Coils , Frequency and Depth logic

[Huemac]

Although I have been reading about different metal detectors , coil sizes , frequencies .....etc...etc...I am very much new
at this hobby.
I wonder if somebody can help me and answer the following questions or direct me to read a book or info.
1 . - What is the logical association of the different coil sizes , frequencies and depth for gold prospecting , cache and
meteorite hunting ?
2 . - For instance what coil size and what frequency is best for maximum depth ?.....Good and bad ground conditions ?
3 . - What combination of coil and frequency is good for gold prospecting and meteorite hunting ?

Any thoughts , ideas , or advice will be greatly appreciated.

Huemac

 

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[AUDuke]

There is no easy answer to to your question. Big coils detect deeper than small coils on larger objects, smaller coils are more sensitive to smaller objects. Higher freqs are generally more sensitivity to lower conductors ( small gold), lower freqs can detect a little deeper. Double D coils handle mineralized ground better than concentric coils, and everything is a compromise

 

[RPG]

There is no easy answer to to your question. Big coils detect deeper than small coils on larger objects, smaller coils are more sensitive to smaller objects. Higher freqs are generally more sensitivity to lower conductors ( small gold), lower freqs can detect a little deeper. Double D coils handle mineralized ground better than concentric coils, and everything is a compromise

Good answer.

 

[Huemac]

AUDuke :
You are right , there is no easy answer and there are many variables , nevertheless your answer is very good , one
thing l need now is to practice and practice ( trying different combinations of coils and frequencies )..if budget permits.
I do not know yet , but GW I was thinking of buying a Minelab X-Terra 705 to do some prospecting , meteorite and
cache hunting , however if there is another metal detector that costs less and does the same thing l am all ears.
Since l have been hunting for minerals for quite some time now , here in the USA and Mexico , l am looking forward
to pass the detector over the rocks maybe l will learn something.

Huemac

 

[RPG]

There are some very good "OLD" detectors out there that will do everyting you you are looking for in a metal detector. Compass made some machines that are still being used today. Lucky Larry is the pro on the older units for what you are looking for. Maybe he'll see this thread and reply.

 

[mtntrekr2]

I have an xterra 70 with a medium freq. 10.5 DD coil. I have found everything from silver earring backs to buckshot BB's to wristwatch band pins. I have the sens up high and yes it does chatter a little. But I have also found several tiny pieces of gold chains and clasps. I am more than happy to and a 14K lobster claw to my stash.
If your budget lets you, knowing you would be looking for gold and meteorites-iron or nickel. I would suggest a high freq. DD coil designed for gold and iron-relics.
Many will say a smaller coil is good for hunting in trashy areas trying to get between the junk targets and others like them for getting in the thick brush.
But if you are planning to hunt open fields and or desert I would want a large coil to be able to cover some ground.

Just my thoughts
Joe

 

[discovery_bound]

So, if I am understanding this correctly, a lower operating frequency will go deeper and is more for all purpose hunting, and a higher frequency is not as deep but is geared for specific hunting. For example, I seen an add for the new Gold Bug detector and noticed it's set at around 19kHz, and my Tesoro umax is around 10.2kHz. Am I on the right track, or way off? I have been curious as to the different operating frequencies, and is probably a really good thing to know if I decide to upgrade or get a backup unit.

HH
DB

 

[homefires]

Coils DD, Concentric, Coaxial will produce a usable signal as deep as they are wide.

A 10" coil will go deeper then say a 5" coil but will not detect objects as small as the 5" coil can.

The Transmitted and Received signal from the smaller coil is higher power and is easier to deal with.

Frequency has two functions working here.

1. The lower frequencies can penetrate the ground better then the Higher ones. Higher Frequencies are Absorbed by the Ground.

2. Lower Magnetically Conductive objects like GOLD will produce better response with the Higher Frequencies then Lower.

This does not talk about size but you will find it useful.


http://www.treasurefinders.net/types_of_coils.html

 

[Reg]

Hi DB,

This frequency thing is not as simple as it sounds. Yes, higher frequencies are usually used for low conductive objects, but usually refer more to machines operating at 50khz or higher. Typical coin hunting machines today are in the 10Khz and 20Khz will operate much more the same to each other than a 10Khz be for one item and a 15khz for another. Other factors come into play as much as the frequency.

