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Metal Detectors
Detectors with ground balancing are the best type of detectors for prospecting. Some models of prospecting detectors have better depth and sensitivity than many of the old types. They are most useful for detecting small nuggets, which would have been missed by the old detectors. Garret, Minelab and Whites prospecting models are popular.
Metal detectors will respond to any type of conductive or magnetic material. The metal detectors transmitting coil produces a primary electromagnetic field. When a conductive object encounters a primary electromagnetic field, currents flow through the surface of the object, called eddy currents, each producing its own secondary electromagnetic field. The secondary electromagnetic fields distort the primary electromagnetic field. A receiving coil within the metal detector receives the distorted primary electromagnetic field signal, ultimately producing an audio signal in response to the strengths of the secondary electromagnetic fields. Materials of greater conductivity produce larger and stronger secondary electromagnetic fields, and therefore audio signals, than smaller objects. For a given conductivity, the sizes and strengths of the secondary electromagnetic fields are controlled by the surface area facing the primary electromagnetic field rather than density or mass of the object. Larger surface areas produce larger fields and responses.
Manufacturers of metal detectors classify targets as either metal or mineral. Metal targets include all conductive, non-ferrous metals. Examples are silver, gold, copper and aluminium. Mineral targets consist of ferrous metals, magnetic minerals and conductive ground minerals. Examples of mineral targets are steel, iron, magnetite, iron oxide ground minerals and wet salt. These have lower conductivities than most metal targets. "Hot rocks", often encountered in the field, are concentrated forms of conductive iron oxide.
The audio signal produced usually varies according to the type of target. Gold tends to produce short, sharp signals while ferrous objects produce broad signals. A double blip will be produced on long thin objects, such as wire or nails. It is wise to do a bench test of different objects to determine their different responses. Some objects that can be used are a nail, silver coin and gold ring. With field experience, audio response from various targets, including "hot rocks", will become familiar so that identification will be easier.
Ground conditions affect the operation of metal detectors. Heavily mineralized and dense ground conditions cause the primary electromagnetic field to compress, resulting in loss of depth. Wet ground allows greater penetration of the primary magnetic field, providing better depth. Magnetic and conductive minerals (mostly iron oxide minerals) in ground soil produce background signals that can mask target objects. The ground cancel is used to decrease the effect of minerals in mineralized areas.
Headphones should always be worn when prospecting otherwise the batteries will drain quickly. Signals are also easier to comprehend with headphones. Some prospectors have an audio boost fitted to amplify small signals, while also suppressing very loud signals. Some audio boosts also operate with a more sensitive tone. They definitely make it easier to detect small signals. Hipmounts reduce strain when detecting for long periods or using large coils.
