I was looking for old coins on the shore of a local river back when I first got into metal detecting and I kept getting false hits even though I was using discrimination. I couldn’t figure it out, which especially bugged me because
I usually have great luck with rivers. I went back to that river last week, now with much more experience, and immediately realized the problem… HOT ROCKS! To help you not make the same mistakes I did, I have put together some tips and tricks for identifying and dealing with Hot Rocks when Metal Detecting.
What Are These “Hot
Rocks” That Detectorists Keep Talking About?
Hot Rocks (sometimes known as “Cold Rocks”) are rocks, pebbles, or
sediment, which contain higher or lower amounts of conductive or nonconductive
minerals relative to the ground around them. Specifically, related to what your
metal detector is manually or automatically ground balanced.
Types of Hot Rocks Found While Metal Detecting
There are two main forms which Hot Rocks can be classified into. Firstly, Positive Hot Rocks, which contain higher amounts of conductive material. Secondly, Negative Hot Rocks, also known as “Cold Rocks” which contain higher amounts of nonconductive material.
Negative Hot Rocks – Think Magnetite
Negative Hot Rocks are very nonconductive as they contain (usually) high concentrations of Magnetite. Magnetite is an Iron Oxide (Fe3O4 to be exact) and will often cause the rock or sediment which it is in to become dark black in color. Additionally, it will cause the rock or sediment to become heavy due to the high atomic weigh of the Iron Oxide Molecule. One of the most common ways to encounter this type of Hot Rock in North America is “Black Sand”. This is just as it sounds, sand which is dark, or black, in color, and abundant in Magnetite.
Magnetite, however, can be used for
more than just disrupting your metal detectors ground balancing systems. In
places where Magnetite is common, it is also not uncommon for the soil to also
be gold-bearing. This is especially noticeable in streams and dry-washes of
gold-bearing regions due to the Magnetite being collected into a more
Tips for Identifying and Dealing with Negative Hot Rock Interference
Negative Hot Rocks, at shallow
depths, will exhibit a false-metallic audio response. This false-metallic audio
response of the metal detector will sound less definite and more general than
responses from “real” targets. This is often accompanied by a delayed
acquisition and then nulling of the audio response when you move your coil away
from the area and back to the target location. Some other characteristics of
the Negative Hot Rock audio response are: the response being unrepeatable or
only repeatable when swinging the coil one direction, the signal being
completely unable to be pinpointed, and in cases where the metal detector will
display a signal on a screen these signals will not be present on that devices
If you are using Manual Ground
Balancing then, besides hearing these false-metallic audio signals, you will
also have a lower detection depth in highly non-conductive sediments. One way
to fix this issue is by using the Non-Silent Search type accompanied with the
All-Metal discrimination mode, then re-balancing your metal detector. You will
notice a lowering of the Threshold tuning level before re-balancing which is an
indication of this process needing to be done.
If there are large inconsistencies
in the nonconductive Hot Rocks or Negative Hot Rocks at deeper depths then the
Threshold tuning level may become completely null. In this case, when using
All-Metal discrimination, nothing can be done to mitigate the effect on the
metal detector. Instead, in this case you will need to readjust the Manual
Ground Balancing until a small increase in the audio threshold is noticeable
when lowering the coil to the ground. This an adjustment which is known as
“Positive Offset” to the Ground Balance. This helps your metal detector
compensate for any sudden decreases which could be caused by the inconsistencies
or deeper signals.
If you happen to be using
Silent-Search, you will not notice any change in performance unless you stumble
upon a large Negative Hot Rock at a shallow or medium depth level. In this case
your metal detector will exhibit the previously described false-metallic audio
signal which can be ignored once you feel competent in recognizing the signal.
To reduce these false-metallic
audio, or non-audio, responses the best thing to do is to use the
discrimination features on your metal detector to eliminate ferrous responses.
This, of course is only available on Very Low Frequency metal detectors.
Furthermore, using these discrimination features may have an effect on your
coil’s sensitivity or responses to metallic objects which exhibit a
false-ferrous metallic signal. In the case that you are using a Pulse Induction
metal detector, Hot Rocks of any kind (besides non-Graphitic Positive Hot
Rocks) will not affect your metal detectors efficiency / responses.
Positive Hot Rocks – Think Maghemite
Positive Hot Rocks are very conductive as they contain (Usually) high concentrations of Maghemite. Maghemite is an Iron Oxide (Fe2O3 to be exact) and will often cause the rock or sediment which it is in to become red, reddish-orange, or yellow color. Although, Maghemite is an Iron Oxide, just like Magnetite, it exhibits lower ferrimagnetic properties (aka higher ferromagnetic properties) than Magnetite.
However, it is also possible that
Positive Hot Rocks are very conductive because they contain high amounts of
sulfide minerals. These minerals are Pyrrhotite (aka Magnetic Pyrite Fe(1-x)S
(x = 0 through 0.2)) and Bornite (aka Peacock ore, CuFeS4). Pyrrhotite will be
the same color as Maghemite but Bornite will be a copper-red or brown color
unless it has been tarnished. In which case, it will turn to a range of blue
and purple shades.
While these are the most common
Iron-Bearing Positive Hot Rocks, these is a tricky subsection of
non-Iron-Bearing positive Hot Rocks. These can be any rock or sediment material
which has high concentrations of copper ore, bauxite (aluminum), manganese,
gold, nickel, or commonly graphite. Graphite is a highly conductive
Carbon-based substance which you may know from the fact that most pencil leads
are now made of the Graphite substance.
Tips for Identifying and Dealing with Positive Hot Rock Interference
The non-Iron-Bearing forms of Positive Hot Rocks which are not Graphite, due to their distinct metallic properties, are difficult to deal with when it comes to ground balancing, although some tips will be given later in this article. When it comes to the Graphitic Positive Hot Rocks, however, there is a simple solution to dealing with its interference on your metal detector.
Turn down the sensitivity of the metal detector if is manually ground balanced and simply reset the metal detector if it is automatically ground balanced. Graphitic Hot Rocks will have a positive audio signal, be black or darker in color, and will leave a mark on your skin and other materials when touched.
