Ground-penetrating radar (applications)

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Ground-penetrating radar (GPR) is an active geophysical subsurface exploration method. The method uses the reception of radiated electromagnetic waves that have entered the ground and returned after reflecting off objects in the subsurface. The method can be compared with active seismic acquisition which uses reflected acoustic waves. Although both use reflected waves, they are fundamentally different and thus have distinct applications. GPR data can be acquired in a number of ways: using small-area ground-based instrumentation such as is displayed in Figure 1 to the left, using aircraft-based instrumentation to gather data for a larger area, and using satellite-based observations to gather regional-scale data. A processed GPR dataset can be interpreted much in the same way as a seismic reflection dataset. Traces of returning electromagnetic waves are juxtaposed to create an image of reflectors in the subsurface like that in Figure 5 below. For more information on GPR interpretation and processing visit:Ground-penetrating radar. GPR data can be acquired continuously with a moving source-receiver pair, unlike seismic data, which requires an array of sources and receivers with distinct shots. This continuous data collection is what allows for the possibility of massive scale surveys. The possible scale of GPR study areas allows for many diverse applications, spanning from finding a pipe in a front yard to measuring the thickness of the ice caps of Mars.

In research and the public sector

Radio Glaciology

Electromagnetic radiation will reflect off of a boundary when a large change in electrical permittivity occurs across the boundary. Electromagnetic radiation is heavily reflected and attenuated in materials that are conductive such as air and water. Ice is a non-conductive and permits the travel of electromagnetic radiation relatively unimpeded. Because of this, The study of ice sheets and glaciers is perfectly suited for GPR acquisition.[1] Researchers in glaciology, climatology, sedimentary geology and planetary science are all curious about the deep structure and dynamics of glaciers and ice sheets. There are many active research groups studying ice in this way. Commonly the data is of regional scale in the case of ice sheets and local in terms of glaciers. Changes in permittivity are large between ice, air, water, and sediment. This means that EM waves will reflect and return to the instrument when traveling through ice and meeting a layer of air, ice of different permittivity, water, or sediment. This property of GPR allows for investigation of ice caves, tubes, and glacial drainage systems.[2] Layering in ice is seasonal and cyclical. As ice melts and refreezes over a period of time, solutes, dust, and other inclusions get concentrated into a layer. These layers are separated by periods of rapid ice growth in which there is little to no melting. This means layering can be used as an analogue for climatic periods when paired with ice core data.[3][4] Proessed GPR data of large ice volumes can therefore be interpreted to reveal both structure and climate history. Because GPR penetrates ice easily and seismic does not, it is useful for imaging shallow geology below ice bodies. GPR is collected in many ways for radio glaciology including satallite (Figure 3), airborne, and on-ground "sled aquisition" as shown in Figure 2. GPR is a prime target for studying planetary bodies in the outer solar system because they often have large amounts of ice, and the data can be collected via remote sensing. These studies are largely targeting Europa and Mars.

File:PIA21137 - Radargrams Indicating Ice-Rich Subsurface Deposit.jpg
Figure 3- NASA SHARAD orbiter detects ice in mars subsurface.

Hydrogeology

Hydrogeology is often investigated with GPR as there is a high permitivity contrast between dry and wet materials. Because of this, subsurface ground water is easily "seen" by GPR. Waterlogged rocks and sediments reflect a lot of electromagnetic energy and appear well in GPR data. Environmental scientists use GPR to measure the movement and size of groundwater bodies.[5] It is also used in municipal work in order to find and track groundwater sources for public use. Combining these two interests, GPR can track contaminated bodies of ground water in order to inform solutions to protect the public from consuming it through environmental remediation.[6]

Geologic interpretation

File:Mastiff and Panama Remote Controlled Vehicle MOD 45159064.jpg
Figure 4- Pictured is a Mastiff fitted with Choker Mine Rollers following on behind the Panama remote control Land Rover which carries a ground penetrating radar used for route clearance operations.

Most sediment and shallow rock bodies are relatively conductive, this means that GPR is attenuated quickly in the shallow subsurface. GPR penetration can be up to 50 ft in the most resistive rock formations (granite, limestone) and but a few centimeters in moist or clay rich soils. This limits the ability of GPR to be utilized for effective study of the deep subsurface. The high frequency of electromagnetic waves, however, results in very high resolution data, capable of resolving much smaller objects and features than seismic. In this way GPR is mainly utilized for finding caves and sinkholes, as well as shallow igneous and ore bodies.[7]

Government use

GPR can easily image metallic objects and current-carrying wires. This means a main application GPR is often used for is the location of conductive utilities such as metal piping and wiring. It is not effective for locating concrete sewers and plastic based piping which are non-conductive utilities. GPR can be used to define landfills or map their extent. Law enforcement and forensics groups often use GPR to locate graves, evidence, and stolen goods. The military uses GPR to locate and disarm underground mines, detect explosives and ordinance, as well as discover tunnels and bunkers. GPR is also used by law enforcement to find unmarked graves.

