Disturbed Ground?

Out with the F75 LTD today hunting pasture land on our place. Running bP mode, disc 4, sens 60, 4 tones,GB 63. Got a broken high tone. The target indicated 9" deep in pinpoint mode. I removed about a 6" plug of earth and swept over the hole with the 6x11 DD coil...Nothing...No signal at all. I almost dismissed the initial broken tone as an iron false that went away whenever I reoriented what I thought was a bit of iron by removing the 6" plug. Before I replaced the plug, I decided to check the hole with my Sunray probe. Sure enough I got a high tone (60-62 VID) in the bottom of the hole. After removing about another 3" of dirt from the bottom of the hole, I recovered a thin brass button about the diameter of a penny with the back missing. I used my Sunray probe to gauge the depth of the hole which turned out to be 8 1/2 inches after I measured my mark on the probe when I got back home.

My question is, why did I lose the signal on the F75 LTD after removing the plug of earth? The button was still buried right in the center of the hole and definitely was not disturbed by my initial removal of the 6" plug. I read a post a day or two ago where Jim Hillis was explaining how disturbed earth can affect a detector's signal reception. Was this what I encountered? Really strange to me that I lost the signal after removing the plug. Could something else explain this?



Edited 1 time(s). Last edit at 04/22/2010 03:45AM by dgc.

This has happend to me many times.
If I'm sure I had a good target I'll continue to dig until I find something.

If I'm not mistaken the less filtering the better when dealing with disturbed ground. Pf process over bP may have been the better choice in this instance. Whether or not it would have seen the target in the open hole could depend on many factors. Tom D correct me if I'm wrong.

Tom Z

It sounds like it was masked...

LowBoy

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If you don’t dig it, then how are you going to know what you’re missing!
How can you have your pudding if you don’t eat your meat!

Removing dirt should have HELPED the situation a little.......especially if the dirt has some minerals........ so as to give a more 'clear' shot to the target. I would still suspect that the target fell back into the hole......and went on edge. . . . . . although you very clearly state that the target was still in the hard-pack dirt ... deeper in the bottom of the hole. Hmmmmmmmmmm.

And the 'bp' mode should provide a much more Sensitive 'help' (assist) ......vs.......the 'pf' mode.

Yes it was definitely in the hard pack Tom. I removed all loose dirt and had a good tone with my probe afterward. I then removed another 3 inches or so of dirt before I got to the button which was right in the center of the hole. Really strange. Maybe the open hole was some how causing a negative detector response. The hole was rather narrow. Maybe 4 inches in diameter as I use a small but stout shovel to dig initially and then use my Lesche as I remove final dirt to recover a target. Found three more buttons today and one of those acted just like the one we are talking about i.e. no tone with the LTD after first plug was removed. I will say that there are a lot of cattle in the pasture and they leave a lot of liquid and solid calling cards. I have heard the salts in urine can create a problem for detectors. Maybe that's got something to do with it.

I remember talking about "ghost" signals when I first got my F75 and was told by Fisher "If you get a signal, and then lose it once you dig a plug, take another 3" out of the hole". I am very familiar with coins getting tipped on edge, or falling back down into the hole, but I can say for a fact, with the F75, here in Wisconsin soil (which is realively mild), this scenario happens a lot. Good signal, remove plug, signal disapears. It usually only happens on deeper targets, say 8" and beyond. I always thought it had to do with the change to the ground matrix and the "air gap" now over the target, but after your reply now I am not so sure. Any other reasons that you can think of as to why this happens? It is common with the F75/LTD machines, from what I hear. Thanks.

Interesting phenomenon. I can only speculate 'matrix change'; yet, I'm uncertain about this answer.

Scully Wrote:
-------------------------------------------------------
> I remember talking about "ghost" signals when I
> first got my F75 and was told by Fisher "If you
> get a signal, and then lose it once you dig a
> plug, take another 3" out of the hole". I am very
> familiar with coins getting tipped on edge, or
> falling back down into the hole, but I can say for
> a fact, with the F75, here in Wisconsin soil
> (which is realively mild), this scenario happens a
> lot. Good signal, remove plug, signal disapears.
> It usually only happens on deeper targets, say 8"
> and beyond. I always thought it had to do with
> the change to the ground matrix and the "air gap"
> now over the target, but after your reply now I am
> not so sure. Any other reasons that you can think
> of as to why this happens? It is common with the
> F75/LTD machines, from what I hear. Thanks.

Now you and markg have both said you have experienced the same thing Scully. Must be something to it. And yes, whenever this has happened to me, the targets have always been fairly deep (7 to 8 inches). I wonder if Fisher could give us an explanation since they seem to be aware of the phenomenon based on what they told you to do?
Tom, maybe with your connections at FT you could get us an explanation. Always good to know as much as possible about the detectors we use.

