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