Now, the older 6khz units do better on higher conductive objects so they do better on silver and copper and do lag on gold or the more common meteorites the stonies. The iron meteorites can be detected just fine on the lower frequency VLF's. In fact, some of the old timers still use one of the older Whites units for this type of meteorite hunting. On the opposite side of the coin, areas such as Gold Basin AZ where the common meteorite found is the stony type, the Goldmaster series of gold machines has been popular along with other gold detectors. The reason is the stony type meteorites are low conductor, so the 50khz detectors simply work better.

Now, to add to the can of worms, there are flat areas at Gold Basin that are covered with hotrocks that drive VLF's crazy making it almost impossible to use them for meteorite hunting, so PI's are often used because the hotrocks are ignored.

If you are going to hunt gold and meteorites and still want to use a VLF, make sure you get one that has an all metal mode that is easily selectable and a reasonably fast autotune or an adjustable speed autotune (this is called SAT on some detectors). It helps to have a ground balance adjustment also.

There are a few people who try to hunt for meteorites or gold using a motion mode discriminator but there are many limitations to doing so even if one is using it at zero disc. Once in a while a person will try to use the TID (target ID) feature, but my recommendation is to forget that. It simply will not work with any reliability. TID was designed for coins where each similar denomination coin is much like the other in size, shape and chemistry.

Don't overlook basic detectors such as the Tesoro Vaquero which was basically designed to be a coin hunting detector. Actually, it does quite well in low mineralized ground for gold and can detect small gold down to a grain. At least, mine does with the smaller coil. The fast autotune feature works fine and the all metal mode will work fine for meteorites also. The down side is in bad ground, the ground balance adjustment is not as refined as it might be. So, it is a little touchy. BTW, this detector was recommended by Steve Herschbach as a low priced unit that works for gold.

The truth is, most detectors can be used for both, but some will do a little better on different sizes. The key is know your detector inside and out. Know how to adjust it for the best response for the conditions and objects you are seeking.

Practice, practice, practice to find what works best. If you don't have small gold use small lead or small brass objects. If you don't have a weak stony meteorite, use a piece of an old red brick. Generally, the old red fired brick gets its color from the iron oxide used in making it and that will cause the brick to generate a signal. Some will generate a stronger signal than others, so when possible sort through some to find different responding ones.

Hopefully, this post will give you a few more things to think about.

Reg

 

[DGDancer]

You'll find alot of thoughts in the field of Frequency verse's depth as well as Freq's verse's metals that react best for detection.
No real good books on the subject but books and people familure with the subject all typically agree that lower frequencies
work best for penetration~ the denser the medium is your trying to sense thru the more resistant it is to electromagnetic waves
of shorter wave length ie higher frequency. This also goes in hand with the relative conductivities of various metals as thier
conductivity also reacts better at certian frequencies. Size of targets and thier shapes are yet another variable.... but I want to
keep it simple....Higher frequencies are better for shallower targets especially of small size or low conductivity. But its all relative....

I'll take apart your questions best I can. But I will say they all ask essentially the same thing as question 1 so thats what I'll comment on.
I'm going to concentrate on VLF detectors for you questions because the second type
of detector commonly used is what is called a PI, Pulse Induction, unit and it operates on a very different principle. Reg touched
on this type of unit in his comments about PI's but I feel your not looking at these units as of yet.
Also I'm talking about mono coils in general, DD coils operate the same but depth is half the coil size and they naturally
reject ground contidions to an extent.~ they are a strange creature.

1 . - What is the logical association of the different coil sizes , frequencies and depth for gold prospecting , cache and
meteorite hunting ?
This is actually two questions as the third part is purely opinion as to finding gold, meterorites and caches:

Lodgical association of coil size verse's frequency for depth. First is understanding is that the EM field generated by a coil, antenna,
is approximate in size as related to its diameter. This means a 4 inch coil has a lodgical field size of 4 inches and a 12 inch
coil would have a field of 12 inches. This is the baseline of how deep a detector will go. The EM field shape will also follow
the shape of the coil thus changing its effective depth and ultimate sensing window. Round verses Eliptical shaped coils.

