Ground-penetrating radar ( GPR ) is a geophysical method that uses radar pulses to describe subsurface. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequency) of the radio spectrum, and detects the reflected signal from the subsurface structure. GPR can have applications in various media, including rocks, soil, ice, freshwater, sidewalks and structures. Under the right conditions, practitioners can use GPR to detect subsurface objects, changes in material properties, and voids and cracks.
GPR uses high frequency radio waves (usually polarized), usually in the 10 MHz to 2.6 GHz range. The GPR transmitter emits electromagnetic energy into the ground. When energy meets buried objects or boundaries between materials that have different permittivity, they can be reflected or refracted or redistributed to the surface. The receiving antenna can then record the variations in the return signal. The principles involved are similar to seismology, except that the GPR method applies electromagnetic energy rather than acoustic energy, and energy can be reflected in the boundaries in which subterranean electrical properties change rather than underground mechanical properties as well as seismic energy.
The electrical conductivity of the ground, the transmitted center frequency, and the radiated power can all limit the effective range of GPR investigations. Increased electrical conductivity weakens the introduced electromagnetic waves, and thus the penetration depth decreases. Because the attenuation mechanism is frequency dependent, the higher frequency does not penetrate as far as the lower frequency. However, higher frequencies can provide better resolution. Thus the operating frequency is always a trade-off between resolution and penetration. The optimal subsurface depth penetration is achieved in ice where penetration depth can reach several thousand meters (to the basement in Greenland) at low GPR frequency. Dry sandy soils or large dry matter such as granite, limestone, and concrete tend to be resistive rather than conductive, and penetration depth can reach 15 meters (49 ft). In wet and clay soils and materials with high electrical conductivity, penetration may be only a few centimeters.
Ground-penetrating radar antennas are generally in contact with the ground for the strongest signal strength; however, antennas launched by GPR air can be used on the ground.
Cross borehole GPR has evolved in the field of hydrogeophysics into a valuable tool for assessing the presence and quantity of ground water.
Video Ground-penetrating radar
Histori
The first patent for a system designed to use a continuous-wave radar to locate buried objects was proposed by Gotthelf Leimbach and Heinrich LÃÆ'¶wy in 1910, six years after the first patent for the radar itself (patent DE 237 944). Patents for the system using radar pulses rather than continuous waves were submitted in 1926 by Dr. HÃÆ'¼lsenbeck (DE 489 434), which leads to an increase in depth resolution. The depth of the glacier was measured using ground penetrating radar in 1929 by W. Stern.
Further developments in the field remained uncommon until the 1970s, when military applications began to encourage research. Commercial applications followed and the first affordable consumer equipment sold in 1975.
Maps Ground-penetrating radar
Apps
GPR has many applications in a number of fields. In Earth science is used to study the bedrock, soil, ground water, and ice. These are some utilities in finding gold nuggets and diamonds in alluvial gravel beds, by discovering a natural trap in buried flow beds that have the potential to collect heavier particles. The Chinese lunar lunar Yutu has a GPR at the bottom to investigate the soil and moon's crust.
Technical applications include nondestructive testing (NDT) of structures and sidewalks, the placement of hidden structures and utility channels, and study of soil and bedrock. In environmental remediation, GPR is used to define landfills, contaminant clumps, and other remediation sites, while in archeology is used to map archaeological and grave features. GPR is used in law enforcement to find clandestine graves and buried evidence. Military use includes mine detection, unexploded weapons, and tunnels.
Radar bore radar using GPR is used to map the structure of a drill hole in underground mining applications. The modern borehole directional radar system is able to produce three-dimensional images of measurements in one drill hole.
One of the other major applications for ground penetrating radar is to look for underground utilities. The standard electromagnetic induction mounting apparatus requires utility to be conductive. These tools are not effective at finding plastic channels or concrete storms and sanitary sewers. Because GPR detects variations in dielectric properties beneath the surface, it can be very effective for finding non-conductive utilities.
GPR is often used on the Channel 4 Time Team television program that uses technology to determine areas suitable for inspection by excavation. In 1992, GPR was used to return the £ 150,000 cash stolen by Michael Sams as a ransom for the property agent he abducted after Sams buried the money in a field.
Archeology
Land penetration radar surveying is one of the methods used in archaeological geophysics. GPR can be used to detect and map archaeological artifacts, features, and subsurface archaeological patterns.
The concept of radar is familiar to most people. With ground penetrating radar, an electromagnetic pulse signal - directed to the ground. The subsurface and stratigraphic objects (layering) will cause the reflection taken by the receiver. The travel time of the reflected signal indicates its depth. Data can be plotted as a profile, such as a planview map isolating a certain depth, or as a three-dimensional model.
GPR can be a powerful tool in favorable conditions (ideal uniform sandy soil). Like other geophysical methods used in archeology (and unlike excavations), it can find artifacts and map features without the risk of damaging them. Among the methods used in archaeological geophysics is unique both in its ability to detect some small objects at relatively large depths, and in its ability to distinguish the depth of anomalous sources.
The main disadvantage of GPR is that it is severely constrained by less ideal environmental conditions. The fine sediment (clay and dust) is often problematic because of its high electrical conductivity causing a loss of signal strength; rocky or heterogene sediments spread GPR signals, attenuate useful signals while increasing foreign noise.