As I mentioned, the signals from the non-Graphitic Positive Hot Rocks have their own distinct signal because they are all metals which could be found in thing which would be considered “real” targets. This makes them the most difficult type of Hot Rock to deal with by far, and also means that in some cases there is no other way to deal with them than to dig them up and find out for yourself. In areas where these types of Hot Rocks are abundant, this has been known to be a serious problem.
I have never experienced this, although I have been told they may be more common than you would think. There are some ways which very experienced detectors have claimed to be able to discern between these Hot Rocks and “real” targets although they take years of constant practice to theoretically be learned. I say theoretically because there is no proof that these methods are actually efficient at making a certain distinction.
Tips for Eliminating the Effects of Mineral Salts when Metal Detecting
Along with Positive Hot Rocks, high levels of mineral salts can cause sediment to have a higher conductivity than would be expected. This is because various mineral salts can exhibit conductive properties which produce false-magnetic signals from your metal detector.
The most common place to find these mineral salts is on the beaches of the ocean as they have been concentrated by the evaporation of ocean water. Also, because this is one of the most common places where metal detectorists will go for metal detecting (if they live near an ocean) so the statistic is somewhat skewed, but is effectively useful for determining some “best practices” when you know you may encounter these mineral salts.
Mineral salts will not pose an
issue for you if they are completely dry. This means that if the soil or sand
is completely dry then they will not cause any false-metallic signals to your
metal detector. However, if they are wet then they can become conductive, and
if they are concentrated enough, they can present as a metallic signal. These
false-metallic signals can be discerned from true-metallic signals as I
described above, and also can be completely eliminated when using a Pulse Induction
I was on a metal detecting outing with a couple of friends of mine at the beach the other day and I couldn’t get a single hit because the sand was mineralized, but my friends were finding tons of coins. I asked them how and they told me that their metal detectors were Pulse Induction, I responded with “What is a Pulse Induction metal detector?”.
What is Pulse Induction in Metal Detecting?
A metal detector coil technology (the other is Very Low Frequency) which creates an electromagnetic field and then allows it to collapse (this is the pulse). Pulse Induction metal detectors offer no discrimination features, however are often used because they are not affected by mineralized ground.
How does a Pulse Induction Metal Detector Work?
Pulse Induction metal detectors work by sending a high amperage signal through a, usually, copper coil to create an electromagnetic field. This electromagnetic field is then allowed to collapse which in turn creates a voltage spike that is able to be detected by the receive coil.
If the electromagnetic field is in contact with a metallic object then the voltage spike is affected. This happens because the metallic object stores a small amount of the energy from the electromagnetic field within itself in the form of Eddy currents.
Eddy currents are formed by the natural response of a conductor to a varying magnetic field, this is the reason which Pulse Induction metal detectors create an electromagnetic field and allow it collapse. The collapse is opposite in direction and polarity to that of the initial pulse. This is also why Pulse Induction is named as such, because the Eddy currents are Induced by the metal detector rather than simply existing as a function of the metallic object.
Very Low Frequency (also known as an Induction Balance metal detector), in contrast to Pulse Induction, is a metal detector coil technology which works by allowing a constant alternating current to pass through a copper coil. With a Very Low Frequency metal detector the transmit coil and the receive coil must be separate due to the constant nature of the electromagnetic transmission. This is different from a Pulse Induction coil which can have the transmit and receive coil be the same coil which simply serves both functions.
The signal which is received by a Pulse Induction metal detector is sampled by a mini-processor and then averaged into another signal. This new signal is then converted to direct current and further processed by a beat frequency oscillator (only used in older detectors, otherwise this function is done by a mini-processor). A beat frequency oscillator is a device that was originally developed to make Morse code audible. In a metal detector it is used to make the signal of the receive coil audible to the user.
Pros and Cons of a PI Metal Detector?
Pros of PI Tech Metal Detectors:
Mineralization? No Problem
The main reason why Pulse Induction technology was invented was to solve a common problem that users were having which is that of the interference of highly mineralized ground on signal reliability. Pulse Induction metal detectors are not affected by mineralization due to the fact that these non-metallic ferrous minerals are often not very good conductors.
Generally, Greater Depth
In contrast to their Very Low Frequency counterparts, Pulse Induction metal detectors have the ability to detect metal at much greater depths. This is often a feature of the fact that Pulse Induction coils can be larger due to the initial signal having more power than a Very Low Frequency signal does.
Only Option for Salt Water
This advantage comes as a function of Pulse Inductions ability to ignore mineralization and Very Low Frequencies inability to do so. The salt, and other minerals, in salt water make it almost impossible to use almost any Very Low frequency metal detector on the beaches of the oceans or in the oceans themselves.
Best for Gold Nuggets
Although it is uncommon for most users to need this feature, if it is gold that you are trying to find then you may experience mixed results with a Very Low Frequency metal detector. However, due to the rejection of ground mineralization and powerful search depth and recognition of Pulse Induction even very small gold nuggets are detected.
Cons of PI Tech Metal Detectors:
Like most newer technologies Pulse Induction is still generally more expensive than their Very Low Frequency counterparts. If this is not an issue then there may be some high-end Pulse Induction metal detectors which minimize the negatives, however if the high price is an issue then you might want to look for a Very Low Frequency metal detector instead.
Make and Model
Garrett ATX Deepseeker
Minelabs GPX 5000
JW Fishers Pulse 8X
Very Low Frequency metal detectors have been around longer and are more ubiquitous among manufacturers. There are some which have a higher price tag but many can be found for around a hundred dollars.
The one thing that everyone who has used a Pulse Induction metal detector knows is that it has no discrimination features. Due the way Pulse Induction detects metal it is unable to determine anything other than whether it is there or not.
This means that you need to be ready to be digging on every hit. If the area you are searching is riddled with pop-can tabs or nails, or any variety of junk, a Pulse Induction metal detector will not be able to help you filter these annoyances out.
