DetectingWales.com

Hi all, if I had a penny for every post I have read over the years on different forums and from different people on the question of relative coil depths then I would be a wealthy man.
The very same question is also asked about machines, so I thought I would do a little diagram that will hopefully explain why, regardless of machine and coil combination, VLF motion machines have a limit that is governed not by price but by the laws of physics.
This law equates to the fact that all soil is mineralised to some extent, this is read by the machine as a target as it has it's own conductivity. This is why we have ground balance, either manual or automatic, to basically discriminate this target out before we begin detecting.
Unfortunately this conductivity is cumulative, the deeper the machine goes the more mineralisation it sees, unfortunately for us the items we search for, lose their strength the further away from the coil they get.
Eventually the weakening signal from the target equals the strengthening signal from the surrounding mineralisation, making the target totally camouflaged by it's surroundings, as shown (badly) in the picture below.

(http://i693.photobucket.com/albums/vv294/Chef_02/targets.jpg)

This "effect" is obviously dependant on both the size (conductivity) of the target and the amount of mineralisation present. If you watch "Test" videos, especially from Eastern Europe and Turkey you will notice that the vast majority are showing targets buried in sandy soil, sandy soil is well drained and has therefore had most if not all of it's minerals washed out of it. Similarly air testing involves no mineralisation at all and so the machine can use full power on the target.

So why are the more costly machines seen as "better"?
Well they are more finely tuned to identify signal that are on the margins of this target/mineralisation area, on lesser machines it's a little like trying to read Braille with leather gloves on. Plus some do have more power to find deeper larger conductors.

Why then do larger coils go deeper?
They don't, well at least on the normal hammered and Roman coins we seek but do punch a lot deeper on larger conductors, some are also more finely tuned to the working frequency of the machine which again helps in those marginal areas between target and mineralisation.

The above explanation is why Pi machines naturally detect deeper as they are not effected by mineralisation and are limited  practically entirely by the target strength.
This situation I'm afraid to say will never change as it's how VLF motion machines work, so until a totally new system comes along, always question what the manufacturers claim ;)

Excellent explanation there Geoff I think that will go a long way to dispel some myths.

That is greatly explained and should help a lot of people understand better. At least until the same questions are posed by newer members.

I have some VERY mineralised fields. A real problem to detect and that could be one reason not much has come up. Signals are confused, changing all the time. Since last going some fields have been ploughed, rolled and seeded. Standing on the high part I could see the soil and was amazed to see how it changed colour. Some quite large areas were red with the iron that had leached out.  On these areas even the Garrett bleeped just touching the soil and the threshold was non existant.
I dug more iron than anything else even though the signals were for everything except.
I'd defy anyone to do well in those conditions no matter what machine or coil they may have.
Having said that I did manage to winkle out a small button, an iron scale weight, a copper slidy thing off a steelyard scale and a tiny copper hinge.

Val.

p.s.Well explained Geoff, good post.

Good on you for trying Val, check your "red" soil areas as these can quite often be a result of burning, unfortunately this doesn't help in any way as it only magnetises the soil even more but if you can retrieve any iron or non-ferrous from that area, it may help date your side. ;)

Def. iron Geoff. When I say big I mean 3/4 of the field.  I know it was very old pasture/scrub till the present farmer took over 3 years ago. He's the first one to plough them since Adam was born. In some of the dig holes, even now, if you go down deep enough, you can see clumps of old boggy stuff that hasn't rotted away.
I remember reading that this sort of ground would be highly mineralised from lying dormant for years.

Val

Yes if it's underlying clay you get iron pan on top of it, which I've got on a few fields and it's a nightmare.

Great explanation Geoff - I like the graphics too :)

I remember a former DW member giving me a field demonstration on a hammered farthing - great signal at 3" - almost nothing at 4".

I know that the land I search on is highly mineralised - lots of coke / lots of iron / lots of lead ore and the like.

It'll be good to see the difference between PI and not PI :)

Hi Geoff, could you stick this on DS as well. Great explanation and might help a few members gain a better understanding of how it all works  :)

Done ;)

well said Geoff and in simple terms as well

Cheers Geoff! ;D

Excellent diagram Geoff very K.I.S.S for interlectually challenged people like myself!  ;) ;)

An excellent post Cheffy - clear and concise. I have tried to explain this to numerous "non detecting" friends in the past when they ask the age old "how deep do they go?". I think I'll print this and just hand it to them in future! ;D

Its certainly in the running for "Post of the Year". By the way the prize this year is a days detecting with Taff Laff!

See you Saturday
Neil

Second prize is two days with him..  ;) :D

If I ever get around to giving a talk to Cardiff Archs (they have asked me if I will present a lecture to them), then I think that slide (with permission) would be an excellent visual aid.

