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A metal detector is a device for locating metal objects by exploiting the physical phenomenon of magnetic induction.
People often think of metal detectors as a tool for hunting for treasure.
While that is one application for them, they have many more applications than that, many of them pre-dating the treasure hunting phenomenon they are often associated with these days.
The practice of metal detection, for example, is widely used in the broader fields of security, such as in airports to detect hidden weapons on the passengers of an aircraft.
Also, metal detectors are used in the military field for mine clearance, in recreation for the search for various buried objects, in archeology for the search for old objects, and, marginally, in the medical field for the detection of metals before using an MRI machine.
But, yes, metal detectors are also widely used to find buried treasure.
Now, in order to limit damage to the archaeological and historical heritage, the use of metal detectors is regulated in various countries including France, Spain, and Belgium, for example.
Please use the following table of contents to navigate this article, as it is quite lengthy.
Table of Contents
Alright, let’s dig in!
Metal detector used to find unexploded bombs in France after the First World War (1919).
In 1881, the president of the United States, James Garfield, was victim of an violent attack with a gun. None of the 16 surgeons who treated him could locate the bullet that had remained in his body.
Alexander Graham Bell was summoned to his bedside and used a primitive metal detector to search for the ball, but without success.
During the First World War, metal detectors were used by surgeons to locate or even extract (with an electromagnet ) metal elements that had penetrated the body (the eye in particular).
In particular, a remote metal developer was used, in the form of an “audioscopic fingerstand”, based on the principle of Hughes’ electric scale; an earphone said phone placed on the ear of the surgeon indicates to him by means of a sound more or less strong if his finger with the fingerstall approaches or away from the metal object to extract.
According to H, Guilleminot, a large version of this detector would have even been tested to find unexploded ordnance in the ground, with results “which seem to be very encouraging,” said Guilleminot.
Gerhard Fisher is the inventor and builder of one of the world’s first broadband metal detectors, the “Metallascope”. It will be successful after founding in 1931 the company Fisher Labs to commercialize his invention.
Józef Kosacki is the inventor during the winter of 1941 – 1942 of a portable metal detector, the Polish mine detector. It was used for the first time by the British army for mine clearance in North Africa.
Operation and Physics
A metal detector works by exploiting a well – known physical phenomenon: electromagnetic induction. Only conductive objects, and in particular metals, are the seat of induced currents (eddy currents).
A metal detector is composed of two coils:
-the transmitting coil is powered by a sinusoidal alternating electric current.
-the receiver coil reacts to the magnetic field induced by the eddy currents and produces an electric current which has a phase shift with respect to the current of the transmitting coil.
A processor analyzes the intensity of the induced current as well as its phase shift to effect the detection. This analysis allows him to perceive the depth, the size and the conductivity of the object.
The larger the metal object, the more conductive it will be and the greater the frequency shift.
Similarly some metals are better conductors (money in particular) and the frequency shift will be even greater.
It is by studying this frequency shift that the detected object can be “discriminated”, that is to say, succeed in distinguishing and finding the nature of this metal.
But in practice, it’s a little different. Indeed a metal detector looks for metal but also its different alloys.
The electrical conductivity of an alloy is different from one alloy to another and its value can be close between two alloys of very different nature.
Thus 18-carat gold can be confused with aluminum foil and if aluminum foil is removed with the discrimination button, gold is also eliminated. Low-grade gold (14 karat) can even be visualized on the detector’s meter as iron.
In the end, the discrimination is valid only for small ferrous (small pieces of barbed wire or small nails) but not for large ferrous masses (kind horseshoe whose loop form is easily detected).
However, there are metal detectors capable of indicating the presence of a large ferrous mass without error.
The iron gives indeed a particular signal easily recognizable on an oscilloscope. Because of the variety of alloys, discrimination can not be considered completely reliable.
If all the metals were pure, discriminating would not be a problem.
The detection of a metal in the air by an electronic device is always done according to the same physical principle: the magnetic induction.
An electrical conductor carrying current produces a magnetic field in the surrounding space but the reverse is also true. A variable magnetic field produces a current in a conductor.
Through a coil, Faraday’s law gives the value of the electromotive force produced by the variation of the magnetic field.