Earth Field Effect Signal
Pulse Induction metal detectors are somewhat susceptible to the Earth’s magnetic field. The way this effect presents itself to most users is at the end of every swing it may sound like there is a hit but this is not true. It is easy to get accustom to but can still be annoying to deal with.
Pulse Induction vs Very Low Frequency Metal Detector: Which is Better?
There is no short answer to this question because, in my opinion, they serve completely different purposes. I mean they both detect metal, don’t get me wrong they both do that. What I mean is that they both have advantages and disadvantages to their respective coil technologies.
For Very Low Frequency metal detectors their advantages are more useful for the average user. If you are a beginner or simply someone who doesn’t use their metal detector in a variety of places than this is the one you will want to get. Discrimination features and the general proliferation of Very Low Frequency technology makes these metal detectors a great option.
However, if you are a looking for gold nuggets in Western-Australia, or if you are looking on a salt-water beach, or if you have any of the specific needs that a Pulse Induction metal detector was built to serve, then a Pulse Induction detector is the best option. Another reason to get a Pulse Induction metal detector is if you are looking to search deeper in the ground as this is often where historical remnants can be discovered.
The decision of which coil technology to choose comes down to: price-range, geography of the area where you will be metal detecting, the type of target you are looking for, and your level of experience with metal detectors. Neither metal detector technology is better than the other one all of the time, or in all scenarios.
Where to Find a Good Pulse Induction Metal Detector?
Just like with Very Low Frequency Metal Detectors, there are a myriad of companies which sell Pulse Induction Metal Detectors. However, there are three pulse induction metal detectors which are standouts within their price range and are often chosen by consumers.
Garrett ATX Deepseeker – Most Popular
The Garrett ATX Deepseeker (Amazon link to read more) is an extremely popular Pulse Induction metal detector and not just because it is made by, perhaps, the most successful metal detector brand. This metal detector is water-proof up to 10 feet of submersion, features automatic ground balancing, and a gigantic 10-by-12-inch coil.
Minelab GPX 5000 – The Best PI Metal Detector
Minelab is a company known for making the best Pulse Induction metal detectors in the world. If price is not an issue then this is the Minelab GPX 5000 (Amazon Link) Pulse Induction metal detector is the one to get.
This machine has every feature you could need and some that you don’t know you need yet, but is still easy enough to use that a beginner could start using it out of the box. This is somewhat a byproduct of the machines automatic ground balancing and enormous LCD display.
JW Fishers Pulse 8X Version 2 – Underwater King
The JW Fishers Pulse 8X is built on UNDERWATER salvage ad scanning technology. Want the best for diving in low to no visibility? Get a JW Fishers 8X the standard for this kind of work.
The JW Fishers Pulse 8X can detect all metals and, importantly, even in area with ground mineralization. Moreover, the JW Fishers Pulse 8X can be used underwater and in salt-water. The waterproof feature is rated to withstand up to 250 feet of depth.
The price may seem lofty for most consumers who are used to Very Low Frequency prices, but this is actually what a medium-grade Pulse Induction metal detector costs.
When to Use a Pulse Induction Metal Detector?
It may be confusing to know when you should switch from a Very Low Frequency metal detector to a Pulse Induction metal detector and vice versa. However, when you understand what things too look out for and what things you need to think about before you go out searching, it can be easy.
Firstly, if you know you will be searching in salt water or on salt water beaches, if possible use a Pulse Induction metal detector. The salt in the water, and the other minerals in the sand, will create signals that can confuse Very Low Frequency metal detectors and possibly cause you to miss hits or get hits that don’t exist. If you don’t have access to a Pulse Induction metal detector the discrimination feature on Very Low Frequency machines can be used to somewhat mitigate this problem. However, the best option in Pulse Induction.
Similarly, if the soil in the area you are going to be searching is mineralized you may want to consider Pulse Induction. You can tell that soil is mineralized by crumpling it in your hand and seeing if the consistency is mostly small rock like particles, or sand like particles, instead of a fluffy biological waste product. Again, Very Low Frequency detectors can be used in this scenario but a Pulse Induction machine may be better.
Moreover, if you are a beginner, or someone who is just getting a feel of the hobby, just buy a Very Low Frequency metal detector. You will have a better experience, most of the time, due to the manufactures attempt to make a machine that works best in all scenarios. A Pulse Induction metal detector will often cost more and be geared to a specific use case that you might not fully understand how to utilize.
Wow! The thrill of finding silver – there’s nothing like it! It’s not just the monetary value, but silver in the ground often does not tarnish, so it pops out of the dirt in brilliant glory! Here’s how you can fine-tune your detecting skills to collect more silver for yourself.
The Sweet Sounds of Silver (Metal Detecting)
a) Detector Sounds – An Important Skill
Using your own ears to distinguish between detector sounds is a basic skill, but it deserves a second look when your goal is digging up silver.
Remember the classic “blub-uh-dub” sound you get from a soda can pull tab.
Compare that to the clear, steady “twang, twang, twang” you get from a copper penny.
The difference between the pull tab and the coin is about the same as the difference between a copper penny and silver.
Silver produces a louder, consistent signal, a rock-solid “zing, zing, zing!”
(I know, my description of detector tones is not Pulitzer Prize winning material, but you get the idea.)
I suspect you already have these sound differences stored in your memory banks, but it’s valuable to focus your attention on their qualities as you set out to find silver.
b) Detector Settings – Getting it Right!
Just as your ears can learn to distinguish detector sounds and what they mean, your detector does exactly the same thing. Modern detectors accurately measure phase shift caused by the target, which helps determine the conductivity of the coin or jewelry.
Silver is a pure element and an excellent conductor of electricity. It’s a better conductor than copper. This is what enables the robust sounds in your headphones described above. Figure 1 shows how electricity flows more easily through silver.
You can adjust the settings on your detector to alert you when you have a rich signal that indicates silver. More specifically, you can turn off the beeps that represent less precious metals. The relatively inexpensive ACE series of Garrett metal detector features notch filtering, where you can select to hear only the most promising targets.
Figure 1 Silver has one “loose” electron in its outermost ring. This makes electron flow easier. Impure metals impede the flow of electricity, so the detector signal is not as clear.