I did an internet search on losing signals while metal detecting. Came across this video. No explanation but it appears to be the same phenomenon. Watch closely. He definitely has a good signal initially and the loses it after removing some dirt.

[www.youtube.com]

Twice, Bill alluded to the target being in the side of the hole once some dirt was removed. The first time he mentioned it......they were both talking at the same time. . . . but you can still hear Bill's inference. You will also notice that when the coil was down in the hole at a diagonal.......searching a sidewall...... you will then hear the audio hit. I may be wrong.....but it looks like the target was in the side of the hole.

Yes maybe so Tom but he sure removed a lot of dirt (after his initial dig) before he got any kind of repeatable tone. Some of the beeps appear when he bangs the coil into the side of the hole. I've seen this many times when a coil is jarred. In any event, if you find out anything on the loss of signal phenomenon some of us F75 LTD users have experienced, would sure appreciate your passing it along. As always, thanks for all the info you exchange freely here in this excellent forum. Its gonna be stormy around my area for the next day or so, so all my detecting will be "virtual" lol.

I've experienced this type of signal also. Good or semi good signal initially and then it disappears with the detector. I started using a Garrett pinpointer this year and it's the best tool I've had since I started detecting. I'm getting older and lazier and it helps keep me from digging so much but it also helps me keep from hitting targets with my shovel. It also helps find some of these disappearing targets. I still have to continue digging sometimes to locate the disappearing target because it's too deep for the pinpointer and find it is usually off to the side of the hole somewhere. Using the 5" coil helps because I can get it in the hole and move it around until I hear the target and again it's usually off to the side of the hole.

Would like to know what's going on with these targets.

A pin pointer would have been nice

LowBoy

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If you don’t dig it, then how are you going to know what you’re missing!
How can you have your pudding if you don’t eat your meat!

In my experiences.........most of the time, the target falls deeper into the hole..and on edge. Yes, many times the target is still in the hole.... but in a side wall. Yet, this still does not explain the phenomenon of when the target is still positively in the bottom of the hole.....a few inches deeper.

I surmise; when the coil is sweeping over a linear 'set' of mineralization/dirt whilst aproaching the hole , , the detector electronically stabilizes to this linear condition...............then the coil passes over the hole and a 'delta' (differential) different 'set' of conditions incurs (less dirt/less minerals)................................and then resumes back to the initial 'set' of sweeping mineralized dirt (mild or otherwise) as the coil clears the hole and is now sweeping solid dirt again; = Causes a electronically measured 'change' (delta) with the metal detector; subsequently, 'masking' the true/real target.......as the actual metal targets signal strength may be WEAKER than the 'delta' (change) from the hole........and the detector remains quiet. This 'change' is slower with a open hole............vs...........the abrupt change of a metal object.............so the detector may remain quiet OR........more commonly.......... give a signal on the open hole. This is especially true in high mineralization dirt conditions. Can easily happen on wet-salt beaches. -- If you dig a hole in the wet salt without any metal targets around...........and you simply sweep the coil fairly fast over this hole; the detector will see this 'delta'......and possibly report it as a target. It's the same effect as lowering the coil too fast......or raising the coil too fast over wet salt sand (or mineralized dirt). You may get a 'beep'.

Said a bit differently; If you are relic hunting in fairly mineralized dirt and you are sweeping the coil steadily over the ground.........the electronics does know of the mineralization..........but because it's a 'steady' input........the detector remains quiet. Now...........whilst sweeping, you hit a metal target.........and the detector will 'see' this sudden delta/change............and report it.

NASA-Tom Wrote:
-------------------------------------------------------
> Causes a electronically measured 'change'
> (delta) with the metal detector; subsequently,
> 'masking' the true/real target.......as the actual
> metal targets signal strength may be WEAKER than
> the 'delta' (change) from the hole........and the
> detector remains quiet.

Tom, your explanation lines up with my observations. As I've witnessed, and Scully reported also, this phenomenon only occurs when we are dealing with a fairly deep (marginal) target. After reading your explanation I think about it this way. I start a hunt by ground balancing. The ground matrix at this point is, by detector design, made to be right at the threshold of detector response. Afterwards as I move along the detector sees a fairly flat ground response and remains quiet, operating right on the detector response threshold. Over an open hole the detector SUDDENLY sees LESS ground (minerals). If there is a WEAK target in the hole, the response of the detector to the open hole may be stronger than the WEAK POSITIVE response of the target and the target goes undetected (masked by the open hole).

Correct! (( I was concerned my explanation was 'wordy'/difficult )).

Scully I had that happen today with my F75.

But I also have numerous recoveries where I picked the target out of the hard pack down in the bottom of the hole.

A coin or button that gets tipped on edge is nearly impossible for a machine to detect at deeper depth, but I also always use a Pocket Probe pinpointer so I catch these right away.