Recently I have been thinking about depth and the size of DD coils and how one affects the other. For instance I have seen it stated that the elliptical coils offer better coverage lengthwise but sacrifice depth to do so. For example a round 6 inch DD would get more depth than a coil that is 6"wide and 8" in length but the latter coil gets better coverage by virtue of it's length. I'm not even sure what the difference in depth would be if any but I would think they would be close. also...
Paragraph 3 of page 99 of "The Minelab Explorer & E-trac Handbook" by Andy Sabisch states:
"Another interesting characteristic of Double-D coils is that unlike concentric coils , smaller coils do not have an appreciable loss of detection depth when compared to the stock coil. This means you can still find deeper targets with a small coil while eliminating the possibility of missing something due to target masking. Larger Double-D coils are also not prone to the typical loss of sensitivity to smaller targets that one sees on larger concentric coils. Many users of the larger coils such as the 15" Coiltek WOT or the new Detech / EXcelerator S.E.F. coils consistently report finding small artifacts or even coins on edge at impressive depths"
Do you think this is true? If I continue to use my SEF 6x8(which is easier to separate and pinpoint with)and not the stock 11" Pro-coil, that I will get comparable depth?
Having tested about a dozen Coils for the sovereign GT, I found the best Coils for them are the Coiltek Coils, Here is what I got from just 4 DD Coils
Using a Silver Hammered Cut Half Penny,
Type...........Size.........distance
BBS 800.....=8"........= 5 1/4"
BBS 1000...=10".......= 6"
Little Joey...=6x10"...= 7 1/2"
Platypus.....= 8x12"...= 6 1/2"
So the theory of big verses small is a double edged sword and is more related to Brand, and when you look at the Little Joey being a 6X10 it is the only Coil that provides the depth equal to the formula's 6x10 = 6+10=16 /2 = 8" (possible depth)
The Depth loss compare to size Is.
BBS 800 = 2 3/4"
BBS 1000 = 4"
Little Joey = 1/2"
Platypus = 3 1/2"
Now when you think that many claim that Elliptical Coils loss depth compared to their Round equivalent The 8" ML 800 coil looses to 2 1/4" to the Little Joey which is equal in size to the ML BBS 800 and the ML BBS 1000 looses 1 1/2" to the Little Joey, The SEF and Excelerator Coil results were very much mixed in and around the same figures as the Standard Coils, But because they are not quite as sensitive they seem to handle mineralization better than the standard coils but EMI was a little worse.
I know this is an old thread, But what Andy say's here is Not quite right In that On small Items All small coils will see small Items deeper Except for DD's and what a Small 4x6 DD will see at 1" inch A 6" inch Concentric will see at 3" inches, And the same applies to when you compare the 950/9.5 concentric coil to the 10" D2.
This all changes once the Item gets larger than say A 1/4" X 1/4" bit of flat lead, Then Both coils are equal, It is often said that it is unfair or wrong to compare a Concentric with a DD because the concentric is bound to be deeper, But with the 950 coil and the 10" D2 Coil this is not the case, In and Air Test using a Quarter they are Both equal and It is only on Tiny Items that the 950 Concentric pulls away by 3 times the distance, Even when you compare coils that are meant for prospecting like the 4x6 DD It is A lot less sensitive compared to the 950/9.5 Concentric coil, As in what the 4x6 see's at 1.5 to 2" inches the 950 will see at 3 to 4" inches,
Now many folks claim that their ground is too hot to run a Concentric Coil in Gold baring ground, Well that might be true to some extent but that all changes too, Now the 4x6 see's bits at 1 to 1.5 inches, Now it is going to loose "say" 30% depth in hot ground that brings use to about 1" of depth, Now throw the 5.3 (6") concentric in to the mix, We already know that they can see bits at around 3 to 4 inches Now add the mineralization loss of say 50%,,, That leaves us with a depth of about 1.5 to 2" inches and the final figures tell us that by using a concentric coil you are going to Gain between 50 to 100% more depth over a DD of equal size In hot mineralized soil.
Now too many people use DD's as a Crutch to justify bad detecting habits and blame the machines and Coil or Both, Once their ground meter reads above halfway they reach for the DD and then bag the machine as being not very deep, Or because their mate swears by them then it becomes a must have Coil and by doing any of the above a person is Wrecking any chances they have of finding deeper targets, Not to mention having a miserable day trying to swing a huge coil just to gain an extra inch or two,
There are a couple of things to remember about coils that relate to the "possible depth",
A DD can give a Coin depth equal to it's size,, So a 6" DD = 6 inches, A 4x6 = 5 inches ( 4+6 = 10 divide by 2 = 5 ) A 6x10 = 8 (6+10= 16 / 2 = 8) and so On,
Now with a concentric it is easy to dig coin at the depth the same size of the coil, BUT they can reach depths of at leased 1.5 X their size in depth, But the Average depths achieved are more like 1.3 X the size of the Coil, So the 6" CON = to about 8" but 9" is not unheard of the 950/9.5 coil is 9.5" X 1.3 = 12.35, which I have achieved on a regular basis, and the 12"/300 Coil is 12 X 1,3 = 15.6/16" and the 15" Concentric is 15" X 1,3 = 19.5". Now remember these Depths are only Averages, With the 950 coil I have hit Coins between 11 and 13" all day and with the 12"/300 Coin they have come out at around 14" In heavy Iron and fertilized ploughed Farmland, So I do not have much faith in all the DD stories because side by side testing says otherwise,
Now to work out what size DD you will need to match the Achievable Depths of Concentric Coils then if you look at the Answers of all the above Sums, because those are the Coil sizes that you will need remembering that a DD can not see further than its size As a Rule, There are times they will go deeper but not on a regular basis like the Concentric Coils do,
In prospecting Country I know of a few people who have done extremely will using the 6" Concentric coil in hot ground because it can handle the high mineralization because of it's small size but it retains most of the Depth because the machine is not seeing as much of the Ground, I have personally tested this coil in some of the hottest ground on the planet and so have others and they found the Gold others were missing.
Other thing to remember is Discrimination DD's Love Flat Iron and Crown Caps.
I own Both DD's and Concentric Coils, I love how the Concentrics go very deep and can see those Tiny Bits etc But I also like how the DD's handle Hot Ground, But there is a right time and a wrong time to use both, The Only coils that give me doubts as to which one to use Is the 950 Coil or the 10" DD because they are both very well matched when it comes to depth But when it comes to tiny bits the 950 pulls streets ahead.
hope this helps
J