Figure 2 shows detector displays which enable you to select only the high-end phase-shift coins. The ACE 300 uses notch filters. The more expensive detectors use reference code numbers, or Target Identification (TID) numbers to indicate the coin. Fore example TID number of 18 is almost always a nickel, and a quarter registers at 79. You can see in Figure 2 the area of high value on the face-plate next to the word “silver”
In practice, I find it best to combine the above two methods. I usually set the detector to find the more valuable coins, eliminating nickels and zinc pennies. I then keep my ears open for the tell-tale zipping sound of the silver targets. This method is best because you will sometimes find a silver ring, which registers lower on the discrimination scale but still gives you that distinctive zipping sound of silver.
Figure 2. Face-plate images for the Garrett ACE 300 and White’s MX Sport showing settings for silver.
Metal Detector Quality
There’s no diplomatic way to say this: The expensive detectors are much better at finding silver coins and jewelry than the cheaper models. Sure, you can often find the valuable targets with a $100 detector, but for the dedicated dirt-fisher, the advanced engineering of the high-end machines are your best bet.
a) Discrimination Ability
The more advanced detector models have more robust signal processing features and better discrimination, which allows you to separate treasure from trash. The technology for this sport has advanced markedly in recent years, to where you can now measure not only phase-shift, but target conductivity, target identity, and depth of the coin.
I was on a 3-hour club sponsored hunt in a park using a relatively decent $500 detector. My buddy following behind me covered the same ground as I did and he came up with a silver quarter at 9 inches deep. This happened three times in the same day. He got two silver quarters and a silver dime in locations I had covered just minutes ahead of him. His machine was $1,200 model.
b) Signal Processing
The latest technology in detectors means that the newer, more powerful detectors add a host of features that help you find coins and jewelry in a more productive manner.
Precise notch filtering.
Coil frequency selection.
Run multiple frequencies at the same time.
Specific conductivity settings.
Lower frequency settings lets you find objects that are deeper in the soil, and the higher frequencies help find smaller objects.
c) Battery Voltage – More is Better
Likewise, be sure you buy batteries that are listed at 1.5 volts each. Some manufacturers have switched to selling batteries listed at 1.2 volts each, instead of the standard 1.5 volts.
TIP: Change your batteries frequently. This applies to all detecting modes, but it helps substantially in silver hunting. A detector that takes four AAA batteries at 1.5 volts each should register near the upper limit of 6.0 volts DC. If each battery drops to 1.1 volts, that is a loss of 27% in power output.
Metal Detector Coil Type for Silver
a) Metal Detector Coil Diameter
In general, the larger the coil the deeper you will be able to detect coins. Very large coils, however can be heavy and difficult to handle for long periods of time. The standard round coil will produce a magnetic field that is essentially bowl shaped. An elongated coil will produce a field that is shaped like an oval bowl.
b) DD Coil – Best for Silver Detecting?
Your best bet for silver fishing is the double-D (DD) coil. This arrangement is actually two coils whose fields interact in such a way as to produce flattened spade-like magnetic field. Figure 3 shows the shape of the coil field for a round coil versus a double-D.
The flatter, fan-shaped scan area of the double-D coil makes it easier to pinpoint the coin, and the field penetrates deeper into the soil. Conclusion: The DD coil works much better.
Figure 3. (Left) Field shape of a round coil, or a DD-coil seen from the side. (Right) Field shape of a DD-coil seen from the end. The flatter scan area goes deeper and makes it easier to pin-point the coin in the ground.
Best Locations for Finding Silver with a Metal Detector
Your hunting location is just as important as your detecting gear. Here are some tips for finding silver.
a) Old Maps – Great for Metal Detecting
Silver currency started disappearing from circulation in 1964, when the mint switched to silver plated coins. Knowing where people lived before that date will be a big help in scanning the most productive areas.
Get an old map of your town, one that was published say in 1950.
Compare that to a modern map. Do you see where the old parks were located? Where the fairgrounds once stood?
Choose the 1950 locations instead of the new housing development that went in just 10 years ago.
Focus on the main roads into and out of your town.
Choose locations in open areas in and near the older sections of the city.
You will have a much better chance of finding silver in the areas that were populated long ago.
b) People and Places – More Dropped Coins
Use your brain in selecting a place to hunt. Here’s an (oversimplified) example. I live in a town at the base of the foothills of the Sierra Nevada mountain range. The old covered wagon trails snake through a narrow pass that ends in a wide valley. Guess where the wagon train, and later millions of motorists, will take a break, stretch their legs and lean back against an old oak tree.
A few miles to the west of that pass is a small town which happens to be in gold country. The early Wells Fargo Bank had an assay office in town where they would exchange gold for hard cash. The town town council recently decided the old main street was too narrow for modern traffic. They pulled up the sidewalks for repaving the streets and walkways. Guess who was there with a metal detector for the two weeks between the sidewalk removal and the pouring of the new walkways?
Well, you might not have such ideal locations or opportunities where you live, but the mental process will be similar. Search in old areas where people often congregated.
Important Tips for Finding Silver While Metal Detecting
Silver coins are often worth more as collector items than simply the value of the silver. You can destroy much of that value by gouging a deep scratch on the surface when you retrieve it.
a) Hand-Held Probe – Pinpointer
Do yourself a favor and buy a hand-held probe. They make pinpointing and coin retrieval about twice as fast.
Scan the target from one direction, say facing north.
Scan again at 90 degrees, say facing west.
You should now be able to pinpoint the exact coin location.
If you have one on your detector, check the depth reading.
If you have a hand-held probe, try to fine-tune the target area.
Place your digging tool about 2 inches to the side of the target.
Press the digger at least one inch deeper than the depth reading.
Crank out the dirt slug and scan with your hand-held probe.
Grab the coin from the edges without rubbing dirt across the surface.
Let your face break out in a gleeful smile.
