METAL DETECTORS - HOW DO THEY WORK? Metal detectors are versatile machines. They can be used as a scientific tool or simply for entertainment and fun. A of the people who use them are just as keen to talk about their favourite metal detector, as they are to get on site and use them. It is highly likely that feed back from detectorists discussing the merits and de-merits of their machines has resulted in the improvements and innovations to be found on the modern detector. It must be just as hard for the technical people working in laboratory conditions to translate the users slang as it is for us to understand the jargon they use! In these pages we hope to 'bridge the gap' and explain briefly in ordinary terms how metal detectors work. Do we know how they work? After all, anyone can switch on and go! That's true but a little understanding of what they do and how they do it can pay off in the field. In the end it is up to the individual. IN THE BEGINNING - BFO. A simple design based on a transistor radio. The presence of metal near the coil disturbed the generated frequency, which became audible in the radio. Although never totally abandoned as a concept, they were soon replaced by more sophisticated technology. VLF (Very Low Frequency) Transmit & Receive (TR) There are two coils of wire in the search head for this type of detector. One is the transmitting coil which has a small electric current passed through it. This current creates an electromagnetic field. The direction of the current flow is reversed several thousand times every second; the transmit frequency "operating frequency" refers to the number of times per second that the current flow goes from clockwise to anticlockwise and back to clockwise again while travelling through this coil. When the current flows in a given direction, a magnetic field is produced whose polarity (like the north and south poles of a magnet) points into the ground; when the current flow is reversed, the field's polarity points above of the ground. Any metallic (or other electrically conductive) object which happens to be nearby will have a flow of current induced inside of it by the Influence of the changing magnetic field, in much the same way that an electric generator produces electricity by moving a coil of wire inside a fixed magnetic field. This current flow inside a metal object in turn produces its own magnetic field, with a polarity that tends to be pointed opposite to the transmit field. Receiver A second coil of wire inside the search head, called the receive coil, is arranged (in a number of different configurations) so that nearly all of the current that would ordinarily flow in it due to the influence of the transmitted field is cancelled out. Therefore, the field produced by the currents flowing in the nearby metal object will cause currents to flow in the receive coil which may be amplified and processed by the metal detector's electronics without being drowned by currents from the much stronger transmitted field. The resulting received signal will usually appear delayed when compared to the transmitted signal. This delay is due to the tendency of conductors to impede the flow of current (resistance) and to impede changes in the flow of current (inductance). We call this apparent delay "phase shift". The biggest phase shift will occur for metal objects which are primarily inductive; large, thick objects made from first rate conductors like gold, silver, and copper. Smaller phase shifts are typical for objects which are primarily resistive; smaller, thinner objects, or those composed of less conductive materials. Strangely, Some materials which conduct poorly or even not at all can also cause a strong signal to be picked up by the receiver. We call these materials "ferromagnetic". Ferromagnetic substances tend to become magnetised when placed in a field like a safety pin which becomes temporarily magnetised when picked up with a bar magnet. The received signal shows little if any phase shift. Most soils and sands contain small grains of iron bearing minerals which causes them to appear largely ferromagnetic to the metal detector. Cast iron (square nails) and steel objects (bottle caps) exhibit both electrical and ferromagnetic properties. It should be made clear that this article describes an "Induction Balance" metal detector, sometimes referred to as "VLF" Very Low Frequency (below 30kHz). This is one of the more popular technology's in use at the present time, and includes the "LF" Low Frequency (30 to 300kHz) machines produced for gold and other prospecting. Discrimination Because the signal received from any particular metal object has its own characteristic phase shift, it is possible to classify different types of objects and tell between them. For example, a silver sixpence causes a much larger phase shift than an aluminium pull tab does, so the detector can be set to signal on a sixpence yet remain quiet on the pull-tab, and/or show the identification of the target on a meter or display screen. This process of distinguishing between metal targets is known as "discrimination". Easily the simplest form of discrimination allows a metal detector to respond with an audio output when passed over a target whose phase shift exceeds a certain (usually adjustable) amount. Unfortunately, with this type of discriminator the instrument will not respond to some coins and most jewellery. That is if the discrimination is adjusted high enough to reject common aluminium rubbish for example pull tabs and bottle caps. A more useful scheme is called "Notch Discrimination". With this kind of system, a notch in the discriminate response allows the metal detector to respond to targets within a certain range (such as the 10p/ring range) while still rejecting targets above that range (pull tabs, bottle caps etc) as well as below it (iron nails, foil etc). The more sophisticated notch metal detectors allow for each of several ranges to be set for either accept or reject responses. White's Spectrum XLT for example, provides 191 individually programmable notches. A metal detector may provide a digital readout, meter indication, or other display mechanism which shows the target's possible identity. We call this feature a Visual Discrimination Indicator, or V.D.I. Metal Detectors with this capability have the advantage of allowing the operator to make an informed decision about which targets he/she chooses to dig rather than relying solely on the instruments audio discriminator to do all the work. Most, if not all, V.D.I. metal detectors are also equipped with audio discriminators. Metal detectors are able to distinguish between metal objects based on the ratio of their inductance to their resistivity. This ratio gives rise to a predictable delay in the receive signal at a particular frequency. An electronic circuit called a phase demodulator can measure this delay. In order to separate two signals, such as the ground component (or mineralisation) and