The presence of a metal object near a coil changes its inductance, which is identified electronically by the change in electromotive force across the coil.
The first detectors operated according to the frequency beat principle but they performed poorly. The very low frequency technique gave a better sensitivity, but in the 1960s impulse induction was developed and is currently still the most used.
Frequency detectors were the first to appear because they are simple to implement but they are also the least sensitive.
The principle is the frequency beat. It uses, in fact, two oscillators , one fixed, the other sensitive to changes in the magnetic field.
The modification of the magnetic field of a coil influences, as we saw, on its inductance and thus, if an oscillator is built around this one, this one will have a frequency which reacts with the magnetic field and thus the presence of metal.
To use it, it suffices to compare the signal coming from this oscillator with a reference signal; the latter represents the signal of the first oscillator which would not be modified by the presence of metal.
The compared signal can be used to light a diode or be connected to an amplifier to hear through a loudspeaker the frequency difference if it is between 20 Hz and 30 kHz.
Very low frequency
The frequencies used are less than 20 kHz. This detector is composed of two coils, a transmitter and a receiver.
The emitter coil traversed by a sinusoidal current generates around it a magnetic field; when a metallic object passes into this magnetic field, eddy currents appear within it.
These currents in turn generate a magnetic field that tends to compensate for the magnetic field created by the transmitter coil.
The receiver coil will react to the magnetic field emitted by the metal object, an induced current will cross it. This current treated by the electronics makes it possible to know if there is or not a metallic object.
This detector makes it possible to discriminate metals and ferromagnets. The signal perceived by the receiver coil is out of phase with the transmitted signal.
The phase shift depends on the metals and thus makes it possible to discriminate them. For a prospector, the goal is to get rid of small iron objects first and foremost.
Discriminate the aluminum runs the risk of missing valuable targets alloy such as ridge , the electrum , the gossip (that were used through the ages to make currencies) and even the gold.
The oscillator frequency on which the detector operates conditions its quality of response to precious metals as well as its resistance to soil effects.
Thus, the higher its frequency (above 10 kHz and well beyond, around 20 kHz), the more it will be sensitive to scrap and disturbance of the soil and less he will feel the precious metals.
Below 10 kHz or even lower, the devices become insensitive to reducing ground noise of their performance 7 .
Very low frequency metal detectors, also called VLF (very low frequency), can be used with two types of detection disks: concentric disks and wide scan disks (also called double D disks).
The difference between these two types of disks comes from the internal arrangement of the transmitting and receiving coils. In a concentric disc, the two coils do not overlap, whereas in a wide scan disc they overlap on a small surface.
The pulse induction sensor requires only one coil. These detectors are very powerful in research in great depth. They can detect up to 1.50 m under the ground for small objects and up to about 3.50 m for large metal masses.
A powerful current pulse is sent into the coil. Each pulse generates a very short magnetic field. When the pulse ends, the polarity of the field reverses and collapses suddenly which causes a current spike, the return pulse.
This lasts a few microseconds and causes another current through the coil. The process is repeated. If the detector is above a metal object, the pulse creates an opposite magnetic field in the object.
When the pulse stops, the magnetic field of the object increases the duration of the return pulse. A test circuit makes it possible to control the duration of the return pulse.
Compared with the start length, the circuit determines whether another magnetic field has extended the decay time of the return pulse.
These detectors do not necessarily have the oval-shaped shape of conventional low-frequency detectors.
There are 1 × 1 m or 2 × 2 m frames that can be lifted with both hands, which can only detect fist- sized masses, so not the small parts.
But there are smaller frames, close to the size of a conventional tray, which they detect parts and other small objects, always in pulsed induction technology.
This type of detector is also in the form of two small frames connected by a bar attached to a conventional detector dashboard.
Their performance in discrimination is much less important than those at low frequency. On the other hand, they are much less sensitive to soil effects.
The first models were static (move from one detection zone to another and then remain static for detection), we now find semi-static where we can move very slowly on the ground.
Ground penetrating radar
The penetrating radar floor, also used to detect metal, now sells in portable apparatus for leisure detection as professional.
This equipment remains very expensive, but detection depths can reach 15 meters. The interpretation of the results is very delicate and these devices should only be used by specialists.
