Geoscience Reference
In-Depth Information
dissipates as part of it is reradiated to the outside of the object. Closed objects are said to have a reso-
nant frequency based on the size of the object, and the electrical properties of the object and the sur-
rounding material. However, the ability of an object to resonate depends on the wavelength (velocity
of the object, divided by the frequency of the wave) with respect to dimensions of the object. The
length of time that an object resonates is determined by the permittivity contrast between the object
and the surrounding material.
Diffraction and resonant scattering are complicated phenomena that depend on the properties
of the incident wave (including polarization, amplitude, and frequency content) and the size, shape,
and electrical properties of the scattering object. Diffraction scattering can be seen on some GPR
records where the boundaries between media are sharp (e.g., engineering investigations of founda-
tions), but resonant scattering is very difficult to discern in all but the most ideal conditions.
7.2.2 gPR R
e c of R d i n g
Considering the wave scattered from the object in Figure 7.3, if a receive antenna is switched on at
precisely the instant that the pulse is transmitted, then the pulses will be recorded by the receive
antenna as a function of time. The first pulse will be the wave that travels directly through the air
(because the velocity of air is greater than any other material), and the second pulse that is recorded
will be the pulse that travels through the material and is scattered back to the surface, traveling at
a velocity determined by the permittivity (ε) of the material. The resulting record measured at the
receive antenna is similar to one of the time-amplitude plots shown in Figure 7.3b, with the “input”
wave consisting of the direct wave that travels through air, and the “output” pulse consisting of the
wave reflected from the buried scattering body. The recording of both pulses over a period of time
with receive antenna system is called a “trace,” which can be thought of as a time-history of the
Trace
Amplitude
Direct arrival
Reflection
Traces at Different Locations
2D Cross section
Direct arrivals
Recorder
Closely spaced
Reflections
Receiver
electronics
Multiple traces
Transmitter
electronics
Multiple traces
along multiple
lines
Transmit
antenna
Receive
antenna
Object
Object
3D Block
“pseudo image”
(c) Traces combined to form
a cross section, and cross
sections combined to form
a 3D block
(a) Single location yields
a trace of data
(b) Multiple locations yield
a cross section record of
time-distances traces
fIGURe 7.3
Steps in the GPR process: (a) formation of a single time trace of data, showing the direct arrival
and reflection from the object; and (b) multiple traces for a wiggle-trace display, with each trace representing a
different position on the earth's surface. Multiple traces can be color coded, with different colors (gray scales)
assigned to amplitudes, and the result displayed as a cross section, or as a three-dimensional block view as
shown on the right side of the figure.