the target component of the receive signal, as well as to determine the likely identity of the target, we use two such phase demodulators whose peak response is separated from each other by one fourth of the transmitter period, or ninety degrees. We call these two channels "X" and "Y". A third demodulated signal, we call "G", can be adjusted so that its response to any signal with a permanent phase relationship to the transmitter (such as the ground) can be reduced to zero regardless of the strength of the returned signal. Some metal detectors use a microprocessor to monitor these three channels, determine the target's likely identity, and assigning it a number based on the ratio of the "X" and "Y" readings, whenever the "G" reading exceeds a predetermined value. This ratio can be found with a resolution of better than 500 to 1 over the full range from ferrite to pure silver. Iron targets are orientation sensitive; therefore as the search coil is moved above them, the calculated numerical value may change dramatically. A graphic display showing this numerical value on the horizontal axis and the strength of the signal on the vertical axis is extremely useful in distinguishing metal rubbish from more valuable objects. This display is called the "SignaGraph" (TM). Ground Balance As stated before, most soils and sands contain some amount of iron. They may also have conductive properties due to the presence of salts dissolved in the ground water. The result is that a signal is received by the metal detector caused by the ground itself. This may be thousands of times stronger than the signal coming from small metal objects buried at modest depths. Fortunately for us, the phase shift caused by the ground tends to remain fairly constant over a limited area. Therefor it is possible to arrange things inside the metal detector so that even if the strength of the ground signal changes considerably, like when the search coil is raised and lowered, or when it passes over a mound or hole, the metal detector's output remains constant. Such a metal detector is said to be "ground balanced". An accurately ground balanced detector makes it possible to "pinpoint" the find spot with a good deal of precision as well as to estimate the depth of the targets in the ground. If you choose to search in a non-discriminate, or "all-metal" mode, accurate ground balance is an absolute must!. The most simple form of ground balance consists of a control knob which the operator adjusts while raising and lowering the loop until a good balance is achieved. Although this method is good, it can also be time consuming, and some people find it to be difficult or confusing. More advanced metal detectors will perform ground balance automatically, typically by a two-step sequence in which the metal detector is balanced with the coil raised, then balanced once more with the coil lowered to the ground. The most sophisticated ground balance metal detectors will gradually adjust themselves as changes in the composition of the ground occur. We refer to this as "Tracking Ground Balance". A good tracking metal detector allows you to balance once, and then be able to search for the rest of the session without having to balance again. A word of caution here!. Many metal detectors which are advertised as having "automatic" ground balance are actually factory preset to a fixed balance point. It's a bit like having a radio that can pick up a couple of stations but not those above or below a fixed point! Motion/Non Motion Types The signal from the ground may be much stronger than the target signal, but the ground signal tends to remain the same, or change very slowly, as the search coil is moved. The signal from the target, on the other hand, will rise quickly to a peak and then subside when the search coil is sweeps past it. This opens up the possibility of using methods to separate ground from target signals by looking at the rate of change of the receive signal rather than looking at the receive signal itself. Metal detector modes of operation which rely on this principle are called, not surprisingly, "Motion" modes. The most important example is a mode called "Motion Discrimination". If we wish to isolate the target signal well enough to determine the target's identity, the ground balance alone is not enough. We need to look at the target from a couple of different perspectives, sort of like the way distances can by measured by triangulation if you have more than one viewpoint. We can only be ground balanced from one particular "viewpoint"; the other will contain some combination of target and ground signal. Fortunately, we can use the motion technique to minimise the effect of the remaining ground signal. At the present time, all discriminating and V.D.I. metal detectors require search coil motion to be effective. This turns out not to be much of a disadvantage in practice, since you have to move the search head anyway in order to cover any ground. After you have located a target in the motion discrimination mode, you will most probably want to locate it more precisely for easy recovery. If your metal detector is equipped with a depth meter, you will also want to measure the target's depth. "Pinpoint" locating and depth measurement are done in what is called the "All Metal" mode. Since discrimination is not required to perform these functions, search head motion is not usually required, apart from that motion required to get the coil over the center of the target. More precisely, the speed at which you move the coil is not important. The All Metal mode (also sometimes refereed to as the "Normal" mode) is therefore called a "Non Motion" mode. Silent search The development of SAT (self adjusting threshold) made it feasible to maintain a threshold just below the audible, this is why modern detectors do not have a background or threshold sound but only make a noise when they pass over a potential target. There are a few potential points of confusion in this. Because some detectors are equipped with a feature called "Self Adjusting Threshold", or S.A.T., which gradually increases or decreases the audio output in an attempt to maintain a quiet but audible "threshold" sound. This helps to smooth out audio changes caused by the ground or inadequate ground balance. S.A.T. may be very rapid or very slow depending on the metal detector and how it's tuned, but strictly speaking, S.A.T. implies a mode of operation in which motion is present. For this reason you will hear certain metal detectors described as having a "True Non Motion" mode; this means of course, an All Metal mode without S.A.T. Another sometimes confusing thing is that some discriminators allow for adjustment down to the point that the discriminator responds to all metals, in other words, it's a discriminator that doesn't discriminate. This is something very different, however, than the All Metal mode previously described. For this reason we often refer to it as a "Zero Disc" mode.