Coil Bridging?

I just read this statement in a post on another forum.

The small DD coils have a sharper response to targets but require more difficult "compass direction" turning of the coil to defeat bridging. This is due to the front to back on edge rectangular detection field. The concentric coils have less bridging on deeper coins because of the funnel shaped detection field. It's really a matter of personal choice as to which one to use.

Would someone please explain what "bridging" is?

Thanks

To me it sounds like the poster is comparing two isolated incidents using the different coil types. By 'bridging' I think he is referring to the "blade" shape detection field picking up multiple targets that happen to be inline with the field. Tom posted about this a while back stating that moving the coil forward or back just a tiny bit will lose a target due to masking from another target that is inline.. ie: hits only in the sweet spot. This is true and why we need to overlap a DD just as much as a concentric if we want to get the deeper or severely masked items. The footprint of the Fisher Tek DD goes a long way in limiting this type of masking but its still there.

Even when well centered with either coil you may or may not get a hit at off angles. The variety of in ground scenerios of target orientation and masking are countless and every one unique. All coils 'mask' in multiple target scenerios but the technique is the same.. overlap overlap overlap.

JMHO

Tom



Edited 1 time(s). Last edit at 02/14/2010 12:46PM by Jackpine.

Yes, the analogy here is correct, IRT engineering term 'bridging'; however, , , , , like the concentric coil, , , , , there still lies a 'funnel/cone-effect' .......even with the elliptical DD's fore-and-aft rectangular electromagnetic footprint. The fore-and-aft EM footprint STILL has a cone-effect (on the fore-and-aft axis); subsequently giving a (aka) 'sweet-spot' in the dead-center of the coil........especially at depth. I'm not sure if Minelab.......or the Minelab users started this rumor/assumption of a 'square' (or rectangular) footprint at depth. This is untrue.........and has nothing to do with Minelab, Fisher or any of the others of whom utilize a DD coil. It's a principle of physics......dictated by coil config. The coil does not know if it's connected to a Minelab/Fisher/Tesoro/Tek/White's/Troy.......control box. EM energy is EM energy.

Yes, rotating your body around a target.........especially with a DD coil .... will accentuate/expose this phenomenon to an extreme.....when the coils footprint starts to 'bridge'/detect/pick up another target under the coil at the same time it is continuing to detect the original target. You may hear a target with good resolution/audible clarity......then start rotating your body around the target whilst continuing to sweep......subsequently entering into a bridging target.....and wonder why the ID starts to dramatically change. THIS IS WHY it is critical to detect pay-dirt from SEVERAL different directions (angle-of-attack/approach)......AND overlap your sweeps!

Does this clear up anything?

Tom

So I take it then that in heavy trash areas one is better off using the smaller "sniper" type concentric coil instead of the DD for better separation at depth? Now I understand why the guys in the Whites forum who really know what their talking about prefer the 5.3 concentric coil over any of the DD coils for trashy areas.
Thanks alot, Ive been wondering about this for along time.

Many thanks for all the replies.

So bridging is basically adjacent secondary metal objects in the EM field of the coil pattern. Compass turning hopefully finds a sweet spot that eliminates the secondary objects from the field.

I've searched the internet for a technical discussion of DD coils and what I found didn't go into any detail. I couldn't find a diagram of the pattern of a DD coil. Does anyone know where I might educate myself on DD coil physics and patterns? I'm familiar with the inverted cone shape of a concentric. Is the pattern of a DD coil much narrower along the axis of coil motion?

Aaron Wrote:
-------------------------------------------------------
> Now I understand why the guys
> in the Whites forum who really know what their
> talking about ...

* wow *

Aaron. NO!!! In searching trashy areas.....two primary things must be taken into consideration IRT coil size/selection:

1. You want to minimize "VOLUME" of dirt being analyzed in trashy areas. A concentric coil will see/analyze a larger volume of soil .....comparable coil "size-for-size".....compared to its DD counterpart.

2. You will nearly always have better adjacent target separation with a DD coil......vs......concentric coil. Bridging can happen with both types of coils; moreso with concentric coils.......due to greater VOLUME of dirt being analyzed at any given point in time.

The more soil analyzed.....the greater increase in chances that more targets will be in the same 'analyzed' volume of soil; a bad thing.

IF WE CHOOSE TO REMAIN IN THE 'ELECTROMAGNETIC MEDIUM' MODE/METHOD TO DETECT TARGETS (making no effort for a paradigm shift) COIL TECHNOLOGY COULD/SHOULD ADVANCE TEN-FOLD. There's a lot of room for improvement. (Understanding....it'll be a difficult undertaking).

Tom

dgc Wrote:
-------------------------------------------------------
> Many thanks for all the replies.
>
> So bridging is basically adjacent secondary metal
> objects in the EM field of the coil pattern.
> Compass turning hopefully finds a sweet spot that
> eliminates the secondary objects from the field.
>
> I've searched the internet for a technical
> discussion of DD coils and what I found didn't go
> into any detail. I couldn't find a diagram of the
> pattern of a DD coil. Does anyone know where I
> might educate myself on DD coil physics and
> patterns? I'm familiar with the inverted cone
> shape of a concentric. Is the pattern of a DD coil
> much narrower along the axis of coil motion?


