Metal detectors sense the presence of any material that conducts electricity, making the majority of all metals detectable. The ability of a detector to successfully signal a target depends heavily on the metal’s physical properties and the technology used to process the signal. Hobbyists, security personnel, and geologists rely on this electromagnetic principle to distinguish between metallic objects, ranging from ancient coins to buried infrastructure. The complexity lies in the device’s capacity to identify which metal it is and filter out undesirable targets.
The Physics of Electromagnetic Detection
All standard metal detectors operate on the principle of electromagnetic induction, allowing them to sense electrically conductive materials beneath the surface. The device’s search coil generates a rapidly changing primary electromagnetic field that radiates outward into the ground. When this field encounters a metallic object, it induces tiny circulating electrical currents within the metal called eddy currents.
These eddy currents generate their own localized secondary magnetic field. The detector’s receiving coil senses this secondary field, noting the disruption in the original primary field. Modern devices like Very Low Frequency (VLF) detectors analyze the phase shift between the primary and secondary fields. Pulse Induction (PI) detectors use a single coil to send short magnetic bursts and then listen for the subsequent decay of the secondary field.
Categorizing Targets: Ferrous and Non-Ferrous Metals
Metal detection distinguishes targets based on magnetic properties, dividing materials into two groups. Ferrous metals contain iron, such as steel, cast iron, and oxidized scrap. These metals are magnetic and respond strongly to the detector’s primary field, making them readily detectable.
Non-ferrous metals do not contain iron, including gold, silver, copper, aluminum, lead, and brass. These metals are not magnetic, but they are highly conductive. Since non-ferrous metals are generally sought after for value or historical interest, the detector’s primary goal is to accurately distinguish these targets from abundant magnetic iron debris.
Conductivity and Target Identification
Target identification depends significantly on the metal’s electrical conductivity, which measures how efficiently it accommodates electric current flow. Non-ferrous metals vary widely in conductivity; silver is the most conductive, followed by copper and then gold. This variation dictates the strength and phase of the generated eddy currents, providing information about the target metal.
Highly conductive metals, such as large pieces of silver or copper, produce a strong, stable signal. These targets often register at the high end of the Visual Discrimination Indicator (VDI) scale. The VDI is a numerical range, typically 0–99, used by modern detectors to quantify the target’s electromagnetic response before excavation.
Lower conductive non-ferrous targets, such as small gold nuggets or thin aluminum foil, register lower on the VDI scale. Their response often overlaps with less desirable targets like pull tabs and iron contamination. Detectors use the VDI number and the signal’s phase characteristics to perform “discrimination.”
Discrimination allows the operator to selectively ignore signals from specific numerical ranges, typically those associated with iron or common junk. However, the conductivity response is not unique to a single metal. For example, a large piece of low-conductive gold can produce a signal similar to a small piece of highly conductive aluminum. Experienced detectorists rely on the VDI number, the signal shape, and the audio tone to make an educated guess about the buried material.
Environmental Factors That Limit Detection
Even metals with favorable conductivity can have their detection significantly limited by external environmental factors. The depth of the object has a profound effect, as the strength of the secondary magnetic field diminishes rapidly with distance. For instance, a highly conductive coin easily detected at six inches may be too weak to register accurately if buried at twelve inches.
Ground mineralization is a major variable, referring to the concentration of naturally occurring magnetic minerals, such as iron oxides, in the soil. When the detector’s primary field interacts with these minerals, the ground creates a strong electromagnetic response that can mimic or mask the signal from a buried object. Highly mineralized soil produces excessive background noise and reduces the machine’s overall depth penetration.
Modern detectors combat mineralization through ground balance, which electronically filters out the consistent background signal from the soil. The object’s size and orientation also affect the signal; an object lying flat generates a stronger, more stable VDI number than the same object lying on its edge. Furthermore, the long-term interaction of buried metal with the soil can create a “halo effect” where leached metal particles distort the VDI reading.