Metal Detecting

ANSWERS TO SOME QUESTIONS FROM THE 'NEW DETECTORIST'


What causes a metal detector to produce a signal/sound ?

With the exception of Pulse Induction detectors (which contains a single search coil) a detector’s search coil is actually two coils in a single unit comprising a ‘transmit’ and a ‘receive’ coil. When switched on and set-up correctly with no metal influence present, the two coils produce a balanced magnetic field. When a target comes within range of the search coil's transmitted magnetic field, a small electrical eddy current is produced on the surface of any target/find. This in turn produces a secondary magnetic field which causes an imbalance in the search-coils magnetic field output thereby producing an audible response from the detector.

On salt wet sand my detector repeatedly indicates a target but nothing is found when I dig a hole

Detecting on dry sand with most metal detector holds few problems for detectorists. however, on salt wet sand, spurious signals can occur. There are several causes of such signals but they are usually due to the electrolytic reaction of salt water on the sand rendering negatively charged minerals to become ‘positive'. This has the effect of overwhelming the search coil’s balance causing the detector circuitry to produce a signal response. Any change in the height of the search coil whilst operating over wet sand will create such an imbalance between the transmit and receive coil windings resulting in spurious signals, thus it's important to maintain a constant search coil height but it's almost impossible to eradicate the effects completely. Reducing sensitivity or increasing the level of discrimination can sometimes be effective but they in turn reduce depth capability.

Why do some stones, coke and other non-metallic items produce a signal?

This phenomenon can be experienced on both beach and inland sites and is due to the electrical properties of a wide range of items. Coke, some pottery and earthen-ware are examples. Highly mineralised stones are generally referred to as 'hot rocks' having ‘capacitive’ rather than ‘conductive’ properties. When such targets come within range of the magnetic field, (search-coil output). They absorb and momentarily retain, a small amount of the electrical charge produced by the magnetic field, this in turn, generates a secondary magnetic field creating a change in the coil’s state of balance and resulting in a positive target response.

Tip:- To differentiate between the response from a 'hot-rock and that from a metal target, the target should be checked from several different angles. The audible signal produced by a 'hot-rock or a similar capacitive object, will be audibly identical each time regardless of the angle of approach (N.S.E.W). This will rarely be the case when locating metal targets unless they are uniformly circular in shape - e.g. complete coins/washers etc. Hot-rocks also produce a signal response which has a slight but noticeable sustain which indicates a 'hot rock' is responsible.

Will a larger search-coil locate deeper targets?

There is much conjecture on this subject but it’s commonly felt that any depth increase over and above the ‘standard sized’ coil will be relatively modest. Large coils are said to locate ‘larger’ targets at greater depth than a standard coil of say 8 or 9 inches which is why large coils are generally preferred for searching for hoards and larger artefacts. Large coils are less sensitive to the smaller desirable objects such as coin fractions (cut halves and quarters etc) however, they come into their own when searching a site of large acreage as they allow increased ground coverage in a similar time frame to that of a standard coil. If shallow finds are being made while using a large coil it would seem prudent to change to a smaller coil.

Can I use any search-coil on my detector which has the same coil connection?

Whilst it may be possible, it would be wise to check with the manufacturer first as there can be several incompatibility sources such as connections, wiring order of the pins and operating frequency variations.


Why does my detector sometimes produce a back-ground humming noise on some fields?

Assuming the anomaly is not due to operator settings, the culprit may be one or more of the following:

1. The presence of high voltage overhead power lines nearby. In rural areas, power supply lines are sometimes laid underground and in both cases the interference level may vary becoming more pronounced nearest the supply source and during peak supply times.

2. Electric Fences - These produce an audible pulsing signal which varies in strength depending upon proximity. Moving away from the source usually improves the situation.

3. Some detectorists report interference from their mobile phone, switching the phone to ‘Airplane' mode should resolve the issue.


Why does a detector sometimes produce a ‘signal’ when it’s knocked or during searching?

This is quite a common effect and there are several possible causes.

