Ground-penetrating radar is a geophysical method that uses radar pulses to image the
subsurface. This nondestructive method uses electromagnetic radiation in the microwave
band of the radio spectrum, and detects the reflected signals from subsurface structures.
GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and
structures. It can detect objects, changes in material, and voids and cracks.
GPR uses high-frequency radio waves and transmits into the ground. When the wave hits a buried
object or a boundary with different dielectric constants, the receiving antenna records variations
in the reflected return signal. The principles involved are similar to reflection seismology,
except that electromagnetic energy is used instead of acoustic energy, and reflections
appear at boundaries with different dielectric constants instead of acoustic impedances.
The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted
center frequency and the radiated power. As conductivity increases, the penetration depth
decreases. This is because the electromagnetic energy is more quickly dissipated 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 ice
where the depth of penetration can achieve several hundred metres. Good penetration is
also achieved in dry sandy soils or massive dry materials such as granite, limestone,
and concrete where the depth of penetration could be up to 15-metre. In moist and/or clay-laden
soils and soils with high electrical conductivity, penetration is sometimes only a few centimetres.
Ground-penetrating radar antennas are generally in contact with the ground for the strongest
signal strength; however, GPR air-launched antennas can be used above the ground.
Cross borehole GPR has developed within the field of hydrogeophysics to be a valuable
means of assessing the presence and amount of soil water.
Applications
GPR has many applications in a number of fields. In the Earth sciences it is used to study
bedrock, soils, groundwater, and ice. It is of some utility in prospecting for gold nuggets
and for diamonds in alluvial gravel beds, by finding natural traps in buried stream
beds that have the potential for accumulating heavier particles. The Chinese lunar rover
Yutu has a GPR on its underside to investigate the soil and crust of the Moon.
Engineering applications include nondestructive testing of structures and pavements, locating
buried structures and utility lines, and studying soils and bedrock. In environmental remediation,
GPR is used to define landfills, contaminant plumes, and other remediation sites, while
in archaeology it is used for mapping archaeological features and cemeteries. GPR is used in law
enforcement for locating clandestine graves and buried evidence. Military uses include
detection of mines, unexploded ordnance, and tunnels.
Before 1987 the Frankley Reservoir in Birmingham, England UK was leaking 540 litres of drinking
water per second. In that year GPR was used successfully to isolate the leaks.
Borehole radars utilizing GPR are used to map the structures from a borehole in underground
mining applications. Modern directional borehole radar systems are able to produce three-dimensional
images from measurements in a single borehole. One of the other main applications for ground
penetration radars to locate underground utilities, since GPR is able to generate 3D underground
images of pipes, power, sewage and water mains. This technology is often referred to as PAT,
short for Pipe Avoiding Tool. GPR is often used on the channel 4 television
program Time Team which uses the technology to determine a suitable area for examination
by means of excavations. In 1992 GPR was used to recover £150,000 in cash that kidnapper
Michael Sams received as a ransom for an estate agent he had kidnapped after Sams buried the
money in a field. Archaeology
Ground penetrating radar survey is one method used in archaeological geophysics. GPR can
be used to detect and map subsurface archaeological artifacts, features, and patterning.
The concept of radar is familiar to most people. With ground penetrating radar, the radar signal
– an electromagnetic pulse – is directed into the ground. Subsurface objects and stratigraphy
will cause reflections that are picked up by a receiver. The travel time of the reflected
signal indicates the depth. Data may be plotted as profiles, as planview maps isolating specific
depths, or as three-dimensional models. GPR can be a powerful tool in favorable conditions.
Like other geophysical methods used in archaeology it can locate artifacts and map features without
any risk of damaging them. Among methods used in archaeological geophysics it is unique
both in its ability to detect some small objects at relatively great depths, and in its ability
to distinguish the depth of anomaly sources. The principal disadvantage of GPR is that
it is severely limited by less-than-ideal environmental conditions. Fine-grained sediments
are often problematic because their high electrical conductivity causes loss of signal strength;
rocky or heterogeneous sediments scatter the GPR signal, weakening the useful signal while
increasing extraneous noise. Three-dimensional imaging
Individual lines of GPR data represent a sectional view of the subsurface. Multiple lines of
data systematically collected over an area may be used to construct three-dimensional
or tomographic images. Data may be presented as three-dimensional blocks, or as horizontal
or vertical slices. Horizontal slices are essentially planview maps isolating specific
depths. Time-slicing has become standard practice in archaeological applications, because horizontal
patterning is often the most important indicator of cultural activities.
