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174:. Fluxgate magnetometers utilize two ferromagnetic cores each wound with a primary coil (in opposite directions) and an outer secondary coil attached to an amp meter. When an alternating current (AC) is passed through the primary coils, it creates two opposing magnetic fields that vary in intensity based on the outside magnetic fields. By floating them parallel to the seafloor, they can measure the changes in magnetic fields over the seabed.
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Magnetometry is therefore useful for finding pits and ditches which have been backfilled with topsoil, with a higher magnetic susceptibility than the surroundings. Roads and structures are also visible from magnetic surveys since they can be detected because the susceptibility of the subsoil material
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used in geophysical survey may use a single sensor to measure the total magnetic field strength, or may use two (sometimes more) spatially separated sensors to measure the gradient of the magnetic field (the difference between the sensors). In most archaeological applications the latter (gradiometer)
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Every kind of material has unique magnetic properties, even those that we do not think of as being "magnetic". Different materials below the ground can cause local disturbances in the Earth's magnetic field that are detectable with sensitive magnetometers. The chief limitation of magnetometer survey
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come in two types: surface-towed and near-bottom. Both are towed a sufficient distance (about two ship lengths) away from the ship to allow them to collect data without being affected by the ship's magnetic properties. Surface-towed magnetometers allow for a wider range of detection but have lower
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Magnetometers react very strongly to iron and steel, brick, burned soil, and many types of rock, and archaeological features composed of these materials are very detectable. Where these highly magnetic materials do not occur, it is often possible to detect very subtle anomalies caused by disturbed
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configuration is preferred because it provides better resolution of small, near-surface phenomena. Magnetometers may also use a variety of different sensor types. Proton precession magnetometers have largely been superseded by faster and more sensitive fluxgate and cesium instruments.
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than most bedrocks or subsoils. This is because of the concentration of iron minerals in the topsoil, often weathered from the bedrock. Environmental processes such as repeated vegetation fires and redox reactions caused by wetting and drying of the soil convert iron compounds to
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In maritime archaeology, these are often used to map the geology of wreck sites and determine the composition of magnetic materials found on the seafloor. An
Overhauser magnetometer (PPM) was used in 2001 to map Sebastos (the harbor of
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magnetometers improved sensitivity, and greatly increased sampling speed, making high resolution surveys of large areas practical. Equally important was the development of computers to handle, process, and display large datasets.
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In terrestrial archaeology, magnetic surveys are typically used for detailed mapping of archaeological features on known archaeological sites. More exceptionally, magnetometers are used for low-resolution exploratory surveys.
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Boyce, Joseph I., Eduard G. Reinhardt, Avner Raban, and
Matthew R. Pozza. "Marine Magnetic Survey of a Submerged Roman Harbour, Caesarea Maritima, Israel." The International Journal of Nautical Archaeology 33 (2004):
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soils or decayed organic materials. Many types of sites and features have been successfully mapped with magnetometers, ranging from very ephemeral prehistoric campsites to large urban centers.
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are employed. The amount of detail is a function of flight height and sample density, in addition to instrument sensitivity. For surveys, drones are used which helps greatly in the process.
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138:. Data collection with proton precession instruments was slow, making high sample density surveys impracticable. Data were manually recorded and plotted. The subsequent introduction of
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Magnetic surveys are extremely useful in the excavation and exploration of underwater archaeological sites. The apparatus used on the water slightly differs from that on land. Marine
189:, basically turning them into dipole magnets. When the stimulus is removed, the protons precess at a rate that can be interpreted to determine the magnetic forces of the area.
185:) that, when agitated by a direct current (DC) or Radio Frequency (RF), cause the electrons to become energized and transfer that energy to the protons due to the
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Boyd, Thomas M. "Measuring the Earth's
Magnetic Field." Introduction to Geophysical Exploration. 30 June 1999. University of Melbourne. 9 May 2009
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Another common type is the newer proton precession magnetometer. This utilizes a container full of hydrogen-rich liquids (commonly
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The magnetic properties of archaeological materials form the basis for a number of other archaeological techniques, Including:
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Several types of magnetometer are used in terrestrial archaeology. Early surveys, beginning in the 1950s, were conducted with
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Le Borgne, E (1960). "Influence du feu sur le propriet´ es magn ´ etiques du sol et sur celles du schiste et du granite".
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Magnetic survey help to prove that a survey area has the potential for more detailed studies and scientific excavation.
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is that subtle features of interest may be obscured by highly magnetic geologic or modern materials.
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Magnetic
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66:. Magnetic surveys record spatial variation in the Earth's magnetic field. In
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The most common type of magnetometer used for marine surveying is the
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Magnetometry largely relies on the fact that the topsoil has a higher
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used in their construction is lower than the surrounding topsoil.
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http://www.whoi.edu/page.do?pid=8415&tid=3622&cid=14847
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Measuring the Earths' magnetic field is a very useful tool in
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Seeing
Beneath the Soil. Prospecting Methods in Archaeology
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Seeing
Beneath the Soil. Prospecting Methods in Archaeology
221:. To cover large areas with uniform data, aircraft such as
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Magnetic detection of archaeological artefacts and features
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Aspinall, Arnold; Gaffney, Chris; Schmidt, Armin (2008).
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Smekalova, T. N.; Voss, O.; Smekalov, S. L. (2008).
78:. Magnetic surveys are used in both terrestrial and
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http://www.earthsci.unimelb.edu.au/ES304/index.html
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484:. London, United Kingdom: B.T. Batsford Ltd.
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23:Magnetic survey of an archaeological site
366:Tite, M. S.; Mullins, C. (August 1971).
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276:"Geophysical survey at Pembroke Castle"
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475:. Lanham: AltaMira Press.
205:Airborne magnetic surveys
64:archaeological geophysics
246:Magnetic susceptibility
158:Marine magnetic surveys
103:magnetic susceptibility
504:Methods in archaeology
499:Archaeological science
349:Annales de Géophysique
278:. Castle Studies Trust
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219:geological mapping
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493:Categories
355:: 159–195.
306:Geophysics
261:References
392:0003-813X
334:0016-8033
227:airplanes
72:artefacts
282:27 April
183:methanol
179:kerosene
140:Fluxgate
86:Overview
76:features
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314:Bibcode
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248:survey
231:drones
229:, and
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