Most soil contains sand, or silica (silicon dioxide), often in the form of quartz, which is extremely hard and will easily scratch the softer silver. Even rubbing the dirt off with your fingers is enough to cause a scar. Best bet is to carry a small medicine container filled with soapy water. Drop the coin in there and wash it carefully when you get home.
c) Avoid Over-Fished Metal Detecting Sites
Many clubs have weekly events where dozens of detectorists descend on an urban park. These sites may be over-fished. They have been scoured many times for coins. Instead, focus on smaller parks, grassy areas near major travel routes, and open fields near the center of town.
Finding Silver with your Metal Detector
There’s an old saying: Treasure is where you find it. You can, however, increase your odds of success by applying some of the ideas discussed here. Good luck!
Vince Migliore is a writer and researcher. He has written numerous magazine articles on metal detecting and three books. His latest book is “The Art and Science of Metal Detecting,” available in paperback at Amazon.
When I first started metal detecting I thought that I just needed to get an expensive metal detector to get better. However, after doing some research I’ve realized that using the correct coil can be just as important as having a good quality metal detector.
How Does a Metal Detecting Coil Work?
Metal detectors operate by sending an electrical current through a coiled copper wire to generate a magnetic field (this coil is called the transmit or TX coil). When this magnetic field encounters metal it induces an equal opposing current which is sampled by the receive or RX coil.
What are the Different Types of Metal Detector Coils?
At a basic level every commercially sold metal detector either works through pulse induction or very low frequency technology.
Pulse induction detects metal through repeatedly allowing a high amperage electric current to pass through a low resistance coil for a very short period of time. This action creates a magnetic field to quickly be created and then quickly collapsed. When the magnetic field collapses it creates a voltage spike which is both high in intensity and opposite in polarity to the original pulse. The receive coil (which is often the same coil as the transmit coil in pulse induction coils) then measures the energy of the pulse at the point where it decays to zero.
When the magnetic field does encounter a target some of the energy of the field will be stored within the object. This will cause the reflected pulse to have less energy and in turn take longer to decay to zero. The reason why having less energy causes the pulse to take longer to decay to zero is that the energy is being transferred through electromagnetic waves. When energy is taken from the wavelength it becomes longer, or rather less frequent, and therefore slower.
Up to the point where the reflected pulse is measured, the signal being sampled, which is collected from the RX coil, contains only the signal from the TX coil which has been reduced in voltage by a resistor as to not overload it. When the reflected pulse is also present the signal being sent from the RX to the sampling circuit is changed. The signal which is sampled is then averaged to create a reference voltage which eventually becomes the voltage of the DC current powering the alert system. A higher voltage will result in a higher volume, or a higher pitch, or even a higher frequency of clicks.
Very Low Frequency detects metal by allowing a constant alternating electric current to pass through a coil to create an electromagnetic field. The polarity of that electric current is reversed thousands of times per second to create what is known as the transmit frequency. Any conductive object which encounters the magnetic field will be induced by the rapidly changing magnetic field to create something known as eddy currents. These eddy currents produce a magnetic field for the conductive object which has a polarity pointed opposite to the transmit field.
The Rx coil is arranged in a way, relative to the TX coil, that causes energy from the magnetic field of the TX coil to not affect the amount of net energy within the RX coil. This means that if a magnetic field was created by eddy currents within a conductive object then that would be the only force acting upon the RX coil. If any current is present then, in the RX coil, it will be sampled and converted to a DC current which powers an alert device. The greater the current in the RX coil the greater the current going to the alert device. This causes it to increase in volume, change pitch, or even increase in frequency of clicks (Much like a pulse inducing metal detector).
Discrimination between types of metal and determination of depth is possible with very low frequency coils. This is because each metal exabits its own unique response to magnetic fields and have varying levels of conductivity. Additionally, because the time between induction and sampling will increase over distance. Ultimately, this data is all processed and then categorized by a mini-processor within the metal detector that has been programmed with a common constant value for these occurrences.
What are the Different Metal Detector Coil Designs?
Although all metal detector coils operate on either pulse induction or very low frequency technologies their design can be manipulated in a few ways. Manipulation of these designs can create detectors which are further specialized for specific use cases. These use cases, which a coil can be optimized to detect, include: depth, mineralization level of soil, amount of area being detected, and number of targets within a given area. The most common modifications are size and shape changes of the coil.
The size of a metal detector coil, no matter the shape, will determine the depth and width of a resulting magnetic field. The larger the coil the greater each of these factors will be. However, that does not always mean that larger is better. In areas where there is a large amount of metal debris a smaller coil will give you greater ability to avoid timewasting targets and instead focus in on potential good targets. In areas where there is a large amount of space, and a small number of targets, a larger coil can increase your efficiency and effective search area both horizontally and vertically.
Shape manipulations of the search coils can also affect performance, although usually with a more specific application in mind than when considering size manipulations. There are five shapes which are available on the consumer market today each of which can be “the best choice” depending upon the scenario.
A Monoloop Coil
A Monoloop or mono coil is characterized by having just one coil for both TX and RX coil applications. Mono coils are only available, and possible, with pulse induction technology. Because of this, mono coils will give you the most sensitive detection and will not be affected by soil mineralization. However, this also means that with a mono coil there won’t be any discrimination of the targets and so you will have to be ready to dig up a lot of pop can tabs.
Concentric coils operate in a similar manner to the Monoloop coil with the major difference being that the TX and RX coils are separate. Due to this separation, concentric coils can be manufactured to work with either pulse induction or very low frequency technologies.
The advantage of having a concentric design is that the two coils have the largest amount of circumference possible which in turn provides the most sensitive magnetic field possible. While pulse induction concentric coils will not be affected by soil mineralization, very low frequency concentric coils will. In fact, they will suffer more than any other coil design.
DD or Double D Coils
Double D coils are formed by the TX and RX coils being placed next to each other in the shape of two opposing capitol letter D’s. The affect this has upon the magnetic field is that the positive detection Is only directly below the intersection of the two coils. The remaining portions of the coils produce equal field which cancel each other out.
