can the % extra depth/ground coverage of differant size coils be determined by calculation.

Hi Tom/All, I have a question for the boffins out there,who im sure can help with this.
His the square root of the differance between any 2 sizes of coil= to the %extra depth/ground coverage of any 2 coils, a friend of mine none detectorist thinks this does. Please keep keep the replys as simple as possible for us laymen,and many thanks in advance of your replys.

yabadabado

4/3 x Pi x R-cubed is volume of a sphere......and is a good standard for concentric coil 'volume'. There's more factors involved; yet, this is a good standard. Coils that are NOT of this configuration are too difficult (too many variables) to determine.

thanks tom still to complecated for me

I've recently been working on home-building some concentric coils, and so have been through the maths of their performance. I came to the conclusion that depth does vary approximately with the square root of coil diameter. This was based on tedious hand calculations, I was planning on writing a program to verify this and to experiment with other parameter changes, but have not done so yet.
This was based on a target distance from the coil approximately equal to the coil diameter, eg. 200mm away from a 200mm coil. If you were looking at the near-field behaviour, eg. a tiny nugget 25mm from a 100mm coil, this square root relationship might not apply.
I see no reason why the above should not apply to double-D coils too.

There are many other variables which can affect this relationship, such as:
* A larger coil will pick up more ground signal too, this may partially null out the target signal gain
* Different coil manufacturers may produce different results,even for the same size coil, eg. if the coil generates more noise due to a differing screening method, then less detector sensitivity can be applied.
*A double-D coil with parallel central coil sections may differ from one with slightly arced sections.
I hope this is of help to you.

A concentric DD coil looks at a different 'volume' of dirt compared to a elliptical DD coil. The 'tightness' of the electromagnetic footprint dictates 'volume'......and depth.

Okay Tom, I'm gonna have to ask you to draw this one out in crayon for me.
If I'm thinking about this right, the volume of a sphere based on an 8" concentric coil is roughly 268.08"^3.
How does depth work as a function of this volume?

Thanks,
Chris

(4/3)*(pi)*(8/2)^3 = 1.333 * 3.1416 * (4)^3 = 1.333 * 3.1416 * 64 = 268 in^3 <-- Neugene, you are correct!

But the detector depth is one dimensional, and related roughly to the diameter of the coil (one dimensional also), modified by many, many factors. Years ago a 8" diameter coil could be expected to find a coin sized target 8" deep in neutral soil. With todays electronics you can get nearly 1.5x that, or 12".

The calculation of volume gets you no closer to the correct answer.

The problem for detector designers is seperating out the wanted target signal from the residual (unwanted) ground signal. The ground signal is a problem because it is large AND varies. It's perfectly possible to make a detector with a normal-size coil detect a coin 80 cm away in air, but put it near soil and it would fall over.
Fitting a larger coil will increase the target signal picked up ( in accordance with the 'square root' relationship mentioned previously), but it will also pick up more ground signal. Depending on whether this signal and it's variation can be 'eliminated' affects how much of the extra depth can be realised.
As an example: a coil will see the 'average' amount of mineralisation below it, but will respond to a non-mineralised (cold) rock in the soil. A larger coil will respond less to that same cold rock, as it is seeing more of the mineralised ground relative to the cold rock. In effect it 'smooths out' the cold rock's signal. Less ground signal variation means more chance of detecting the signal variation caused by a target.

DD coils do respond to the ground differently to concentrics, which is why they are generally favoured for mineralised ground detecting. But the 'square root' relationship for target signal response is still valid for them, as the transmit/receive coils (and the target itself) still have the same electromagnetic rules applied to them.

Ground signal pickup varies with coil distance above the ground, as you know, which is why a smooth even sweep is recommended, as it minimises ground signal variation. Have you noticed the phenomena of ' loop foldback', where the ground pickup drops in the last inch above the ground? This should in theory give deeper detection when 'scraping the surface', as the target signal is larger and the ground signal is smaller. But I feel the ground signal picked up by the coil varies rapidly in this low-coil-height region, and this can negate the gains. Some manufacturers recommend a 1 to 2 inch coil height, I think this may be because the ground pickup varies less here, just above the 'height of inflexion'.

Anecdotal evidence also supports the square root (or similar non-linear relationship) idea. How often do you see such statements as " I upgraded from a 10 inch coil to a 14 inch, but it hardly seems any different!" or " I fitted the 5 inch sniper coil and I can't believe it will still pick up a coin at 8 inches". Also notice how deep Sunray probes go relative to their small size (though they do have a different coil arrangement).

Regards.

great write up pimento u should have been my school teacher, most of that i understood.

Seems to me, if we could possibly, VISUALIZE this somehow, anybody tried to possibly put a metal detector in/near a viscuous fluid filled with fine iron filings ? Similiar to the old magnet below a piece of paper filled with iron filings trick ? To see what happens in 3 dimensions...................??? Anybody got an old fishtank lying around..............??


Tri

Neugene,

The correlation between 'volume' vs 'depth' has numerous ties; yet, does not give 'depth'. ,,, It is the electronics platform that dictates depth (sensitivity).....primarily.

What really throws measurements out the window is the following: A CZ equipped with the following coils can detect a clad dime in inert dirt.....at the following depths/ranges:

5" coil = 10"
8" coil = 12"
10.5" coil = 12.6"

Coil size... vs... depth is not linear; subsequently, no formula can be presented to determine (not even roughly) approx depth capabilities.