Geoscience Reference
In-Depth Information
material electrical properties. In cases where the electrical conductivity is high (e.g., clay soil), it is
impossible to match the impedance, and all antennas over these types of “lossy” materials will ring
like bells.
GPR antennas are designed to maximize the amount of energy that propagates in the ground,
but antennas naturally radiate energy in all directions, including above the surface of the ground.
The effect of this fact is that “reflections” will appear on data from anything above the surface of
the ground that can reflect the electromagnetic energy (e.g., buildings, people, trees, ceilings, walls,
etc.). In order to avoid this effect, the antennas must be shielded from transmitting and receiving a
signal above the antenna.
Shielding
is relatively easy to accomplish for signals with a short wave-
length (high frequency), but it is difficult to achieve good shielding of antennas that transmit and
receive long wavelengths. In practice, it is very difficult to shield an antenna with a center-band fre-
quency below about 300 Mhz. The interpreter of GPR data must be aware of the fact that reflections
from above the ground will appear on the data, and these artifacts will need to be identified so they
are not interpreted as scattering from subsurface objects.
7.4 fIeld opeRAtIonS
In practice, GPR measurements can be made by towing the antennas continuously over the ground,
or at discreet points along the surface. These two modes of operation are illustrated in Figure 7.5.
The fixed-mode antenna arrangement consists of moving antennas independently to different points
and making discrete measurements. The moving-mode arrangement keeps the transmit and receive
antennas at a fixed distance with the antenna pair moved along the surface by pulling them by
hand or with a vehicle. Transmit and receive antennas are moved independently in the fixed mode
of operation. This allows more flexibility of field operation than when transmit and receive anten-
nas are contained in a single box. For example, different polarization components can be recorded
easily when transmit and receive antennas are separate. In the fixed mode of operation, a trace is
recorded at each discrete position of transmit and receive antennas through the following sequence
of events in the GPR system: (1) a wave is transmitted, (2) the receiver is turned on to receive and
record the received signals, and (3) after a certain period of time the receiver is turned off. The
resulting measurements recorded during the period of time that the receiver is turned on are called
a trace, and the spacing between measurement points is called the trace spacing. The chosen trace
spacing should be a function of the target size and the objectives of the survey. Traces displayed side
by side form a GPR time-distance record, or GPR cross section, which shows how the reflections
vary in the subsurface. If the contrasts in electrical properties (e.g., changes in permittivity) are
relatively simple, then the GPR time-distance record can be viewed as a two-dimensional pseudo-
image of the earth, with the horizontal axis the distance along the surface, and the vertical axis the
two-way travel time of the radar wave. The two-way travel time on the vertical axis can be converted
to depth, if the permittivity (which can be converted to velocity) is known. The GPR time-distance
record is the simplest display of GPR data that can be interpreted in terms of subsurface features. A
GPR time-distance record can also be produced by making a series of fixed-mode measurements at
a constant interval between traces on the surface.
7.4.1 f
i e l d
P
R o c e d u R e s
Field procedures for GPR measurements follow the basic design that must be followed for most
other geophysical methods. The field procedures for a specific objective are determined by answer-
ing a number of questions, including the following:
•
What are the objectives of the survey?
•
What is the nature of the subsurface environment?
•
What are the electrical properties of the materials at the site?
