Ground Penetrating Radar ( GPR )
Co-Authored by: Steven J. Tidwell and Christopher
For many project owners, the money invested in subsurface
utility investigation is well spent. Designers have documented
more efficient designs when the final designs are based on actual
site conditions and not on simple reliance on old plans and
records. A contractor can no longer afford to guess what lurks
beneath the ground where excavation or drilling will take place.
With the advent of new materals like fiber optic, plastic conduits/mains,
the challenge to locate and identify utilities has become significantly
more complicated. Add to this the ever-shrinking amount of right-of-way
and you have the formula for trouble.
Ground Penetrating Radar (GPR) is a safe, non-invasive geophysical
method for "looking" underground to locate subsurface
features. GPR can detect a variety of metallic, non-metallic,
natural and manmade underground utilities, storage tanks, rebar,
sinkholes and voids.
emits a series of high frequency, high amplitude electro-magnetic
pulses (radio waves) from a transmitting antenna into the ground.
When the pulses encounter any underground irregularities, a
portion of the energy is reflected back to a receiving element
at the surface. These reflections are collected as digital images
and fed to a portable laptop computer, which then displays a
real-time continuous "picture" or profile of a slice
of the subsurface area, pinpointing the precise location of
the subsurface feature. All substantial features in the subsurface
that differ in electrical composition from the surrounding soils
will produce a reflection. Subsequently, GPR is capable of locating
utilities of all materials.
For greater accuracy, the frequency of the emitted radar wave
can be increased. However, greater accuracy and resolution is
achieved at the expense of depth of penetration. Generally,
as antenna frequency and resolution increases, the maximum depth
of investigation decreases. Depth of penetration is also dependent
upon the geologic conditions of the soils in which the investigation
is being performed. The geology of the soil in which GPR is
used governs the performance of any GPR system. The radar waves
may be absorbed or scattered depending on the properties of
the soil, particularly its electrical conductivity. GPR works
best in low conductivity soils and is less effective in highly
conductive soils. Generally dry, sandy soils have low conductivities,
and wet, clayey, or saline soils are associated with high conductivities.
Because GPR data is immediately available for interpretation
and analysis, qualitative decisions regarding a particular site
can be made on the spot. Data is displayed on a monitor in real
time. Subsurface anomalies are detected, and the operator can
usually deduce by its appearance, and other factors, whether
it is a pipe or utility, or a natural geologic feature.
In addition, GPR data can be quickly and continuously collected
in long survey lines, allowing for greater data collection than
other comparable geophysical investigation methods can provide.
GPR is recognized as one of the most powerful remote sensing
instruments available today, making it a very effective tool
for subsurface locating and mapping investigations. The obvious
benefit GPR can bring to your project is the ability to locate
buried utilities, including non-metallic and metallic structures.
By doing so, GPR can help reduce downtime and the risk of utility
hits. In addition, GPR can also be used for a broad range of
engineering and environmental applications, including:
utility engineering (SUE)
- Utility locating and mapping
- Condition assessment of large-diameter utilities/drainage
- Underground storage tanks and buried drum locating
- Grave site identification and Forensic investigations
- Concrete assessments (rebar spacing, thickness, voids)
- Subsurface void and sinkhole locating
- Examination of structural integrity of roads
- Landfill boundary delineation
- Bedrock surface profiling
- Groundwater table mapping
- Conductive contaminant plume mapping
- Shallow bedrock fracture and fault mapping
- Archaeological investigations
The tasks of skillfully operating GPR equipment and interpreting
the resulting data require in-depth geophysical training and
field experience. In addition, it is necessary for the GPR consultant
to have a working knowledge of the geology of the targeted subsurface
area to fully understand and interpret the results and limitations
of a GPR survey at that location.
Despite the obvious advantages of GPR, it is not entirely foolproof.
It is an art as well as a science. Due to the ease and speed
of GPR data collection one may assume that being able to utilize
the resulting data for practical purposes is equally as fast
and easy. However, due to the ambiguities and intricacies associated
with GPR data, as well as the parameters that affect data acquisition,
this is not the case. The key to using GPR to its fullest potential
is high quality data interpretation, which is the product of
a well-designed GPR investigation performed by an experienced
geologist or geophysicist.
The future use of GPR depends upon an awareness of its abilities
and limitations. At the moment, GPR is being used extensively
by the engineering and construction industry to map utilities
and plan accordingly. The successful usage of GPR for subsurface
utility identification will be the result of the use of other
new technologies in combination, such as vacuum excavation,
and electro-magnetic pipe locators. GPR does not identify the
specific subsurface/utility type; hence, verification is necessary
using other methods. Non-destructive air-vacuum excavation is
used to determine the exact horizontal and vertical location
of utilities. The process involves removing the surface material
over approximately a 1'x 1' area at the determined horizontal
location. The air-vacuum process then proceeds with the simultaneous
action of compressed air-jets to loosen soil and the vacuum
extraction of the resulting debris. The process continues until
the utility is uncovered and physically verified.
Incorporating both technologies makes good engineering and
design sense for any type of design or construction project
- building airports, utilities, transit, or any other public
works construction - requiring excavation around existing underground
utilities or features.
These activities provide "quality levels" of information
or degrees of risk. The higher the level of information, the
less risk involved in accurately plotting the underground facility's
location. The highest level is only obtained when visual conformation
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