detection depth

[green]

Target, 20 inches deep. Best coil size, 40 inch diameter. Target just detectable with the 40 inch coil. How to approximate detection depth with a 20 inch diameter coil?

Keep inductance and current profile the same.

[Koala]

Target, 20 inches deep. Best coil size, 40 inch diameter. Target just detectable with the 40 inch coil. How to approximate detection depth with a 20 inch diameter coil?

Keep inductance and current profile the same.

Are we talking pi or vlf

Depends a lot on the target size.

a small target on its edge is deeper with a smaller coil.

big coils are only deeper on bigger target.

[green]

Are we talking pi or vlf

Depends a lot on the target size.

a small target on its edge is deeper with a smaller coil.

big coils are only deeper on bigger target.

A PI. If a cache was buried 24 inches deep. What would be the best diameter coil to detect it? What would be the smallest diameter coil that would make sense to try?

[Dave J.]

For a target of a given diameter, if you're thinking only of relative voltage induced in the receiver, it's a fairly straightforward (when you know how) axial air coupling calculation. However a target that's "just detectable" is most often limited by a number of factors other than mere voltage induced in the receiver. Also your question as you worded it doesn't make sense: "best coil size 40 inch diameter, target just detectable" means that any other size searchcoil won't detect that target.

So, in the real world your question has no straight answer. The non-straight answer is that if a searchcoil of a given radius has a weak response on a particular target buried at a depth equal to that of the coil radius, an electrically equivalent searchcoil half that radius will kinda sorta more or less usually give a better response. Your mileage may differ.

[EDIT] now the question about a cache 2 feet deep, what size searchcoil will be best. There is no straight answer especially since there is no "given" regarding the effective diameter of the cache or what size of searchcoil is known to detect it. You might do fine with an 8 inch searchcoil and not need "best".

[Koala]

Are we talking of something like a buried ammo box made of metal

Large gold bars

Or individual coins.

Mineralized ground.


Still too many variables.

[Skippy]

Presumably your 40" coil figure comes from the not-that-well-known data available online, particularly the graphs drawn by Eric Foster? The ones with the semi-circular curves?
Based on them, it would appear that a 41" coil or a 39" would be too big/small.

I'll dig out my copies of those graphs and report back later.

Update: That was easy, found the graph, can't locate the instructions, yet ....but:

Target depth with 20" coil will be 17.5"

I'll add the graph later when I find the 'manual'.

[green]

For a target of a given diameter, if you're thinking only of relative voltage induced in the receiver, it's a fairly straightforward (when you know how) axial air coupling calculation. However a target that's "just detectable" is most often limited by a number of factors other than mere voltage induced in the receiver. Also your question as you worded it doesn't make sense: "best coil size 40 inch diameter, target just detectable" means that any other size searchcoil won't detect that target.

So, in the real world your question has no straight answer. The non-straight answer is that if a searchcoil of a given radius has a weak response on a particular target buried at a depth equal to that of the coil radius, an electrically equivalent searchcoil half that radius will kinda sorta more or less usually give a better response. Your mileage may differ.

[EDIT] now the question about a cache 2 feet deep, what size searchcoil will be best. There is no straight answer especially since there is no "given" regarding the effective diameter of the cache or what size of searchcoil is known to detect it. You might do fine with an 8 inch searchcoil and not need "best".

Carl had a thread detecting a cache at 24 inches awhile back. Been awhile since I read it, don't remember anything detecting it. Assume PI detector with GEB(300uH coil, 1 ohm resistance). If you were to wind one coil to detect it, what diameter would you wind? Would it matter if the ground was mild or severe?

Why does it matter what the target is if it's buried at 24 inches? I agree a 8 inch diameter could detect it if it is large enough but what diameter coil has the best chance of detecting it?

[Skippy]

Quote:"Why does it matter what the target is if it's buried at 24 inches?"

Because: one 'solid' target (fist-sized nugget, horse-shoe, plough blade, metal box....) behaves in a significantly different way to a collection of smaller objects (stoneware jar full of Roman bronze coins, canvas bag full of stolen jewellery...). The latter scenario appears to most detectors as just the top layer of objects, and there's virtually no electrical connectivity between the items, so they don't 'join together' to look larger than their individual sizes.
[and for VLF purposes, the use of low-frequency machines doesn't help. If the coins are '8KHz coins', a big load of them is still probably best found with an '8KHz detector'. So the ground pick-up is higher (than, say, a 2KHz machine) limiting depth.]

Regarding the 24" cache, I believe that several machines have successfully detected Gary's (garysdetecting) original cache in Oxfordshire...large-coil Deus being one, and 12KHz freq, too, if I recall correctly.