The result of this is that the coil will be less sensitive but will also be more useful on mineralized ground. DD coils can be produced with either pulse induction or very low frequency technologies although they are much more popular in the very low frequency variant. The scenario where this coil is perfect is one in which the soil is heavily mineralized but discrimination capabilities can’t be sacrificed.
Perhaps the rarest coil design on the market today (only available from Garrett’s GTI series), imaging coils are an interesting improvement on the concentric coil design. The improvement being the addition of a second RX coil within the concentric design. This second coil allows the detector to pull from a greater range of information about a possible target to hopefully allow you to distinguish between something that is worth your time and something that is not.
2-Box Coils are similar to concentric coils in that the TX and RX coils are separate, but, the similarities pretty much end there. 2-Box coils implement a configuration in which the TX coil is placed a few feet away from the Rx coil, both being at each end of the metal detector. This configuration is only used when attempting to detect a very large amount of ground and is only manageable due to the complete separation of the coils. In areas where there is any metal debris this design will suffer. Moreover, this design is undoubtedly the least sensitive coil shape.
What is the best Metal Detector Coil for Underwater Detecting?
First off… make sure to purchase a metal detector which is designed with all of the waterproofing necessary to be used underwater. If you’re not sure if your metal detector is waterproof the information should be listed on the manufacturer’s website.
Something which is important to consider here is what type of water you will be detecting in. There is a big difference between salt water and freshwater metal detecting. In salt water you either have to use a pulse induction coil or a very low frequency coil with a salt water mode. Pulse induction coils are the most sensitive in saltwater and therefore often preferred. But, many manufacturers are coming out with very low frequency saltwater detectors to accommodate those who desire metal discrimination features.
I used to be scared to use discrimination on my metal detector because I didn’t want to risk losing something valuable even if it meant I would get more time-wasting hits. But, after finding countless pull tabs, and nothing particularly valuable, I’ve realized I might have been wrong.
What does Metal Detector Discrimination Mean?
Metal detector discrimination or DISC is a feature, commonplace among relatively newer metal detectors, by which certain metals can be ignored as to avoid common but low value objects. Modern DISC will often start with leaving out any signals received from iron metal, moving on to tin foil, then steel bottle caps, finally moving into some variation of specifically detecting various coins, gold rings, and possibly other precious metals.
However, a great deal of older detectors will simply include a dial which, while still signaling for the metals, will show the user what metal a target may be. Moreover, even older metal detectors might simply come with a dial which is associated with numbers that signify the strength of the signal detected. This number can then be associated with general signal strengths of various objects and types of metal.
How Does Metal Detector Discrimination Work?
It is important to note that while most any VLF (Very Low Frequency) will come equipped with DISC features, PI (Pulse Induction) metal detectors have no discrimination capabilities. This shouldn’t be an issue for most users as PI metal detectors are often expensive in comparison to VLF metal detectors and therefor only purchased by users who would already know that PI metal detectors don’t have discrimination. However, this information should be in the description of the metal detector if you are unsure.
VLF metal detectors work by sending a constant alternating current through a coil which, due to the polarity being reversed thousands of times per second, creates a transmit frequency that is sent into the ground. If this transmit signal encounters metal, a signal which is opposite in polarity to the transmit coil is produced within the object. These currents are known as eddy currents and are a natural response to a metallic object encountering a magnetic field.
DISC works through the same process by which normal metal detection does. The eddy currents produced within the object are first detected by a second coil. These signals are then processed and then amplified to create the sound which lets you know metal has been detected. DISC simply intervenes during the processing of these signals and tells the alert device on the metal detector to ignore certain frequencies or prioritize others.
What is the Difference Between Metal Detector Discrimination and Sensitivity?
While DISC deals with processing signals to determine an objects metallic composition, metal detector sensitivity is simply changing how strong (or rather weak) a signal your metal detector will accept. This means that by increasing your metal detectors sensitivity you may be able to detect objects which are non-ferrous metallic or simply deeper in the ground. While this can be used, in some degree, to exclude certain types of metal it is nowhere near as comprehensive or easy to use as the DISC function. It is for this reason that the two functions are seperate and are deemed to have separate purposes.
Example of How to use Metal Detector Discrimination / Sensitivity: Garrett ACE 400
The Garrett ACE 400 comes equipped with three different +/- sets of buttons: one which controls sensitivity, one which controls discrimination, and one which controls both by simplifying the settings down to the type of object you are looking to find.
The custom function allows a user to create their own desired profile for sensitivity and DISC.
If you’d like to read the actual OWNERS MANUAL for the Garrett 400 that goes into detail for setting the Discrimination and other settings select the button below for a FREE DOWNLOAD. PDF Courtesy of Garrett Metal Detectors
The Zero mode will allow all types of metals. Essentially, discriminating against zero of them.
This will allow you to choose between 8 different sensitivity levels.
This will affect the depth which the metal detector will be able to reach although this will not be shown on the depth scale. This depth scale is only used for showing the depth of objects detected in real-time.
This will allow you to move markers upon a list of various types of metals/objects. To either discriminate against or discriminate for a desired object/metal, press the button below the discrimination dial.
The constant discrimination feature which shows the composition of targets in real-time will still be active, however, discriminated against object shouldn’t elicit an audible response from the metal detector.
The Discrimination Chart of a Garrett ACE 400
This is the chart of options which are available for DISC for or against on the Garrett ACE 400. The metals are presented in the order and position which they appear on the Garrett ACE 400 metal detector.
5 Tips and Tricks for Using Metal Detector Discrimination
1.Using Metal Detector Discrimination to Eliminate Ground Mineralization Interference
Different hunting locations will have different kinds and compositions of soil. In some cases, you will encounter soil which is rich with ferrous minerals that generate eddy currents of their own when affected by a metal detectors signal. While it is unlikely that these signals will be strong enough to elicit an audible response from your metal detector, their effects are not undetected. These signals from the minerals in the soil can overload a metal detectors RX coil with useless target allocation even if a real target is in range of the coil’s detection field.
However, to eliminate these signals from your detection array you simply need to discriminate against Iron signals altogether. Most targets of value will not be affected as Iron is a low value non-precious construction material. In highly mineralized soil, Iron discrimination won’t be perfect but will be undoubtedly useful for finding targets and elimination phantom hits.