Archaeology

File:Calvary line 23.jpg
Figure 5- Ground penetrating radar Depth slice (profile view) showing the domed roof of a large subterranean crypt in an historic cemetery.

GPR is often used in archaeology because it is a nondestructive method capable of resolving small objects. Often it is used to discover and map larger features and patterning, as well as buried buildings and towns such as those imaged in Figures 5 and 6. In well mapped areas, GPR can be interpreted to discover smaller artifacts.[8]GPR can be used on historical structures themselves with high frequency antennas in order to detect and map cracks and degeneration in order to assist in their preservation.[9]

File:Calvary depth slices.jpg
Figure 6- Ground Penetrating radar depth slices of an underground structure. Plaview maps isolating specific depth are constructed from many lines of data collected at close intervals. The structure depicted is a crypt at an historic cemetery in Saint Paul, Minnesota, USA.

In the private sector

Mining

Often radar can be applied in a borehole to gain a three-dimensional image of surrounding rock. this is used in mining applications to map ore bodies much deeper than surface based radar acquisition. Because metallic ore bodies are very conductive, they reflect electromagnetic radiation and appear clearly in GPR data sets. GPR is usually a supplementary survey type and is usually paired with Electrical Resistivity Tomography, Seismic, and direct investigations such as blasting and drilling.

Construction

Radar is often used investigating construction projects. GPR can provide in sight into subsurface infrastructure and near surface geology. Detecting Karst geomorphology in the subsurface prevents later loss of property due to the opening of a sinkhole. GPR can be used on a concrete structure to reveal imperfections and fractures to better maintain aging buildings in the same way that it is used to preserve historical buildings and structures.

External links

https://en.wikipedia.org/wiki/Ground-penetrating_radar

https://www.military-technologies.net/2018/11/22/ground-penetrating-radar-gpr-market-analysis-global-industry-trends-share-key-players-size-forecast-to-2023/

https://ottawacitizen.com/news/world/archaeologists-use-ground-penetrating-radar-to-discover-huge-viking-ship-burial-half-a-metre-under-norway-farmland/wcm/17f57cc3-a8a7-4b49-bb38-9806b4acd262

References

  1. Plewes, L. A., & Hubbard, B. (2001). A review of the use of radio-echo sounding in glaciology. Progress in Physical Geography: Earth and Environment25(2), 203–236. https://doi.org/10.1177/030913330102500203
  2. Walford, M. (1985). Radio-glaciology: Exploration of temperate glaciers. Physics Bulletin., 36(3), 108.
  3. Arcone, S. A. (1996). High resolution of glacial ice stratigraphy: a ground-penetrating radar study of Pegasus Runway, McMurdo Station, Antarctica. Geophysics61(6), 1653-1663.
  4. Rasmussen, S. O., Bigler, M., Blockley, S. P., Blunier, T., Buchardt, S. L., Clausen, H. B., ... & Gkinis, V. (2014). A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy. Quaternary Science Reviews106, 14-28.
  5. de Menezes Travassos, J., & Menezes, P. D. T. L. (2004). GPR exploration for groundwater in a crystalline rock terrain. Journal of applied geophysics55(3-4), 239-248.
  6. Benson, A. K. (1995). Applications of ground penetrating radar in assessing some geological hazards: examples of groundwater contamination, faults, cavities. Journal of applied Geophysics33(1-3), 177-193.
  7. Collins, M. E., Puckett, W. E., Schellentrager, G. W., & Yust, N. A. (1990). Using GPR for micro-analyses of soils and karst features on the Chiefland Limestone Plain in Florida. Geoderma47(1-2), 159-170.
  8. Grasmueck, M., Weger, R., & Horstmeyer, H. (2004, June) Full-resolution 3D GPR imaging for geoscience and archeology. In Ground Penetrating Radar, 2004. GPR 2004. Proceedings of the Tenth International Conference on (Vol. 1, pp. 329-332). IEEE.
  9. N Masini, R Persico, E Rizzo. Some examples of GPR prospecting for monitoring of the monumental heritage. Journal of Geophysics and Engineering 7 (2), 190, doi:10.1088/1742-2132/7/2/S05