Electromagnetics for Metal Detection (M-5)
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Q: How does it work?
A: A large coil sets up a strong magnetic field which induces a corresponding magnetic field within nearby metallic objects. The inducing field is abruptly terminated and the eddy currents within nearby conductors are sensed, often with the same coil used as the transmitter. The output is controlled by the amount of nearby metal and the orientation of the metal with respect to the axis of the coil.
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Q: What is the geologic model?
A: Buried metal (of any type) or other extremely conductive materials.
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Q: What are the requirements?
A: These instruments will detect metallic objects in the ground or on the surface and possibly do a better job on the latter. If the surface is not cleared (move the cars off the used car lot!) no subsurface detection will occur beneath the fences, cars, reinforced concrete, or known utilities. For UXO (unexploded ordnance) work research continues into discrimination between dangerous objects and teakettles.
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Q: What are the pitfalls?
A: The size of the object matters as does its orientation with respect to the coil axis. The instruments are quite robust though some very strong RF fields (under an AM radio station broadcast antenna, etc.) will affect the operation. The major pitfall is sorting out the target from the response due to known surface objects.
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Q: What logistics are needed?
A: Crew size is usually one person though another person can often help with gridding or other tasks to make the effort more efficient. Often these instruments are run in conjunction with a magnetometer or other instrument and a two-person crew is quite efficient.
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Q: What are the deliverables?
A: Plan maps of project location and interfering cultural structures, color contour maps of metal responses, interpretation maps with cultural responses and subsurface responses clearly separated.