On Minelabs website they have some grahics on coil field shapes that are the best approximations I have seen. I think its under the SD GP gold detector coil section. IRT concentrics it is NOT an inverted coin shape field. True, at depth the sweet spot is very small but DD coils are the same.

Tom, Something you rarely see mentioned is the question of iron and how it effects the the EM field shape. True or false, can iron cause a warping of the field in-ground and at times actually enhance rather than hinder detection of items? Hard to prove but theorectically possible?

Tom Z

Jackpine Wrote:

> On Minelabs website they have some grahics on coil
> field shapes that are the best approximations I
> have seen.

Thanks. I found the coil graphics. Helps a lot.

Aaron I like to read all the forums . I see the Whites guys like the
concentrics better. A better combo with there machines .

I think FT and ML have better DD coil and detector combos

Tom Z. So far...........only with the T2/F75's. All other detectors suffer severely when EM is exposed to iron. Signal is severely attenuated. I am NOT saying the T2/F75 is 'magic'..........it's just an inherent 'flaw' with BOTH 'positive' & 'negative' side-effects. Somewhere...............I know I posted this exact phenomenon/encounterings/experience with F75.

In short (because I'm short on time!); I found a 13" deep 1963 silver Roosevelt dime with F75. (((Welcome to FLA's rapid sink-rate soil))). Under normal circumstances......this is not possible; however...... a very small (almost completely decomposed) iron boot tack was at the 11" depth.....just above.....and barely to the side of the 13" deep dime. The small iron tack/nail 'broke squelch' on the detector and a very small portion of the silver dime detection (composite signal) 'broke through' the squelch with the iron. The excessively deep dime signal was cumulatively 'riding the carrier' of the iron (+ dime). The composite (nail & dime) ID was terrible. NO idea of a silver dime. ID's were bouncing between 11 - 96......BUT.....I was certain there existed something more (in addition to) just a piece of iron/iron-oxide. I knew a non-ferrous target was in co-locate.
I know of no other detector platform of which will produce such resultant.

Tom

Thanks for the response Tom and your thoughts. On the dime you mentioned, I have always characterized that type of repsonse in my mind as targets that are on the borderline of the detectors ability to seperate co-located non-ferrous / ferrous. Adding the carrier aspect is interesting. It is MORE apparent with faster machines such as the T2

Tom Z

NASA-Tom Wrote:
-------------------------------------------------------
> Tom Z. So far...........only with the T2/F75's.
> All other detectors suffer severely when EM is
> exposed to iron. Signal is severely attenuated. I
> am NOT saying the T2/F75 is 'magic'..........it's
> just an inherent 'flaw' with BOTH 'positive' &
> 'negative' side-effects.
> Somewhere...............I know I posted this exact
> phenomenon/encounterings/experience with F75.
>
> In short (because I'm short on time!); I found a
> 13" deep 1963 silver Roosevelt dime with F75.
> (((Welcome to FLA's rapid sink-rate soil))). Under
> normal circumstances......this is not possible;
> however...... a very small (almost completely
> decomposed) iron boot tack was at the 11"
> depth.....just above.....and barely to the side of
> the 13" deep dime. The small iron tack/nail 'broke
> squelch' on the detector and a very small portion
> of the silver dime detection (composite signal)
> 'broke through' the squelch with the iron. The
> excessively deep dime signal was cumulatively
> 'riding the carrier' of the iron (+ dime). The
> composite (nail & dime) ID was terrible. NO idea
> of a silver dime. ID's were bouncing between 11 -
> 96......BUT.....I was certain there existed
> something more (in addition to) just a piece of
> iron/iron-oxide. I knew a non-ferrous target was
> in co-locate.
> I know of no other detector platform of which will
> produce such resultant.
>
> Tom

gman wrote:

"Aaron I like to read all the forums . I see the Whites guys like the
concentrics better. A better combo with there machines"

Yes, I too like to look at other forums also. I only have Whites and Fisher machines. So those are the forums im usually on. I know how some guys feel about Whites on this forum some of which may or may not be true. It doesnt really matter to me because there are guys on the Whites forum that have been detecting for over 40 years, like Monte for example. It doesent matter what kind of machine a person with that kind of experience uses, the knowledge he has is very valuble to anyone who detects. I think it is very narrow minded to be critical of a group of fellow detectorists just because of the machine they use.
I only say this because of the seemingly "sarcastic" remark that was left on my last post on this thread.
Happy hunting to all!
Aaron



Edited 1 time(s). Last edit at 02/16/2010 01:08PM by Aaron.

Strong bias = burning passion blindness. ((( Not necessarily a bad thing ))).