Ground-penetrating radar (
GPR) is a
geophysicalGeophysics, a major discipline of the Earth sciences and a subdiscipline of physics, is the study of the whole Earth by the quantitative observation of its physical properties. Geophysical data are used in academics to observe tectonic plate motions, study the internal structure of the Earth,...
method that uses
radarRadar is an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
pulses to image the subsurface. This non-destructive method uses
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
in the
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
bandA band is a small section of the spectrum of radio communication frequencies, in which channels are usually used or set aside for the same purpose.-Examples:...
(
UHFUltra high frequency designates a range of electromagnetic waves with frequencies between 300 MHz and 3 GHz . Also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
/VHF frequencies) of the
radio spectrumA range of Radio frequencies that defines allowable or usable channels for specific radio transmission technologies. Radio Spectrum is typically Government regulated in most developed countries and in some cases sold or licensed to operators of private radio transmission systems....
, 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 transmitting and receiving antennas or only one containing both functions.
Ground-penetrating radar (
GPR) is a
geophysicalGeophysics, a major discipline of the Earth sciences and a subdiscipline of physics, is the study of the whole Earth by the quantitative observation of its physical properties. Geophysical data are used in academics to observe tectonic plate motions, study the internal structure of the Earth,...
method that uses
radarRadar is an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
pulses to image the subsurface. This non-destructive method uses
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
in the
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
bandA band is a small section of the spectrum of radio communication frequencies, in which channels are usually used or set aside for the same purpose.-Examples:...
(
UHFUltra high frequency designates a range of electromagnetic waves with frequencies between 300 MHz and 3 GHz . Also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
/VHF frequencies) of the
radio spectrumA range of Radio frequencies that defines allowable or usable channels for specific radio transmission technologies. Radio Spectrum is typically Government regulated in most developed countries and in some cases sold or licensed to operators of private radio transmission systems....
, 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 transmitting and receiving antennas or only one containing both functions. The transmitting antenna radiates short pulses of the high-frequency (usually polarized) radio waves into the ground. When the wave hits a buried object or a boundary with different
dielectric constantThe relative static permittivity of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. It is the ratio of the amount of stored electrical energy when a potential is applied, relative to the permittivity of a vacuum...
s, the receiving antenna records variations in the reflected return signal. The principles involved are similar to
reflection seismologyReflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or...
, except that electromagnetic energy is used instead of
acousticAcoustics is the interdisciplinary science that deals with the study of sound, ultrasound and infrasound . A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering...
energy, and reflections appear at boundaries with different
dielectric constantThe relative static permittivity of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. It is the ratio of the amount of stored electrical energy when a potential is applied, relative to the permittivity of a vacuum...
s instead of
acoustic impedanceThe acoustic impedance Z is a frequency f independent parameter and is very useful, for example, for describing the behaviour of musical wind instruments. Mathematically, it is the sound pressure p divided by the particle velocity v and the surface area S, through which an acoustic wave of...
s.
The depth range of GPR is limited by the
electrical conductivityElectrical conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current...
of the ground, the transmitted center
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
and the radiated power. As conductivity increases, the penetration depth also 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
resolutionOptical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
. Optimal depth penetration is achieved in ice where the depth of penetration can achieve several hundred meters. Good penetration is also achieved in dry sandy soils or massive dry materials such as
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
,
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
, and
concreteConcrete is a construction material composed of cement as well as other cementitious materials such as fly ash and slag cement, aggregate , water, and chemical admixtures...
where the depth of penetration could be up to 15 m. In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimetres.
Ground-penetrating radar
antennasAn antenna is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless...
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 scienceEarth science , is an all-embracing term for the sciences related to the planet Earth. It is arguably a special case in planetary science, the Earth being the only known life-bearing planet...
s it is used to study
bedrockIn stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
, soils,
groundwaterGroundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in...
, and
iceRadioglaciology is the study of glaciers and ice sheets using radar. It employs a geophysical method similar to ground-penetrating radar that operates at frequencies in the MF, HF and VHF portions of the radio spectrum...
. Engineering applications include
nondestructive testingNondestructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both...
(NDT) 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
archaeologyGeophysical survey in archaeology most often refers to ground-based physical sensing techniques used for archaeological imaging or mapping. Remote sensing and marine surveys are also used in archaeology, but are generally considered separate disciplines...
it is used for mapping
archaeological featuresFeature in archaeology and especially excavation has several different but allied meanings. A feature is a collection of one or more contexts representing some human non-portable activity that generally has a vertical characteristic to it in relation to site stratigraphy. Examples of features are...
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 ReservoirFrankley Reservoir is a semi-circular reservoir for drinking water in Birmingham, England. Its construction was authorised by the Birmingham Corporation Water Act of 1892...
in Birmingham, England UK was leaking 540 litres of drinking water per second. In that year GPR was used successfully to isolate the leaks.
Three-dimensional imaging
Individual lines of GPR data represent a sectional (profile) view of the subsurface. Multiple lines of data systematically collected over an area may be used to construct three-dimensional or
tomographicTomography is imaging by sections or sectioning, through the use of wave of energy. A device used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in medicine, archaeology, biology, geophysics, oceanography, materials science, astrophysics and other...
images. Data may be presented as three-dimensional blocks, or as horizontal or vertical slices. Horizontal slices (known as "depth slices" or "time slices") are essentially planview maps isolating specific depths. Time-slicing has become standard practice in
archaeological applicationsGeophysical survey in archaeology most often refers to ground-based physical sensing techniques used for archaeological imaging or mapping. Remote sensing and marine surveys are also used in archaeology, but are generally considered separate disciplines...
, 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 clayey soils and soils that are salt contaminated. Performance is also limited by signal scattering in heterogeneous conditions (e.g. rocky soils).
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 InstituteThe European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...
introduced legislation to regulate GPR equipment and GPR operators to control excess emissions of electromagnetic radiation . The European GPR association (EuroGPR) 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, FMCW and noise. Systems on the market in 2009 also use DSP to process the data, while survey work is being carried out rather than off line.
GPR is used on vehicles for close-in high speed road survey and landmine detection as well as in stand-off mode.
Wall-penetrating radar can read through
wallA wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air...
s and even act as a motion sensor for
policeA police service is a public force empowered to enforce the law and provide security through the legitimized use of force.The term is most commonly associated with police services of a state that are authorized to exercise the police power of that state within a defined legal or territorial area of...
.
GPR is used for hand held landmine detection to reduce the false alarms experienced by the standard metal detector and systems are available off the shelf (Vallon and L3Com Cyterra)
The "Mineseeker Project" seeks to design a system to determine whether
landminesA land mine is a target triggered explosive weapon. Their non-explosive predecessors have been used on the battlefield since ancient times. Landmines were designed to be used to deter, channel, delay and kill an enemy. They have been used in various formats, for centuries and have featured in all...
are present in areas using ultra wideband
synthetic aperture radarSynthetic-aperture radar is a form of radar in which multiple radar images are processed to yield higher resolution images than would be possible by conventional means...
units mounted on
blimpA blimp, or non-rigid airship, is an airship without an internal supporting framework or keel. A non-rigid airship differs from a semi-rigid airship and a rigid airship in that it does not have any rigid structure, neither a complete framework nor a partial keel, to help the airbag maintain its...
s.
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