In the area of ​​cultural heritage, GPR with high frequency antenna is also used to investigate historic masonry structures, detect cracks and patterns of column decomposition and fresco release.
Military
Military applications from ground penetrating radar include unexploded ordnance detection and tunnel detection. In military applications and other common GPR applications, practitioners often use GPR in conjunction with other available geophysical techniques such as electrical resistivity and electromagnetic induction methods.
Localization of vehicles
A new approach to localization of vehicles using images based on previous maps of ground penetrating radar has been demonstrated. Called "Localizing Ground Penetrating Radar" (LGPR), centimeter accuracy at speeds up to 60 mph has been shown. The closed loop operation was first demonstrated in 2012 for autonomous vehicle rudders and fielded for military operations in 2013. Highway speeds centimeter-level localization during evening snow storms are shown in 2016.
Three-dimensional imagery
The individual line of GPR data represents a cross-sectional view (profile) from below the surface. Several lines of data collected systematically in an area can be used to create three-dimensional images or tomography. Data can be presented as three-dimensional blocks, or as horizontal or vertical slices. The horizontal wedge (known as "deep slice" or "time slice") is basically a planview map that isolates a certain depth. Slicing time has become standard practice in archaeological applications, as horizontal patterns are often the most important indicator of cultural activity.
Limitations
The most significant performance limitations of GPR are in high conductivity materials such as clay and salt contaminated soil. Performance is also limited by signal scattering in heterogeneous conditions (eg rocky ground).
Other disadvantages of the current GPR system include:
- Radargram interpretation is generally not intuitive for beginners.
- Sufficient expertise to effectively design, perform, and interpret GPR surveys.
- Relatively high energy consumption can be a problem for extensive field surveys.
Radar is sensitive to changes in material composition, detecting change requires movement. When viewing stationary objects using ground-penetrating penetrating radar, the equipment must be removed so that the radar checks the designated area by looking for differences in material composition. While it can identify items such as pipes, cavities, and soil, it can not identify specific materials, such as gold and precious gems. However, it can be useful in providing subsurface mapping of potential gem-pad bags, or "vugs." The readings can be confused by the moisture in the soil, and they can not separate the jewel-bearing bags from the non-gem-bearing.
When determining the depth capability, the antenna frequency range determines the antenna size and depth capability. The distance of the scanned grid is based on the size of the target that needs to be identified and the required results. The typical grid spacing can be 1 meter, 3 feet, 5 feet, 10 feet, 20 feet for ground surveying and 1 inch-1 feet for walls and floors.
The traveling speed of a radar signal depends on the composition of the penetrated material. The depth to the target is determined based on the amount of time required for the radar signal to reflect back to the unit antenna. The radar signal runs at different speeds through different types of materials. It is possible to use the depth to a known object to determine a certain speed and then calibrate the depth calculation.
Power settings
In 2005, the European Telecommunication Standards Institute introduced a law to regulate GPR equipment and GPR operators to control the excess emission 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 technology
Ground-penetrating radar uses a variety of technologies to generate radar signals: these are impulses, frequency stepping, sustained frequency-modulated frequencies (FMCW), and noise. Systems in the market in 2009 also used digital signal processing (DSP) to process data during survey work rather than off-line.
A special type of GPR uses an unmodulated continuous wave signal. This holographic under surface radar differs from other GPR types because it records the subsurface holographic plan. The penetration of this type of radar depth is rather small (20-30 cm), but the lateral resolution is sufficient to distinguish different types of landmines on the ground, or cavities, defects, tapping devices, or other hidden objects on walls, floors, and structural elements.
GPR is used on vehicles for high speed highway surveys and landmine detection and in standby mode.
Inpipe-Penetrating Radar (IPPR) and In Sewer GPR (ISGPR) are GPR technology applications that are applied in no metal pipes where signals are routed through pipes and wall channels to detect wall thickness of pipes and voids behind the pipe wall.
Wall penetrating radar can read non-metallic structures such as those shown to ASIO and Australian Police in 1984 while surveying the Russian Embassy in Canberra. Show the police how to keep an eye on people who are in two rooms laterally and through the floor vertically, can see metal clumps that may be weapons; GPR can even act as a motion sensor for military and police guards, Project was first done in 1984 Canberra Australia. (Also used to detect "Ghost" on TV shows. {Air Disorder in locked room})
The "Mineseeker Project" seeks to devise a system to determine whether landmines are present in areas that use ultra wideband synthetic aperture radar units mounted on hot air balloons.
References
- Borchert, Olaf: Receiver Design for Borehole Directional Radar Systems Dissertation, University of Wuppertal, 2008, [1]
Further reading
An overview of scientific and engineering applications can be found at:
- Jol, H. M.. (Ed.) (2008). Ground Penetrating Radar Theory and Applications . Elsevier. CS1 maint: Additional text: author list (link)
An overview of geophysical methods in archeology can be found in the following works: Clark's, Anthony J. (1996). See Beneath the Soil. Method of Prospecting in Archeology . London, United Kingdom: B.T. Batsford Ltd.
External links
- EUROGPR - European GPR governing body
- GprMax - GPR numeric simulator based on FDTD method
- A short movie showing GPR readings, processing, and accuracy
- FDTD Animation GPR propagation example on Youtube
- GPR Electromagnetic Emission Security Information
Source of the article : Wikipedia
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