Ground
penetrating radar (GPR) is a high frequency
electromagnetic
method that uses radio pulses to image
the subsurface.
This non-destructive method produces a
continuous
cross-sectional profile or record of subsurface
features that can
be applied to a number of engineering
and environmental
problems. GPR can be used in a
variety of
materials, including rock, soil, pavements and
structures. It
can detect buried objects, changes in
material, voids
or fractures.
A GPR system
radiates pulses of high-frequency
EM energy into the
ground from a transmitting
antenna. This EM
wave propagates into the ground
at a velocity
that is related to the electrical properties
of the subsurface
materials present. When this wave encounters the interface
of two materials having different properties (i.e., soil and
water, soil and steel), a portion
of the energy is
reflected back to the surface, where it is detected by a
receiver antenna and transmitted to a
control unit for
processing and display.
The depth of
penetration for Ground penetrating radar is
limited by the
electrical conductivity of the ground, and
the transmitting
frequency. As conductivity increases, the
depth of
penetration decreases. This is because the
electromagnetic
energy is more quickly converted into
heat, causing a
loss in signal strength at depth. Higher
frequencies do
not penetrate as far as lower frequencies,
but give better
resolution. Optimal depth penetration is
achieved in dry
sandy soils or massive dry materials
such as granite,
limestone, and concrete. Misunderstanding the
characteristics of the subsurface and the instrument response often
leads to misinterpretation of GPR data by less experienced
operators