1. If the coil cable is wrapped very loosely around the base of the lower stem, it may be within detection range of the coils magnetic field - search coils transmit a 360 degree omni-directional magnetic field so they are equally sensitive to metal objects above the coil as below it!
To prevent this, the first 30cm length of cable (from the coil) should be fed straight up the lower stem, secured by velcro ties or electrical tape (but not plastic cable ties) and then coiled (not too tightly) around the upper stem and secured immediately at the point where the coil cable plug enters the socket at the control box. This will prevent the majority of ‘false signals’ occurring. It's important to set the stem to the operator's preferred length before securing the cable!

2. Some detectors have a coiled metal cable protector integrated with the coil cable/plug at the control box end. Over time they can ‘work’ their way further into the plug resulting in an intermittent short circuit within the coil cable plug. Remedy – occasionally give the cable protector a light ‘tug’ but be careful not to pull it out altogether!


What is a Pulse Induction detector and what are the advantages/disadvantages in using them?

Pulse Induction (PI) detectors are generally regarded as a ‘specialist’ machine for use in specific conditions. They differ from VLF/TR/IB/Multi-Frequency machines in several ways. There are both advantages and disadvantages over the usual VLF/TR/IB/MF detectors.


Advantages:

1. They are amongst the deepest seeking commercially available metal detectors.

2. They are very sensitive to metal targets (very small ferrous targets – tiny nails, pins etc.

3. They’re not affected by salt wet sand.


Disadvantages:

1. They currently lack effective discriminating ability. Some operators claim an ability to differentiate between ferrous/non ferrous signals) but.....

2. They require higher capacity batteries or an increased number of single cells.

3. As a single complete unit they are much heavier machines.

4. ‘Pinpointing’ a target accurately using the search-coil, can be difficult to master effectively.

5. Although ignoring most levels of ground effect, they are still capable of producing spurious signals when encountering particularly highly mineralised targets e.g. large hot rocks.


How does a Pulse Induction machine differ in operation?

In operation a Pulse Machine produces a strong magnetic field by issuing very powerful and rapidly occurring ‘pulses’ of electric current (up to a thousand per second).

Between each generated pulse, the search coil effectively ‘switches off’ and becomes a receiver ‘listening in’ for a response to indicate a target has been located.
Once a target is 'sensed' during the coil's 'switching-off period', a positive target indication is produced as an audio signal.


Confused About Discrimination and TID Meter Numbers, read on.

A better understanding of the Target Identity Meter

Newcomers to the hobby will be forgiven for supposing the ‘TID’ meter is able to correctly classify targets as ferrous or non-ferrous and in some cases, identify certain coin denominations or items of jewellery, unfortunately, this is not the case as you'll see later in this article

The Target Identity Meter (TID), was introduced on American manufactured detectors around the late 1970’s early 1980’s and it’s scale is specifically calibrated for use in the USA where the quarry is modern coinage the oldest of which will date no earlier than the 1790’s or thereabouts. It is for this reason, the TID Meter is much better suited for use in the USA rather than the UK or Europe. In contrast to our Americans cousin’s 300 years history of coin production, we in Britain have been producing a wide range of coinage in a variety of pure metals and alloys, over a much longer period, about 4,500 years! In recent times, American manufacturers are said to have addressed the meter calibration necessary to take account of the difference in the level of mineralisation between the USA and UK/Europe.

So the meter is not to be relied upon when it comes to classifying metal targets as ferrous or non-ferrous, simply because there are far too many external factors affecting. That said, the TID meter is a useful tool boasting some welcome display features designed to keep the operator more informed. Available features will vary between manufacturer models but some examples are:
Battery Condition, Mineralisation Levels, Target Pin-Pointing but a range of other selectable features may be available to the operator. Many meters include a range of discrimination settings accompanied by a visual range of colourful icons representing target ID ‘possibilities’! It should be remembered that no one target will produce the same target response due to the variation of ground minerals and other external influences

TID’s react according to the electrical conductive properties of target composition which is assessed from a processed signal produced by the search-coil and further analysed by the detector’s processing circuits to indicate ferrous/non-ferrous probability. It’s this next bit that causes confusion to the newcomer. It has to be accepted, that between the lowest ferrous reading and the highest non ferrous reading there lies a varied range of possible signal responses which vary according to target composition/condition/depth etc. Some signals produced are often wrongly termed ‘Iffy' signals - they are simply signals which the operator is unable to determine as ferrous or non-ferrous so it's always advisable to dig and note the result!