2. Depth can Affect the Discrimination ID of an Object
The depth of an object, along with the chosen level of sensitivity, can affect which type of metal an object is identified to be by your metal detectors DISC circuit. If an object is detected from the outer edge of a magnetic field then only a portion of the magnetic field will be processed by your metal detectors elimination circuit. Unfortunately, this can result in your metal detector confusing one metal for another which has a similar DISC ID.
Moreover, if a DISC ID is not incorrectly reported there is still an issue of signal reliability. A weak signal such as this can cause a very low tone, which is both difficult to locate and hone in on, to be produced. In some cases, the signal will not even be reported, this is another common source of phantom hits.
3. Smaller Objects can have Inaccurate Discrimination ID
For a similar reason to why depth can affect DISC ID, the size of an object will also change DISC ID. Essentially, the larger an object is the greater the signal from internal eddy currents will be. The stronger a signal is the easier it is for your metal detectors DISC circuit to determine metallic composition.
So, it would stand to reason that a smaller target, coupled with low sensitivity, could create a very weak signal. The weak signal a small object produces is no different than a very deep object and therefor will have similar characteristics. This means that the object could have: an incorrectly reported DISC ID, a low and unreliable tone, or even an inability to be detected.
4. Oxidation can Cause Discrimination ID to Change
The only metal which is unaffected by surface oxidation is gold and various gold-alloys. Everything else is prone to an unavoidable, and inevitable, oxidation. Oxidation, more commonly known as rusting, happens when a reactive element is exposed to oxygen. In this case it is the production of a myriad of metallic-oxides which we are concerned with.
Metallic-oxides which are produced from corroding objects will bleed into the surrounding soil when moisture is present. This is more commonly known as the halo-effect. It is referred to as the halo effect because a targets field of detection will increase in a radiating pattern around the object.
If the soil which you are hunting in is moist, then the halo-effect can be your best friend. Allowing you to easily find smaller and deeper targets which have had their detection fields increased. However, these metallic-oxides will produce different DISC ID’s which might be discriminated against if you are excluding Iron or even Tin.
5. Positioning/Orientation of a Target Affects Discrimination ID
Due to the copious amounts of information modern metal detectors can gather and process, even the orientation of an object, in the ground, can affect DISC ID. A perfect example of this is that of the orientation of a coin. A coin may have a strong signal if it is positioned with the flat side parallel to the metal detector coil. A coin may also have a very weak signal if the flat side is positioned perpendicular to the metal detector coil.
The difference in the strengths of these two signals affects DISC ID in a similar way as both depth and size previously have. If the coin is positioned perpendicular to the coil: the DISC ID could be incorrectly reported, a low and unreliable signal can be detected, and in some cases an object will be completely unable to be detected.
Should Beginner’s use Metal Detector Discrimination?
The answer to this question will depend upon the objective of the user. Many beginners may find it useful to gain experience from finding various pop can pull-tabs, nails, and other low value high presence objects. Finding these objects can teach a beginner how to use the pinpointer function of a metal detector and how to properly dig for an object. It is in this case that is in fact useful to not use any form of metal DISC.
If you are a beginner but you don’t care much for gaining experience, and instead favor finding only valuable targets, DISC can be incredibly valuable. Without the experience and know-how of an advanced user, a beginner can relatively easily find coins, rings, and other valuable objects. All of this while not being bogged down by constantly finding junk.
In either case, to ensure the value of your choice to, or to not, discriminate for or against certain metals, it is important to become familiar with both the discrimination and sensitivity settings of your specific metal detector. The guide provided above, for the Garrett ACE 400, will give you the basics that are translatable to most any modern metal detector. That is to say, “the basics” only. Other metal detectors can, and probably will, have different features, functions. And controls. Learning how to effectively use these things is essential to proper DISC utilization.
When I was just starting out, it took me forever to understand the difference between a Very Low Frequency and a Pulse induction metal Detector. So, you can imagine my frustration when I learned that even within these classifications there are different operating frequencies that metal detectors use. However, I have since learned how this information can help me improve my searching ability, and I am writing this article to tell you what I wish I would have known when I was new to the hobby.
Which Frequency is Best for Metal Detecting?
The best frequency for metal detecting is somewhere in the range of 5 kHz to 15 kHz. This range is where most general-purpose metal detectors are tuned too, and also the easiest to manage for beginners. Nevertheless, you can always get more specialized detectors once you have mastered the basics.
How does Frequency affect Metal Detectors?
The operating frequency of a metal detector refers to the number of electromagnetic waves (measured in Kilo-Hertz per second or kHz per second) that the metal detectors coil is able produce in a set amount of time, generally per second of operation. A metal detectors frequency can range anywhere from 1.5 kHz per second to a whopping 100 kHz per second. However, to be fair most metal detectors operate in-between the frequencies of 5 kHz per second and 25 kHz per second. This means that a standard metal detector is capable of sending between 5,000 and 25,000 electromagnetic waves into the ground every second.
So, how does the operating frequency actually affect the metal detector? Well, in general this depends on a few factors. These factors include, but are not limited to: conductivity of the metal, size of the target, depth of the target, and even the level of soil mineralization. As a general rule, lower frequencies have longer wavelengths and penetrate the ground deeper than higher frequencies. That being said, higher frequencies have greater signal strength and will result in a more sensitive metal detector. Moreover, the more conductive a metal is the lower the frequency should be for your metal detector and vice-versa.
This all still may be confusing so here is an easy to read chart for various scenarios and metals…
Metal Detector Brand
3 kHz – 7 kHz
Garrett Ace 150 operating at a frequency of 6.5 kHz
14 kHz +
Garrett AT Gold with an operating frequency of 18 kHz
10 kHz +
White’s Goldmaster GMT operating at a frequency of 48 kHz
Fisher Labs F75
4 kHz – 8 kHz
Garrett AT Pro
7 kHz and Less
Minelab X-calibur 2 operating at a frequency of 1.5 to 25.5 kHz
7 kHz and Above
Minelab Xterra 305 operating at a frequency of 7 kHz and 18.5 kHz
Multi-Frequency Metal Detectors vs Single Frequency Detectors?
