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Magnetic survey, one of the tools used by exploration geophysicists in their search for mineral-bearing ore bodies or even oil-bearing sedimentary structures and by archaeologists to locate and map the remains of buried structures. The essential feature is the measurement of the magnetic-field intensity and sometimes the magnetic inclination, or dip, and declination (departure from geographic north) at several stations. If the object of the survey is to make a rapid reconnaissance of an area, a magnetic-intensity profile is made only over the target area. If the object of the survey is to delineate already discovered structures, the surveyor sets up a grid over the area and makes measurements at each station on the grid. The corrected data is then entered on a scale drawing of the grid, and contour lines are drawn between points of equal intensity to give a magnetic map of the target area that may clearly indicate the size and extent of the anomalous body.The SI unit of measure for magnetic field strength is the tesla. As this is a very large unit for most practical uses, earth scientists commonly use the nanotesla (nT) as their working unit of measure. Engineers often measure magnetic fields in Gauss (1 Gauss = 100,000 nT, 1 Gauss = 100,000 gamma).
The Earth's magnetic field (the magnetosphere) varies both temporally (there is a daily variation of around 30 nT at mid latitudes and hundreds of nT at the poles) and spatially (from around 20,000 nT near the equator to 80,000 nT near the poles) for various reasons, such as the inhomogeneity of rocks and the interaction between charged particles from the Sun and the magnetosphere. Geomagnetic storms can cause much larger variations, but, on average, the Earth's magnetic field is relatively weak. A simple magnet from a hardware store produces a field hundreds of times stronger.
Magnetometers are distinct from metal detectors, which detect metallic objects by detecting their conductivity. Magnetometers can detect only magnetic (ferrous) metals, but can detect such metals at a much larger depth than a metal detector; magnetometers are capable of detecting large objects, such as cars, at tens of meters, while a metal detector's range is rarely more than 2 meters


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