In simple terms: Low conductivity Targets = Low Numbers and Low Audio Tones.

Medium Conductivity Targets = Mid Scale Numbers and Medium Tones.

High Conductivity Targets = High Scale Numbers and High Tones.

If only it was that simple but - oxidisation, heavy mineralisation and proximity to iron contamination are all factors which affect ID indications.

Here’s just a few factors which in any combination, are capable of affecting the ability of a detector to produce an accurate ferrous/non-ferrous assessment of a target. Symptoms may include fluctuating meter numbers and a change in audio tones.

1. Deep Iron.
2. Target Depth
3. Target Condition/oxidisation/deterioration/applied surface coatings - enamelling/paint.
4. The proximity of ferrous objects to the target – nails/rusty iron etc.
5. Electrical Interference from Electric Fences, Power Lines (including below ground) Mobile
Phone Masts.

Note: Large iron, ferrous washers/iron rings or holed iron is liable to produce a ‘good’ ie. non-ferrous indication!! It can be seen by this, that a TID meter is far from the perfect tool but it's value largely lies in it’s ability to provide additional information which should be tempered with operator experience to enhance the probability of identifying a target as being ferrous or non-ferrous.

Some Additional Information Worth Noting


Metal detectors being of non standardised construction, circuit design, operating frequencies and other features creates a wide variety of performance issues for detectorists to contend with in their interpretations and operation. Add to that the influences produced by changing levels of on-site ground minerals, electrical interference and the possibility of a capacitive or rusty object near a responding target, all conspire to confound a machines ability to correctly identify the metallic composition of a target by meter indication alone despite manufacture’s claims, it’s just not possible!



There are other influencing factors too such as electric fences, pylons, overland/underground power lines and mobile phone ‘hub’ masts which seem to be on the increase everywhere. It must also be noted that some machines operating on a similar or harmonic frequency may, create electrical interference with other machines. In fact, in selecting a specific frequency change on some machines, will also pick up positive signals produced by a nearby detectorist's machine which can be a little confusing until realising what the cause is!

Some audio responses produced by targets including those arising from non metallic or ‘capacitive’ targets such as coke, some pottery types and natural stones (especially on the beach) can confuse the newcomer but they can be quite easily identified as capacitive targets tend to produce a signal with a slight sustain or sometimes a repeatable double ‘blip’ and the response will be identical regardless of the angle of approach to the target, this is in sharp contrast to the response produced by a non ferrous or ferrous target.

Systems to prevent/reduce the effects of unwanted interference in modern search coils and their cabling have improved over the years but there are reports whereby some machines still react by producing a positive indication when searching over wet grass, some tree roots and when encountering ‘cow pats’ depending on their particular mineral composition.

With time and practice, a detectorist will be able to make a better than ‘probable’ ID of targets from the audio response they produce. This ability is achieved over time and with practice involving many hours spent digging, assessing and retrieving finds over a protracted period. Similarly, it becomes possible to make a reasonable assessment of target size, shape and likely metal to add another useful layer for information for identifying what the target item is liable to be. A detectorist acquiring these skills will prove far more accurate than reliance upon a meter’s target ID numbers.

With time and practice, a detectorist will be able to make a better than ‘probable’ ID of targets from the audio response they produce. This ability is achieved over time and with practice involving many hours spent digging, assessing and retrieving finds over a protracted period. Similarly, it becomes possible to make a reasonable assessment of target size, shape and likely metal to add another useful layer for information for identifying what the target item is liable to be. A detectorist acquiring these skills will prove far more accurate than reliance upon a meter’s target ID numbers.

A Detectorist acquiring these qualities will prove far more accurate at correctly identifying signals than reliance upon Target Identity Meter numbers. Revised 24/04/25