Choose fontsize:
Welcome, Guest. Please login or register.
News
celticspikey
October 13, 2019, 02:09:26 PM
 PMd you Val
Neil
October 08, 2019, 02:26:21 PM
 At ya Nobby
Neil
October 07, 2019, 02:32:36 PM
 Back Nobby
gesza
October 06, 2019, 08:34:19 AM
 Oh yes I remember val 
Val Beechey
October 05, 2019, 07:39:18 PM
 Funny the first time. Remember ?
Val Beechey
October 05, 2019, 07:38:46 PM
 Gestation
Val Beechey
October 05, 2019, 07:38:31 PM
 Gestrat

View All

 

Currently there is 1 User in the Chatroom!





Click here if you
need van signs


Or here if you
need magnetic signs


Or here if you
need a
Corporate Video Production Company in Milton Keynes

See our
privacy policy here


Pages: [1] 2 3
  Print  
Author Topic: Don't be fooled by the hype!  (Read 24940 times)
Chef Geoff
Archaeological and Hardware Advisor
Dark Lord
**********
Offline Offline

Posts: 9368



WWW
« on: March 27, 2013, 08:51:57 PM »

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.



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 Wink
Logged
Tafflaff (Rob)
Deputy Administrator
Superhero Member
*****
Offline Offline

Posts: 5731


Etrac user 55


« Reply #1 on: March 27, 2013, 08:59:09 PM »

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

There is only so many times one can turn the other cheek.
cardiffian
Hero Member
*****
Offline Offline

Posts: 845

Minelab Explorer, Deus


« Reply #2 on: March 27, 2013, 09:36:08 PM »

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

2014 
Hammered                 5         
Roman bronze          10
William 111 shilling
Disc brooches             2
Med. silver annular brooch
Val Beechey
Superhero Member
******
Offline Offline

Posts: 6030



« Reply #3 on: March 27, 2013, 09:41:34 PM »

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.
Logged

Ever Optimistic, it's out there somewhere - And I Found it
Chef Geoff
Archaeological and Hardware Advisor
Dark Lord
**********
Offline Offline

Posts: 9368



WWW
« Reply #4 on: March 27, 2013, 09:51:23 PM »

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. Wink
Logged
Val Beechey
Superhero Member
******
Offline Offline

Posts: 6030



« Reply #5 on: March 27, 2013, 10:01:33 PM »

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
Logged

Ever Optimistic, it's out there somewhere - And I Found it
Chef Geoff
Archaeological and Hardware Advisor
Dark Lord
**********
Offline Offline

Posts: 9368



WWW
« Reply #6 on: March 27, 2013, 10:33:15 PM »

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.
Logged
probono
Superhero Member
******
Offline Offline

Posts: 2434


Nihi nisi sub sole


« Reply #7 on: March 27, 2013, 10:35:54 PM »

Great explanation Geoff - I like the graphics too Smiley

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 Smiley
Logged
Korky III
Rally attendee
*
Offline Offline

Posts: 19


« Reply #8 on: March 27, 2013, 11:26:09 PM »

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  Smiley
Logged

Fisher 1266x Pro
Garrett Ace 250

It's there, somewhere!
Chef Geoff
Archaeological and Hardware Advisor
Dark Lord
**********
Offline Offline

Posts: 9368



WWW
« Reply #9 on: March 27, 2013, 11:36:27 PM »

Done Wink
Logged
Dryland
Superhero Member
******
Offline Offline

Posts: 3824


Keeper of the Lights


« Reply #10 on: March 27, 2013, 11:37:02 PM »

well said Geoff and in simple terms as well
Logged

If money is the root of all evil,why can't we spend parsnips ?
Korky III
Rally attendee
*
Offline Offline

Posts: 19


« Reply #11 on: March 28, 2013, 05:15:40 AM »

Cheers Geoff! Grin
Logged

Fisher 1266x Pro
Garrett Ace 250

It's there, somewhere!
WeCan-Octavian
Superhero Member
******
Offline Offline

Posts: 1032


''On it like Sonic''


« Reply #12 on: March 28, 2013, 07:23:21 AM »

Excellent diagram Geoff very K.I.S.S for interlectually challenged people like myself!  Wink Wink
Logged

''By order of the long haired general'' I now use a Minelab !!!!!
Neil
Administrator
Superhero Member
*****
Offline Offline

Posts: 4947



« Reply #13 on: March 28, 2013, 08:53:10 AM »

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! Grin

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
Logged

There comes a time in every rightly constructed boys life when he has a raging urge to go somewhere and dig for hidden treasure.

Mark Twain 1835 - 1910

If anyone wants to sell any S c r a p gold or sovereigns, regardless of condition -  ask me for a price first please.
ysbytymike
Hero Member
*****
Offline Offline

Posts: 733


Bliss V3, Headhunter Pro, Teknetic T2, C-Scope CS7


WWW
« Reply #14 on: March 28, 2013, 08:57:20 AM »

Second prize is two days with him..  Wink Cheesy
Logged

Life is all about questions - Death brings all the answers   (md)

Pages: [1] 2 3
  Print  
 
Jump to:  


Home
SimplePortal 2.3.3 © 2008-2010, SimplePortal