Electrical resistivity tomography
ERT or electrical resistivity imaging (ERI) is a geophysical technique for imaging sub-surfaces structures from electrical measurements made at the surface, or by electrodes in one or more boreholes. It is closely related to the medical imaging technique electrical impedance tomography, and mathematically is the same inverse problem. The technique evolved from techniques of electrical prospecting that predate digital computers, where layers or anomalies were sought rather than images. Early work on the mathematical problem in the 1930s assumed a layered medium (see for example Langer, Slichter). Tikhonov who is best known for his work on regularization of inverse problems also worked on this problem. He explains in detail how to solve the ERT problem in a simple case of 2-layered medium. During the 1940s he collaborated with geophysicists and without the aid of computers they discovered large deposits of copper. As a result they were awarded a State Prize of Soviet Union.
When adequate computers became widely available the inverse problem of ERT could be solved numerically, and the work of Loke and Barker at Birmingham University was among the first such solution, and their approach is still widely used. Applications of ERT include mineral prospecting, monitoring of ground water flow and archeology.
Ground-penetrating radar
GPR is a geophysical method that uses radar pulses to image the subsurface. This non-destructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) 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 transmitting and receiving antennae. 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 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, and the transmitting frequency. As conductivity increases, the penetration depth also decreases. This is because the electromagnetic energy is more quickly dissipated into heat energy, 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 dry sandy soils or massive dry materials such as granite, limestone, and concrete where the depth of penetration is up to 15 m. In moist and/or clay laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimetres.
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Ground-penetrating radar antennae are generally in contact with the ground for the strongest signal strength; however, GPR horn antennae can be used 0.3 to 0.6 m above the ground.
Applications: GPR has many applications in a number of fields. In the Earth sciences it is used to study bedrock, soils, groundwater, and ice. Engineering applications include Nondestructive testing (NDT) of structures and pavements, locating buried structures and utility lines, and studying soils and bedrock. In environmental remediation it is used to define landfills, contaminant plumes, and other remediation sites. In archaeology it is used for mapping archaeological features and cemeteries. It is used in law enforcement for locating clandestine graves and buried evidence. Military uses include detection of mines, unexploded ordnance, and tunnels.
Magnetometer
A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits. Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Magnetic anomaly detectors detect submarines for military purposes.
A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of the earth's magnetic field.
They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so the both the inclination and azimuth of the drill bit can be found.
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Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the earth's magnetic field. Magnetometers can be divided into two basic types:
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