In detection, there exists on the middle and high-end detectors a potentiometer called “ground effect” or ground.
This allows the user to manually adjust the detector, depending on the mineralization encountered on different soils composed of a soil loaded with mineral oxides, so that it is always calibrated to a neutral stability level.
In this way, the device is not hindered by false signals inadvertent that provoke these oxides. It is good to know that this kind of earth is loaded with natural metallic particles; the load is positive and so-called “positive”, unlike the range that is loaded with salinity.
Since salt is not a metal, the charge will be negative. So the usefulness of such a setting on the beachfront will have no effect on the false signals generated by the magnetic field caused by the high salt content.sand .
Now some detectors have in addition to this setting a switch or other button called beach or range mode that allows the device to extend the range of adjustment in the negative charge so as to be free from the magnetic field created by salinity.
In conclusion, these manual adjustments will achieve the best power of penetration into the ground.
On the other hand, it is sometimes possible to find on the same ground different karstic layers, so necessarily a more or less mineralized soil.
As a result, the user will have to permanently adjust his soil effects according to the nature of the soil prospected.
On some detectors, the ground effect setting is calibrated by the manufacturer.
This setting is calibrated high enough that the device is stable on different soils weakly mineralized: no doubt a good compromise but at the expense of a few centimeters of penetration power.
This setting is probably the best that a detector can have, since it is the device that automatically performs the adjustment with a microprocessor that continuously analyzes and calibrates the best ground effects depending on the nature of the present field.
Of course, if such a system represents a real advantage and user comfort for the prospector, it is still at the expense of the detected target response time, which time, in milliseconds, is very important in detection.
On the dashboard of a detector can be found the following items to proceed to the settings of the detection parameters:
A VU meter or needle galvanometer generally graduated from 0 to 100 to indicate the signal intensity of the detected object, analyze its size and attempt a hypothesis on the nature of the detected object.
The firm Garrett Electronics has introduced to the market of digital analytical VU meters in the late 1990s.
A discriminating potentiometer to define the quality of the components of the object (from the iron nail to gold).
With the threshold function ferrous, a potentiometer can be compared to a discrimination whose range covers only ferrous.
A receiver or potentiometer tuner to make up ground effect, to match the quality of the land (presence or absence of iron in the soil or salt on a beach, both disrupting detection).
A sensitivity potentiometer to define how deep the user wants to explore.
A pinpoint function of very precise refinement of the location, this function is visual and concerns only the most modern detectors; otherwise it must be done by ear by distinguishing the rising of the declines of the sound signal of the frequency emitted above the detected object.
There are pinpoint rods on the market that are pointed in the hole to locate the exact epicenter of the detected object. It is also used on clods removed from the hole to check if the object has not already emerged.
A button for recording a detection frequency, the device only then searches for this one and only detection frequency.
A second press of the button allows the device to return to normal multi-detection mode. This same button allows the rebalancing of the basic settings after any modification.
A volume knob to set the sound volume at the speaker output.
A jack for connecting a headset.
A low battery indicator.
A cane attaching the case to the disc; it must be undetectable so as not to disturb the detection.
On the most modern detectors, there is no more potentiometer but the settings are made on a menu visible on LCD and modifiable by pressing buttons.
The dashboard is not necessarily on the detector rod but can be worn on the hipmount belt.
Minesweeper of the engineer company of the 13th half-brigade of the Foreign Legion in Djibouti in 2005 fiscal year, with a metal detector.
The aim of military demining is to enable units to clear their way through a minefield or to secure a military environment (camp, base) where mines have been hidden, humanitarian de-mining that tends to make mined land accessible, and safe for surrounding populations.
Metal detectors are sometimes used professionally in the field of archeology.
Indeed, the fundamental objective of this one is not the collection of metal objects but their discovery in the framework of methodical digs like the programmed search , by documenting the context with which they were associated.
Metal detectors are therefore used marginally for verification purposes or in emergency contexts as part of a pre-emptive search .
In addition, metal detectors are subject to uncontrolled use or “treasure hunts”, which may conflict with the preservation of heritage and considered clandestine excavation .
For the association Happah, which fights against the looting of the archaeological heritage:
“Recreational metal detector prospecting often has catastrophic effects on the integrity of archaeological sites. Most practitioners of the detection of leisure, the “detectorists” (tens of thousands) do not have authorization of prospection. Only a few dozen people have it in France.