Most metal detectors are single frequency detectors. This means that they operate at one frequency and only that frequency. These are typically, either at or below 10 kHz per second or above 30 kHz per second. The former is good at ground penetration and finding highly conductive targets. The latter is better at finding smaller low-conductivity targets, but also struggles with ground penetration.
However, there is a small percentage of newer machines which can actually detects at two or more frequencies at the same time. These metal detectors are known as simultaneous multi-frequency detectors. Like I said this could be happening at the same time, or possibly at an imperceptibly fast interchanging pattern.
These detectors are just as reliable as single frequency detectors, but obviously more capable due to their improved range of detection. The only downside is that these machines can sometimes be difficult to use, and they definitely cost more than the average metal detector. However, if you are somewhat skilled and money is not a huge issue, this new technology is definitely better (as long as the manufacturing quality remains unchanged) and worth the investment.
Some of the newer simultaneous multi-frequency detectors can even detect a greater number of frequencies than ever before. Previously, the standard was either one frequency or three frequencies (aka Selectable Three Frequency). But, a new technology called Full Band Spectrum can detect up to 28 total frequencies all at once. This ensures you detect any number of targets regardless of size, composition, or depth.
Best Frequency for Finding Coins?
Coins are generally closer to the surface and of a higher conductivity, this means that the best frequency’s for searching for coins are anywhere from 10 kHz and below. That isn’t to say that some rare gold or silver coins won’t benefit from higher frequency detectors due to their specific composition.
Best Frequency for Finding Jewelry?
Although jewelry is often marketed as gold or silver, or even platinum, the reality is that these are often alloyed with other metals that have higher conductivity so a midrange frequency detector is acceptable for looking for jewelry. Somewhere from 10 kHz and Above should do just fine.
Best Frequency for Gold Prospecting?
Gold is mostly only found is small pieces, and of course in remote areas such as western Australia. However, if it is gold you are after then you will need a machine that is 14 kHz and above. The higher the better as gold has a very low conductivity so it will be difficult to detect otherwise.
How does Ground Mineralization Affect Frequency?
Due to the effect that ground mineralization can have on the normal processes of a metal detector it makes sense that it would also affect the frequency you should be using for detection. First thing first however, no matter the frequency you should be using a Pulse Induction metal detector if possible as it won’t be affected by the mineralization. That said, a lower frequency is most likely the best option for a highly mineralized area. The higher the frequency is the more it will be affected by the ground’s composition and even temperature fluctuations.
Following this logic, when you are treasure hunting at the beach, especially a saltwater beach, it is best to use lower frequency Pulse Induction metal detectors. The moisture in the sand, along with the salt and any other number of minerals, can cause false signals that effect high frequency machines and Very Low Frequency machines the most.
How does Search Coil Size Affect Frequency?
The larger a search coil is, generally, the lower the frequency which it produces will be. Furthermore, the smaller a search coil is, generally, the higher the frequency it produces will be.
This means that as a general rule large search coils are good at ground penetration but it will be lacking in sensitivity. Also, this means that as a general rule the smaller a search coils is the worse ground penetration it will have, but the better sensitivity is will have as well.
How a Metal Detector Creates an Electromagnetic Field?
A metal detector creates an electromagnetic field by sending electrical energy through a coiled piece of copper. The various specifications of this process affect the frequency which is produced but the general process is the same no matter what the operating frequency is. An electromagnetic field is produced by the interaction of an electrical field and a magnetic field. These two fields have to be at the same frequency, and also at an opposing direction. This can be seen in the graphic below…
The effect of these two fields coming together is an electromagnetic field which can affect metallic objects in the ground and therefore can be detected by the metal detector.
The Best Frequency for Metal Detectors: Revisited
At the beginning of this article I said that the best frequency for metal detectors is 5 kHz – 15 kHz. While this is true for general purpose metal detectors, hopefully I have given you the information to determine which frequency is the best for your specific use case. I say this because, now that I have explained metal detector frequencies, it is clear that 5 kHz – 15 kHz is not always the best range to have when metal detecting.
For example, if you are searching for gold you need a metal detector which is operating at a frequency that is at least 14 kHz but preferably as high as is possible. The reason for this is that gold is often only found is small pieces. Furthermore, gold is not very conductive. Both of these factors contribute to the reason why a high frequency metal detector is the best for searching for gold.
Moreover, if you are searching for silver then it is preferable to have a metal detector which is tuned to have the lowest frequency possible. A metal detector capable of transmitting frequencies around 3 kHz in fact. Of course, putting this use case squarely out of the range of my original recommendation. But, silver is highly conductive (the most conductive metal in fact) and so it is necessary to use a lower frequency detector to find targets of this kind.
Further judgement calls can also lead you to decide that my recommendation is not the best for your specific case. If you know that what you are looking for is very deep in the ground then the lowest frequency possible is the best frequency to use. Somewhere 5 kHz and below. Inversely, if you know what you are looking for is near the surface, it is possible that a higher frequency would be better. Somewhere 10 kHz and above.
Looking for a versatile Metal Detector with a wide range of Frequencies? The GARRETT AT PRO is a solid choice with a proven track record.
Even further, if you know the area which you will be searching in is either highly mineralized or moist or both, a lower frequency Pulse Induction metal detector may be the best option for your search. But, then of course if this is not a possibility then a Very Low Frequency metal detector which has an operating frequency of somewhere around 6 kHz and below is the best option. The discrimination features which are possible with Very Low Frequency metal detectors can further assist you in removing the effects of ground mineralization as well.
So, the moral of this story, or rather the conclusion of this article, is that for an experienced metal detector there is no one best operating frequency to use all of the time. There is however a best operating frequency to use in any specific scenario. The challenge is knowing what works best and when. However, there is no need to fret, from the information in this article you have all of the knowledge you need to have a good understanding of metal detector frequencies and use cases for those frequencies.