a wave: a propagating wave is described by a frequency, a velocity, and a wavelength, as shown in
Figure 7.1a. If we have two waves and add them together, then we also have to consider the phase, or
time offset, of the wave, as shown in Figure 7.1b. We also know from basic physics that we can add
waves with different frequencies and different phases together, then we can form any wave shape
that we want; and, this wave will still propagate. A wave composed of multiple frequencies, with
a resulting finite time duration (e.g., a pulse of electromagnetic energy), is called a time-domain
wave. A time-domain wave, similar to the waveform commonly applied to GPR systems, is shown
in Figure 7.1c. The most fundamental underpinning principle of GPR is as follows: (a) a pulse of
time-domain electromagnetic energy can be formed by a transmit antenna, propagate into the earth
with a particular velocity and amplitude, and be recorded by a receive antenna, and (b) the energy
of pulse recorded over time provides a time-history of the pulse traveling through the subsurface
(Figure 7.1c).
The theory of GPR is based on Maxwell's equations and the vector form of the wave equation,
which is the same fundamental theory as the seismic method, with a major difference being that the
seismic method is based on the scalar wave equation, and GPR is based on the vector wave equation. In
a practical sense, this means that the propagating GPR wave has both a magnitude and an orientation.
7.2.1 P
R o P a g a t i o n
a n d
s
c a t t e R i n g
The practical result of the radiation of electromagnetic waves into the subsurface for GPR mea-
surements is shown by the basic operating principle illustrated in Figure 7.1. The electromagnetic
wave is radiated from a transmitting antenna, spreads out over time in the form of a spherical
wavefront (Figure 7.1a), and travels through the material at a velocity determined, primarily, by
the permittivity (sometimes called the dielectric constant or electric permittivity) of the material.
Antenna
Antennas
Transmit
T
1
T
2
T
3
Rays
perpendicular
to spreading
wavefront
Receive
θ
r
θ
f
θ
i
ε
1
T
4
ε
2
ε
2
ε
1
(b) Reflection scattering from
a boundary
(a) Spreading wave from antenna,
at increasing times T
1
, T
2
, T
3
, and T
4
(c) Reflection from a pipe
fIGURe 7.1
Simple wave reflection scattering in the subsurface: (a) simplified view of the process of a wave
spreading from an antenna, (b) rays demonstrating by the ray method of a transmitted electromagnetic wave
scattered from a buried layer with a contrasting permittivity, and (c) reflection scattering from a buried pipe.
Permittivity of the host media is ε
1
, and the permittivity of the reflecting target is ε
2
.
