Ground-penetrating radar (applications)
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
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. 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. 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. 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.
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. 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.
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.
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.
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.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.
In the private sector
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.
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.
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