The Environmental Factors That Affect Metal Detectors

A metal detector is a device that responds to a metal substance that may not be readily visible. Most metal detectors consist of an oscillator that produces current from an alternating electromagnetic field. The detectors transmit magnetic fields and analyze return signals from the environment and from the target objects. This magnetic transmitter has a transmit coil with varying electric current. A change in the magnetic field that is being transmitted results in the production of electric currents called eddy currents. Metal detectors are widely used in different field applications such as mining.

Environmental Noise

• Metal detectors are sensitive to environmental magnetic noise. These environmental magnetic sources can include electromagnetic radio waves. However, this is highly dependant on the sensitivity of the metal detector and the amount of information frequency the detector receives. The sensitivity of the metal detector decreases with increasing frequency. The major man-made sources of electromagnetic noise include electrical mains, especially those located near homes, factories and main power lines. Others are TV and switch-mode power supplies that emit low levels of interfering signals, electrical fences and ignition systems that produce broadband frequency interference.

Earth Soils

• Earth soils are magnetizable. This magnetic property of soils is often referred to as mineralization. The degree of mineralization varies depending on the type of soil. This magnetization process results in the production of reactive signals with similar properties to the transmitting field signals. This is in addition to the small fraction of resistive signals produced. This soil property causes the signals to be easily detected by metal detectors. This interferes with metal target signals. The degree of mineralization is weak in soils that are of recent formation. Soils found on the earth's surface were formed a long time ago and therefore have high mineralization levels.

Fluorescent Lights

• Another form of interference originates from fluorescent lights. However, fluorescent lighting in itself is not the cause of problems in metal detectors. Rather, the interference problem originates from defective fluorescent light ballasts. Luminescent materials emit visible wavelength radiations that affect the radiation on infrared receivers. However, direct emission of the gas discharge carries the modulated signal of the lamp ballast. The infrared portion of the optical spectrum of the fluorescent light affects the infrared receivers that is in metal detectors.