Left to right vs up and down

[DaveP]

Your starter for 10 and just curious to see if anyone knows the physics of this.

Equinox 600, searching in all metal, I routinely get buttons (still hate the bloody things) that register anywhere from say 12-18 with high tones when swinging left to right. The numbers and tones stay constant. However, if I move the coil up and down I get strong iron tones and negative numbers mixed in with the high numbers and tones. I assume this is coming from the shank area. It's virtually always a button, shotgun cartridge base or very degraded rifle cartridge base. They still get dug as those high numbers are just saying "you know you can't walk on until you've checked me out".

But, what's the physics of a target passing through the coil left and right vs up and down and, for the same settings, giving obviously different responses?

[Blackadder43]

Actually a great question...
I cant think of any logical reason why this should happen.

Now you have posed the question though i think another experiment is required when you are next out and with one of these signals

Next time you suspect one of these situations where the left to right is the sweet signal, and the up and down is the potential iron signal, then turn 90 degrees to the target and sweep again
This way your original left to right good signal, will now be an up and down movement signal

I have no idea what this would show or prove as i'm only on my second brew this morning, but it should be interesting....

[DaveP]

Actually a great question...
I cant think of any logical reason why this should happen.

Now you have posed the question though i think another experiment is required when you are next out and with one of these signals

Next time you suspect one of these situations where the left to right is the sweet signal, and the up and down is the potential iron signal, then turn 90 degrees to the target and sweep again
This way your original left to right good signal, will now be an up and down movement signal

I have no idea what this would show or prove as i'm only on my second brew this morning, but it should be interesting....

Nope! I tried that yesterday as I was determined to try and sort it out. The day job requires me to have an analytical and experimental mind. The trouble is the left-right signal even when done at 90 degrees is saying "come on, you know you want to find out.... go on, just a little dig ... you know you can't walk on" and just as you raise the coil it goes "fart". Bugger! Move the coil up and down and it's fart, fart, fart, fart ..... fart. Back to left-right and it's a temptress in a low cut ....... well you know
Then dig it out and hey presto - another $£@%^*! button or shottie.

And, I re-checked every hole to see if there was another signal confusing the machine - nope.

[Blackadder43]

Are we using a perfectly round coil or the elliptical shape coil?
As you know both coils act differently in their signal sending, spread and reading on return
This may have an impact?

Edit:
Another possible experiment you have probably already tried, but maybe knocking down to a single frequency and repeating again?

[DaveP]

Are we using a perfectly round coil or the elliptical shape coil?
As you know both coils act differently in their signal sending, spread and reading on return
This may have an impact?

Edit:
Another possible experiment you have probably already tried, but maybe knocking down to a single frequency and repeating again?

Standard 11" round coil.

Single frequency - hmm. It reminds me of a post I made in 'another place'. On my old T2 (elliptical coil) I had noticed that with some 'dig me' signals if you waited a little and kept going over the target an iron number (not always tone) would creep in. It was as if something was slowly building up in the target. I never got an answer to that but it was quite obvious and just required a little patience. And, the signal was very often a button or shottie.

When I'm next out I'll give the single frequency a try on the Nox. I've also sent a message to Minelab to see what they say - assuming they know and reply. It won't really change what I dig but it does change my level of expectation.

[Easylife]

All of the Minelab Equinox coils are DD even the round ones, so that will explain your directional differences. Maybe you assumed that the standard round 11" coil was concentric?

[DaveP]

All of the Minelab Equinox coils are DD even the round ones, so that will explain your directional differences. Maybe you assumed that the standard round 11" coil was concentric?

No, just a round coil vs elliptical. But even if you consider DD or concentric, what's actually going on to give the different signal for the same object?

[Easylife]

I'll try it out later to see if I get the same results. With DD coils the sweet spot is not always centre.

[Easylife]

No, just a round coil vs elliptical.

There should be no difference between shape if both are DD.

[DaveP]

With DD coils the sweet spot is not always centre.

Agree with that and all the images they show of the field pattern would say that too. I'm sure I'm missing the obvious but with a short left-right swing the target will pass through the sweet spot. In the same way I would expect there to be an up-down sweet spot. The bit I'm not understanding is why I can have a non-ferrous target response left to right but have an obvious mixed non-ferrous/ferrous target up and down.
The next experiment should be to pass the coil left-right at different heights and see if the mixed signal can be achieved.

Let me know how you get on

[Pete E]

I would imagine the targets have a small amount of ferrous in them is a button shank...AMy guess is that as the coil approches different directions, the machine "sees" the combinations of metals differently and gives different result???

[Easylife]

Let me know how you get on

I did some air tests with various targets of different size and metal composition. First with the 15" coil and then with the 11" coil and they were all good both ways with no iron grunts at all. Then I got a lump of iron together with a copper item and it was still good both ways. I run Iron bias 0 and Iron vol 1. A toy train wheel with an iron spindle also sounded good and clean both ways when flat but was quite jumpy on it's side and a bit grunty both ways.
To me it just sounds like a case of ferrous being close to the target. A ferrous button shank is unlikely to even register. Larger ferrous or even two or more smaller bits can blank out a good target one way in complete silence yet it can sound good the other way. Maybe if you had tried it at 90 degrees you would have got the same effect? Instead of forward back motion, wiggling side to side whilst moving back until the target just dissapears under the front of coil can be a useful alternative to turning at 90 degrees.

While I was at it I checked out the sweet spot on the coils. On the 15" coil it's 3" forward and about 1 1/4" right of centre, which I already knew as I use that one 99% of the time. On the 11" coil it's just ahead of centre.

[DaveP]

Easylife, thanks for the input- - much appreciated.

I've just done an air test on one of the problem targets - it's a rotted out primer (note to self to clean the keyboard).

Nox run in single frequency - good clean and bright tones from maximum detected distance all the way to almost touching the coil.
Nox in multi-frequency - differing signal left to right, not as clean or bright at same max distance, and as I move towards the coil I get the occasional iron grunt but it occurs at different distances i.e. not consistent in the z plane.

So two things. First, Blackadder's idea of trying some single frequencies in the field seems even more worth doing. And, lifting the coil (happens as I move the machine away to put it down) would fit with getting an iron tone on this target as it's stationary in the dirt and the detector is moving in the z plane.

The next similar target I get in the field I'll go through the gamut of settings and report back.

[Easylife]

Are you using 50 tones? Field 1? Iron Bias setting? Single frequencies usually give a slightly different TID to multi. Multi frequency will give the most accurate TID. Rotted shotties are not the best item to experiment with, a button would be better. As you say experiment more in the field to figure out the cause.

[DaveP]

Nox 600. F1, 5 tones, bias 2.

The rotted shottie is a typical target where I hear the iron tone. Just tried a couple of buttons in multi frequency. First button was not a problem, second one gave iron tones but at two distances from the coil i.e. not all the way through the z plane.
I didn't look at the TID's - just went by tones - my arms aren't that long

The fact that it happens in the field is not a problem - it just lowers my expectations before I start to dig so anything better than a shottie or button is a bonus. I'm just curious to understand the physics of what's happening.

As I mentioned I could get a similar thing on the T2 if I just gave the target a little extra time under the coil.