Therefore, very many archaeological objects (several thousand if coins, fibulae etc. are taken into consideration) are not declared to the SRAand are not published. It is a gigantic loss of information, however crucial for science.
Historical and archaeological sites are then deprived of some of the potential information that they could have delivered in the case of a systematic survey (metal artifacts).”
Different legislative responses have been made to these threats depending on the country.
Finally, let’s talk about some laws related to metal detectors.
Laws Around The World
In Spain, national heritage laws (Leyes del Patrimonio Nacional) regulate the detection of leisure.
Thus, any prospection which does not benefit from an authorization is considered as a crime against the patrimony. Many associations have been created to bring about legislative change.
In Andalusia , the Law 14/2007 of the Andalusian historical heritage authorizes in its article 60 the use of devices of detection of the metal, allowing its use insofar as it does not put in danger the national heritage.
In France, the use of detectors to search for objects of interest to history or art without administrative authorization is punishable by law.
Outside this framework, the use of a metal detector does not require any special authorization, except that of the landowner.
The first detectors of leisure metals appeared in France in 1975. Previously, inventive researchers used mine detectors, modified or not, but their number remained confidential.
The use of metal detectors was first regulated by Law No. 89-900 of 18 December 1989 “On the Use of Metal Detectors”, which was repealed by Ordinance No. 2004-178 of February 20, 2004 which laid the legislative part of the heritage code.
Article L. 542-1 of the Heritage Code states that “no one may use any material that allows the detection of metallic objects to search for monuments and objects of prehistoric interest, history, art, or archeology, without first having obtained an administrative authorization issued on the basis of the applicant’s qualification and the nature and methods of the search “.
Measures included in the consolidated version of February 6, 2016: “Any advertisement or user manual concerning metal detectors must include a reminder of the prohibition mentioned in Article L. 542-1, criminal penalties incurred as well as reasons for this regulation “.
This law is intended to preserve the integrity of the archaeological levels of sites containing metallic objects: the scientific interest of these objects is largely related to their stratigraphic and archaeological context, the study of which is only possible within the framework of excavations methodical.
The use of metal detectors to search for ancient, archaeological or historical objects is strictly prohibited without authorization issued by the Ministry of Culture (Regional Department of Archeology) and validated by the prefecture.
Despite penalties of up to seven years imprisonment and fined 100,000 € or more when several looters are acting in combination, the archaeological sites of looting are still deplore.
The Ringlemere cup was discovered in 2001 by a British amateur using a metal detector. Dating from 1700 – 1500 BC BC, the cup, qualified treasure by the Treasure Act, was bought by the State at the market price and currently resides in the British Museum.
In Great Britain, the government promulgated the Treasure Act in 1996: national museums buy at the market price treasure finds made by amateurs.
In 1997, he set up the Portable Antiquities Scheme, a voluntary program to record the growing number of small archaeological discoveries, which do not constitute a treasure and therefore do not fall under the 1996 law, found by the public, particularly with a detector.
Detector users have also joined together to put a database online on finds and materials used for statistics and historical memory with photos of discovered objects; it is the UK Detector Finds Database .
The law relating to the discovery of a treasure is regulated by the Treasure Act of 1996.
This set of laws obliges whoever discovers a treasure to declare it within 14 days to the nearest coroner . He will investigate and determine if the discovery is part of a treasure.
If so, the discoverer must offer it for sale to a museum at a price set by a commission of independent experts. If a museum is not interested, the discoverer may keep.
Risks and Security
All the big objects are not treasures and the fighting places of the last two wars remind us regularly. By the very fact of their manufacture (iron, steel, lead, copper), the explosive devices detect themselves very well.
The use of metal detectors in these contexts requires great vigilance: it is recommended never to touch or dismount unexploded ordnance as it is always dangerous.
Every year, there are accidents in which there are detection practitioners who have not been able to observe this elementary rule of security.
For these reasons, the use of metal detectors is prohibited in some areas
Metal detectors are used safely, for example at airports to detect hidden metal weapons on the passengers of an aircraft.
Here is one last video showing that someone found a mortar bomb while in a river while using a metal detector.