The frequency used is not real important except in determining what type of metal will react best to the coils EM field.
I suggest reading around and following the opinions of others as to what freq's work best for what targets~ its good stuff to know.
This should allow you to pick the appropriate machien for your needs. But in general almost any detector will work once you
learn it. Most manufactures set thier own freq's and tune thier coils for that freq for best performance of the machien. Many
are specialized as a result.
Conversly frequency also determines how well the EM field of the coil can penetrate dense mediums, ie soils that have high
Iron or Salt contents. Iron and Salt are spread out thru soils and they act alot like a big metal plate when stimulated by the
coils EM field. To negate the effect circuits are built with ground balancing that effectivly negates this condition imposed by
the soil and its contents. The higher the ground balance has to be set the more negating is incorporated into what the detector
ultimatly senses and this can have the affect of Tuning Out targets at increasing depths because the machien will not take
thier signals below a certian point as determined by the ground balance negation..... sound complicated? Higher freq's
react more strongly with the Iron and Salt minerals in the soil requiring more GB~ lower freq's typically do not.
Lower conductivity items, especially small ones, require stronger stimulation to react with the EM field.

Opinion of the best machiens, frequencies and coils for what your curious about *gold, meteorites and caches* run the gamut.
As I said just about any machien can find all of these but its upto the operator to know what thier target is, about what depth
they expect to find it and what the soil conditions are generally like. Your top end machiens can do all of this, some mid range
machiens can do some of this and your low end machiens are upto the user to decide if it works or not.
Want gold~ look at a machien specialized for this. Gold is typically small and 10 to 15kHz is best. Coils~ small ellipticals are good in
shallow creeks and larger ones for deeper soils. DD designs work best for high mineralization but mono's work best for overall depth.
Want meteorites~ just about any machien and any coil.
Want caches~ again just about any machien with a large coil put on it. But if you suspect its beyond 18 inches deep you'll want
to look at whats called a Two Box unit. Some detectors, like the Garret GTI, can be converted to a two box unit.

For all three types of target one should follow the rule of Dig Everything and forget about using discrimination. For small stuff, go slow
and use a small coil.

What would I use:
Gold : if the area is known for alot of trash a Gold Bug 2. Little trash a PI unit, mines a GP3000. Coil size and type will depend on what
I observe for the ground conditions~ high magnatite or salts a DD, otherwise a mono coil. Size~ how deep does the soil seem and is the
gold small. Smaller coils will pick up smaller targets but sacrifice depth.
Meteorites : My GTI or DFX with a large to midsize coil as these tend to be a little deep. Running in all metal mode. My GP works well to
but I like a little discrimination.
Caches: Same as meteorites but because they can be deep and alot of trash is generally associated I would not use the GP. I dont
own a two box but if I want one I can set up my GTI for it.

If you got thru all this reading Thanks

 

[UncleVinnys]

Looks like you have all the good answers there.
I'd just add that meteorite hunting is at the exact opposite end of the spectrum from gold.
For gold, you benefit from higher frequency and smaller coil.
For meteorites, bigger is better, and lower frequency.

 

[Frankn]

Why are you hung up on frequency? There are many more important factors to consider. The problem is most manufactures don't give out this info. How about power output. How about the actual signal processing. As for the DD coils, they have their problem. they are actually two coils side by side therefour they have two prime pinpoints side by side. They cover a slightly larger area,but loose a little on dept. I use a whites XLT with standard 9.5" single coil. In prospect mode I have found many nuggets. And the largest gold hord ever found in England was just found with an XLT with stock coil. There are several for sale on the listings on T-net under $500! Sure low frequencies go further in the ground and water. the Navy uses 2cycles to communicate with it's subs under water and the Air force is up to tetracycles,but for metal detectors, it's all in the circuit board!!

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