It never ceases to amaze me exactly how deep non-detecting people think is the size and depth of hole that is dug in searching for things, although I've been detecting (properly) for a short enough time that I can remember what I thought :)

I know exactly what you mean Alan. I am often asked "how deep can you go". I sometimes jokingly say 6ft. Recently I told a works colleague 10ft. It is amazing that sometimes (as with the 10ft remark) they actually believe me. I shall have to stop doing it because I am going to neglect to tell someone I am joking and I will get the blame when they buy a detector and go around digging those size holes! ;D ;D ;D

A great explanation geoff very easy to understand nice one.

An interesting and topical post Geoff.

I have been helping an archaeological group on an off now for a number of years and it always amazes me how when I arrive on their digs and see how tall their spoil heaps are and that they appear to think our detectors will penetrate 3 - four feet !

Luckily I always carry a rake in which to level them out !  ;D

Well explained hard to see any great improvement in depth untill we have ground penetrating radar instead of coils. Than we would spend all our time digging deep holes. Would we find much more!

sounds like you need to run a large magnet over your fields first val

Isn't that on your list of things to build after the magnatometer? :)

We would find amounts you can only dream about! I detected a 10 acre site on and off for 12 months with my oldest finds being Georgian grots. This land was earmarked for residential development and after 8-10 inches of topsoil had been removed, in the following 12 months I recovered 76 hammered coins.
OK that site was a one off as it was in the centre of a village but it shows what is down there and out of reach.
But of course we would then have the moral dilemma of possibly damaging untouched archaeology, so it's probably better that we have the limitations we do.

I agree with you on the limited depth being a good thing, however every body wishes for more depth and many see good money to be made by providing it. I would prefer time and effort spent on a better ground balance and much better discrimination would follow. Deep holes and my back don't go together.

probono  mag is almost ready just waiting for SOMEBODY ! to get the last few parts then may look at gpr would have the full Time Team kit and not limited to 3 days just have to sort out a day rate? 

Always so good to read your input to the forum. Thank you Geoff.

At my age I find digging deeper holes very exhausting, sad is'nt it.

A great read Geoff, Really informative and easy to understand for simple folk like me :)

I have to agree with some of the comments regarding how deep none detector users think a metal detector will go. You can almost see their dissapointment when you have to say "no it wont find coins at a depth of at least 3 feet :D

Fab post chef ! Well explained so even a numbsty like me can see the logic  :)
Thanks

Great post Geoff

We would find amounts you can only dream about! I detected a 10 acre site on and off for 12 months with my oldest finds being Georgian grots. This land was earmarked for residential development and after 8-10 inches of topsoil had been removed, in the following 12 months I recovered 76 hammered coins.
OK that site was a one off as it was in the centre of a village but it shows what is down there and out of reach.

[/quote]

It is a good post liner ;) I also like the bit I quoted  ;)

hi thankyou for your help on advice for beginners may i ask a question witch metal detector do you think i should get il be useing it on the beach and mines please  :) its my first one

Think it depends on budget but there is good machines out there all doing the same thing. Just get yourself something that's simple too use. I myself started with a garrett ace week after got a minelab Xs and now 4 and half years later I've a ace 150 and 250. MinelabXs. And  c scope. Plus an at pro which im selling for my buddy. A good starter at a reasonable price would be the garrett range for sure.  ;)

OK well there is no such thing as beginners machines only budget machines, metal detecting isn't rocket science and starting off on any machine has it's learning curve and so your choice is going to be driven by cost but if beach detecting is going to take a large part of your time then my stock answer is a second hand Minelab Sovereign (any version) as there is no better VLF beach machine, you should be able to pick one up under £350. ;)

Thank you great piece,I am now a little wiser,and as I am a relative new by I will read it again and again,many,many thanks.

Val...... on that bad site of yours, if you get the opportunity try using P3 and dropping sensitivity down to 70 (yes, that low) it will make a huge difference, you are trying to drive in fog on high beam, try low beam, less is more sometimes.

Thanks for that advice Nigel. This is an old thread but I assume you're referring to the heavily mineralised fields I mentioned.
I do still occasionally get drawn back there, it's only 12 mins away, never find much and can't see why not. Maybe you've given me the answer.
70 - 85 is what Deus recommends for the beach and it works quit well. I'll give it a try.

Some people are discouraged to get metal detectors because they think that products are expensive. However, the good news is that prices of metal hunting devices have gone down, partly because of the increased competition from manufacturers. The price range of basic metal detectors can range from $50 to $300. Top of the line products, which are used by professionals, can be priced from $300 to $900. Whatever your budget, you can consider the company who made the device, included accessories, and of course the main features.

Did we ever hear what happened to Chef Geoff ?....
When he used to participate on this forum he was an absolute Guru with his vast amount of detecting knowledge and expertise.
He wisely advised me on many a topic... anyway,thanks GEOFF wherever you are..👍

Nice, but not quite correct. A bigger coil will punch deeper, including finding small items, as it has a bigger field, this is especially true of concentric coils - think 8" coil, 8" depth. 10" coil, 10" depth, etc.

Also, without bending the laws of physics, there is a way of coping with mineralisation in soil 'building up' the deeper you go, and this is by using more than one frequency, which is why multi-frequency Minelabs can find tiny items, clearly, at great depth.