Ground Penetrating Radar, also known as GPR, is a tool that is used to find Underground Utilities, Underground Storage Tanks (USTs), and in some cases, Graves. The depth and accuracy are dependent on a number of variables, such as soil density, moisture content, and antenna frequency. We use a 350 MHz antenna, which has the potential to reach depths of up to 35 ft (in perfect situations).

1. A Brief Overview Of Ground-
Penetrating Radar

3. Ground-penetrating radar (GPR) is a
geophysical method that uses radar
pulses to image the subsurface. This
non-destructive method uses
electromagnetic radiation in the
microwave band (UHF/VHF
frequencies) of the radio spectrum, and
detects the reflected signals from
subsurface structures.

4. GPR can have applications in a variety of media, including rock, soil, ice, fresh
water, pavements and structures. In the right conditions, practitioners can use
GPR to detect subsurface objects, changes in material properties, and voids and
cracks.

5. GPR uses high-frequency (usually polarized) radio waves, usually in the range
10 MHz to 2.6 GHz. A GPR transmitter and antenna emits electromagnetic
energy into the ground. When the energy encounters a buried object or a
boundary between materials having different permittivities, it may be reflected
or refracted or scattered back to the surface.

6. A receiving antenna can then record the variations in the return signal.
The principles involved are similar to seismology, except GPR methods implement
electromagnetic energy rather than acoustic energy, and energy may be reflected
at boundaries where subsurface electrical properties change rather than
subsurface mechanical properties as is the case with seismic energy.

7. The electrical conductivity of the ground, the transmitted center frequency, and the
radiated power all may limit the effective depth range of GPR investigation.
Increases in electrical conductivity attenuate the introduced electromagnetic wave,
and thus the penetration depth decreases.
Because of frequency-dependent attenuation mechanisms, higher frequencies do
not penetrate as far as lower frequencies. However, higher frequencies may provide
improved resolution.

8. Thus operating frequency is always a trade-off between resolution and penetration.
Optimal depth of subsurface penetration is achieved in ice where the depth of
penetration can achieve several thousand metres (to bedrock in Greenland) at low
GPR frequencies.

9. Dry sandy soils or massive dry materials such as granite, limestone, and concrete
tend to be resistive rather than conductive, and the depth of penetration could
be up to 15 metres (49 ft).
However, in moist or clay-laden soils and materials with high electrical
conductivity, penetration may be as little as a few centimetres.

10. A GPR system is made up of three main components:
• Control unit
• Antenna
• Power Supply

11. GPR Control Unit and Antenna

12. The control unit contains the
electronics which trigger the
pulse of radar energy that
the antenna sends into the
ground. It also has a built-in
computer and hard disk/solid
state memory to store data
for examination after
fieldwork.

13. Some systems, such as the GSSI SIR 30, are controlled by an attached Windows
laptop computer with pre-loaded control software.
This system allows data processing and interpretation without having to
download radar files into another computer.

14. The antenna receives the electrical pulse produced by the control unit, amplifies it
and transmits it into the ground or other medium at a particular frequency.
Antenna frequency is one major factor in depth penetration.
The higher the frequency of the antenna, the shallower into the ground it will
penetrate.

15. A higher frequency antenna will also ‘see’ smaller targets. Antenna choice is one
of the most important factors in survey design.
The following table shows antenna frequency, approximate depth penetration and
appropriate application.

16. GPR Method

17. GPR works by sending a tiny pulse of energy into a material and recording the
strength and the time required for the return of any reflected signal.
A series of pulses over a single area make up what is called a scan.
Reflections are produced whenever the energy pulse enters into a material with
different electrical conduction properties or dielectric permittivity from the
material it left.

18. The strength, or amplitude, of the reflection is determined by the contrast in the
dielectric constants and conductivities of the two materials.
This means that a pulse which moves from dry sand (dielectric of 5) to wet sand
(dielectric of 30) will produce a very strong reflection, while moving from dry sand
(5) to limestone (7) will produce a relatively weak reflection.

19. While some of the GPR energy pulse is reflected back to the antenna, energy
also keeps traveling through the material until it either dissipates (attenuates)
or the GPR control unit has closed its time window.
The rate of signal attenuation varies widely and is dependent on the properties
of the material through which the pulse is passing.

20. Materials with a high dielectric will slow the radar wave and it will not be able
to penetrate as far. Materials with high conductivity will attenuate the signal
rapidly.
Water saturation dramatically raises the dielectric of a material, so a survey
area should be carefully inspected for signs of water penetration.

21. Metals are considered to be a complete reflector and do not allow any
amount of signal to pass through. Materials beneath a metal sheet, fine metal
mesh, or pan decking will not be visible.

22. Radar energy is not emitted from the antenna in a straight line. It is emitted
in a cone shape (picture on left). The two-way travel time for energy at the
leading edge of the cone is longer than for energy directly beneath the
antenna. This is because that leading edge of the cone represents the
hypotenuse of a right triangle.

23. Since it takes longer for that energy to be received, it is recorded farther down
in the profile.
As the antenna is moved over a target, the distance between the two
decreases until the antenna is over the target and increases as the antenna is
moved away.
It is for this reason that a single target will appear in the data as a hyperbola, or
inverted “U.” The target is actually at the peak amplitude of the positive
wavelet.

24. Data Processing

25. Data is collected in parallel transects and then placed together in the appropriate
locations for computer processing in a specialized software program such as GSSI’s
RADAN.
The computer then produces a horizontal surface at a particular depth in the
record. This is referred to as a depth slice, which allows operators to interpret a
planview of the survey area.

26. In many situations, a GPR operator will simply note the location of a target so
that it can be avoided.
For these clients, it may only be necessary to use a simple linescan format in
order to mark the approximate area of the target on the survey surface. Other
clients may require detailed subsurface maps and depth to features.

27. Why AIM?

28. Advanced Infrastructure Mapping, LLC is able to demonstrate a thorough
knowledge and understanding of major SUE activities and is able to provide
these services to the extent desired by the contracting agency.
AIM’s Expertise and Competence

29. Experience – Employees of Advanced Infrastructure Mapping, LLC are well-
trained, experienced, highly motivated, and capable.
Timeliness – The Resources utilized by Advanced Infrastructure Mapping, LLC
provide us with the ability to perform our services in a timely and in
professional manner.

30. Equipment – A wide range of equipment is necessary to detect the variety of
subsurface utilities that may be present.
Equipment utilized by Advanced Infrastructure Mapping, LLC includes state of the art
designating equipment; vacuum excavation or comparable non-destructive locating
equipment; and software systems compatible with those of the contracting agency.

31. Insurance – Advanced Infrastructure Mapping, LLC has adequate insurance
covering all aspects of SUE work.
Minimum amounts should be in accordance with the contracting agency’s
requirements.