[Dave J.]

Carl had a thread detecting a cache at 24 inches awhile back. Been awhile since I read it, don't remember anything detecting it. Assume PI detector with GEB(300uH coil, 1 ohm resistance). If you were to wind one coil to detect it, what diameter would you wind? Would it matter if the ground was mild or severe? Why does it matter what the target is if it's buried at 24 inches? I agree a 8 inch diameter could detect it if it is large enough but what diameter coil has the best chance of detecting it?

Green, everything matters. Ground conditions. Electrical interference. Retune speed. Electrical properties of the target. How the metal detector is ground balanced. Whether or not there are other nearby esp. shallower targets. How the machine processes signals. How the user has the controls of the machine set. How the user sweeps the ground. Whether or not you sweep the thing over the target. Whether or not at that moment you're paying attention. Whether or not you're willing to dig a sound that doesn't jump out at you and yell HERE I AM, DIG ME! All those things interact with the issue of searchcoil diameter.

There are no straight answers your question. The place to begin is not searchcoil diameter, but to understand cache locating.

[Skippy]

Re: Dave's "Understanding cache locating" comment. I get the impression no-one does it, or they don't know anything about it. I started a thread on Dankowski's Forum about it last year, as I've been trying my hand at it, and thought the experienced folk there would have some wisdom to pass on. Zip Nul Nada Sweet.Fanny.Adams. Not one reply...oh well, I tried.

Back to Erics graphs: Here's the graph, I can't find the original explanation.

This is for PI machines, for circular mono coils, not accounting for ground effects.
Basically each solid curve is for one particular size target. Bigger targets have a higher-up curve. So take the point where the 40" coil just detects the large target 20" away. Now move left down that line, and when the coil-size reads 20", you will see a target depth of 17.5", as I quoted earlier.

I have never tested out this data, I'm not a PI guy, and how circular mono-coils compare to elliptic DD's is not my specialised subject : )

[6666]

You need the original explanation to get the best from the graph

[Skippy]

Yes, yes, yes, I know that, that's why I said I wasn't going to post the graph because I couldn't find the accompanying text. But as I didn't know whether I was looking for a .txt file, or a .png, or a .jpg, or .html, it makes it bloody hard to find, OK.
So just the graph will have to do for now.

OK, found it, it was an .mhtml file, never ocurred to check for them. BUT, I can't attach such a file ffs. I will have to think.....this is bloody frustrating, even a tiny text file won't attach, at just 21 K I'm told it's too f-ing big. Come on George, can the limit be increased please.

Retry:

[green]

Yes, yes, yes, I know that, that's why I said I wasn't going to post the graph because I couldn't find the accompanying text. But as I didn't know whether I was looking for a .txt file, or a .png, or a .jpg, or .html, it makes it bloody hard to find, OK.
So just the graph will have to do for now.

OK, found it, it was an .mhtml file, never ocurred to check for them. BUT, I can't attach such a file ffs. I will have to think.....this is bloody frustrating, even a tiny text file won't attach, at just 21 K I'm told it's too f-ing big. Come on George, can the limit be increased please.

Retry:

http://www.geotech1.com/forums/showt...438#post228438 Graph with text. Curves are for constant number of turns not coil inductance.

[green]

Green, everything matters. Ground conditions. Electrical interference. Retune speed. Electrical properties of the target. How the metal detector is ground balanced. Whether or not there are other nearby esp. shallower targets. How the machine processes signals. How the user has the controls of the machine set. How the user sweeps the ground. Whether or not you sweep the thing over the target. Whether or not at that moment you're paying attention. Whether or not you're willing to dig a sound that doesn't jump out at you and yell HERE I AM, DIG ME! All those things interact with the issue of searchcoil diameter.

There are no straight answers your question. The place to begin is not searchcoil diameter, but to understand cache locating.

If I buried a gold nugget at 24 inches deep and you could have it if you could detect it. You select the PI detector and have a choice of 5 coils(all have the same inductance, resistance and resonance) 8, 16, 24, 30, 40 inch diameter. Would you allow me to select the coil for you or would you rather select it and if so which coil would you select?

[Dave J.]

If I buried a gold nugget at 24 inches deep and you could have it if you could detect it. You select the PI detector and have a choice of 5 coils(all have the same inductance, resistance and resonance) 8, 16, 24, 30, 40 inch diameter. Would you allow me to select the coil for you or would you rather select it and if so which coil would you select?

Nice fantasy. I would certainly not allow you to select the coil, I would do it myself. The selection of the coil would depend on the specific circumstances, which your fantasy didn't define. In any case all it is, is a fantasy. People who do cache locating put some work into learning what's involved and they get their hands dirty in the process.