Limitations The most significant performance limitation
of GPR is in high-conductivity materials such as clay soils and soils that are salt contaminated.
Performance is also limited by signal scattering in heterogeneous conditions.
Other disadvantages of currently available GPR systems include:
Interpretation of radargrams is generally non-intuitive to the novice.
Considerable expertise is necessary to effectively design, conduct, and interpret GPR surveys.
Relatively high energy consumption can be problematic for extensive field surveys.
Recent advances in GPR hardware and software have done much to ameliorate these disadvantages,
and further improvement can be expected with ongoing development.
Power regulation In 2005, the European Telecommunications Standards
Institute introduced legislation to regulate GPR equipment and GPR operators to control
excess emissions of electromagnetic radiation. The European GPR association was formed as
a trade association to represent and protect the legitimate use of GPR in Europe.
Similar technologies Ground-penetrating radar uses a variety of
technologies to generate the radar signal: these are impulse, stepped frequency, frequency-modulated
continuous-wave, and noise. Systems on the market in 2009 also use Digital signal processing
to process the data during survey work rather than off-line.
A special kind of GPR uses unmodulated continuous-wave signals. This holographic subsurface radar
differs from other GPR types in that it records plan-view subsurface holograms. Depth penetration
of this kind of radar is rather small, but lateral resolution is enough to discriminate
different types of landmines in the soil, or cavities, defects, bugging devices, or
other hidden objects in walls, floors, and structural elements.
GPR is used on vehicles for close-in high-speed road survey and landmine detection as well
as in stand-off mode. Pipe-Penetrating Radar is an application of
GPR technologies applied in-pipe where the signals are directed through pipe and conduit
walls to detect pipe wall thickness and voids behind the pipe walls.
Wall-penetrating radar can read through walls and even act as a motion sensor for police.
The "Mineseeker Project" seeks to design a system to determine whether landmines are
present in areas using ultra wideband synthetic aperture radar units mounted on blimps.
References ^ Daniels DJ. Ground Penetrating Radar. Knoval.
pp. 1–4. ISBN 978-0-86341-360-5. ^ Wilson, M. G. C.; Henry, G.; Marshall, T.
R.. "A review of the alluvial diamond industry and the gravels of the North West Province,
South Africa". South African Journal of Geology 109: 301–314. doi:10.2113/gssajg.109.3.301.
Retrieved 9 December 2012. ^ Penguin Dictionary of Civil Engineering
p347 ^ Birmingham Mail
^ Conyers, Lawrence B. And Dean Goodman 1997 Ground Penetrating Radar: An Introduction
for Archaeologists. Walnut Creek, CA.: Altamira Press
^ ETSI EG 202 730 V1.1.1, "Electromagnetic compatibility and Radio spectrum Matters;
Code of Practice in respect of the control, use and application of Ground Probing Radar
and Wall Probing Radar systems and equipment ^ An impulse generator for the ground penetrating
radar ^ Zhuravlev, A.V.; Ivashov, S.I.; Razevig,
V.V.; Vasiliev, I.A.; Türk, A.S.; Kizilay, A.. "Holographic subsurface imaging radar
for applications in civil engineering". IET International Radar Conference. Xi'an, China:
IET. doi:10.1049/cp.2013.0111. ^ Ivashov, S. I.; Razevig, V. V.; Vasiliev,
I. A.; Zhuravlev, A. V.; Bechtel, T. D.; Capineri, L.. "Holographic Subsurface Radar of RASCAN
Type: Development and Application". IEEE Journal of Selected Topics in Applied Earth Observation
and Remote Sensing 4: 763–778. doi:10.1109/JSTARS.2011.2161755. Retrieved 26 September 2013.
Borchert, Olaf: Receiver Design for a Directional Borehole Radar System Dissertation, University
of Wuppertal, 2008, [1] Further reading
Conyers, L. B.. Ground-penetrating Radar for Archaeology. Walnut Creek, CA., United States:
AltaMira Press Ltd. A general overview of geophysical methods
in archaeology can be found in the following works:
Clark, Anthony J.. Seeing Beneath the Soil. Prospecting Methods in Archaeology. London,
United Kingdom: B.T. Batsford Ltd. Gaffney, Chris; John Gater. Revealing the
Buried Past: Geophysics for Archaeologists. Stroud, United Kingdom: Tempus.
External links EUROGPR – The European GPR regulatory body
GprMax – GPR numerical simulator based on the FDTD method
Short movie showing acquisition, processing and accuracy of GPR readings
Ground Penetrating Radar Fundamentals FDTD Animation of sample GPR propagation on
Youtube