GEB

[Tinkerer]

Was playing with GEB this mourning. I have a sock with pennies, nickels, dimes and quarters in it, about the size of a baseball. With the sock close to the coil I could rotate it a little and null the signal with the GEB times I had set to balance the clay. The best sample and delay times for one situation might be worst for another. Longer GEB times seemed to help for single coins, maybe not for a lot of them. I'm still guessing what's best.

I have long suspected that the aggressive soil response is composed of several TC's. Something like 50% of the response has a TC of 10us. 20% has 30us. 30% has 5us. This would explain that compensating for a single TC is not perfect. Just guessing, as I have no such soil to make tests.
Your bag of coins represents TC's of a very wide range. Single coils, presented flat to the coil have a specific TC.

The soil under a coil has a very large surface area. The surface area accounts for the amplitude of the response. When Eric Foster made lab tests with soil, he used a very small coil, so that the soil sample was as big as the coil flux area.

[green]

Charted some data to see why the sock of coins nulled(hole). First try with sock, don't know if it would null. Clay slope closer to -1, usually steeper. Doubled Tx on time(160usec), slope steeper but tailed off similar to Tx 80usec. With clay tailing off and sock of coins flattening out I think I see why a hole if GEB time greater than 100usec. Recorded data at three scope settings to improve resolution. Don't know why a flatter slope or why the slope tails off instead of staying straight.

[green]

It's not making sense. I thought if the traces lined up when I changed the scope scales it would be alright to do so. I changed the times, 8usec delay, 5.6usec target sample, 15.2usec delay, 60usec GEB sample. Nulls clay and sock of coins if adjusted. With total time less than 100usec I didn't think the coins would null. In the past I've tried to keep the scope from over scaling when recording decay slopes. Not sure what the problem is yet.

[green]

Added a 2 stage amplifier(x9 each stage) after the preamplifier. Recorded each stage_x275,x2475,x22275 with and without the target. Subtracted the no target recording from the target recording for each gain stage and charted. Reply #32 I changed the scope gain to get the added resolution. When the scope input over scales it messes with the signal. I thought charting the difference of target no target would be alright. It's NOT

[green]

I've been wondering how Tx shape and on time effect ground decay and the decay of a target with a longer TC. Thinking of trying Tx on time of 40usec, 80usec and 250usec. Keep peak current 1 amp. Constant current(turn on and charge to 1 amp as fast as circuit can and hold 1 amp). Constant rate(start at zero and hold rate constant during on time) 40usec(25000amps/sec), 80usec(12500 amps/sec, 250usec(4000 amps/sec). Thinking of charting clay and a US quarter or 5 US quarters stacked. Mark coil for target location, use for all runs. Tried 5 stacked quarters at 80usec, slope a lot closer to clay than one quarter. Maybe someone can tell me what to expect, or has a better suggestion for the long TC target. Looks like GEB can have a big effect on sensing some targets, wondering if there is a Tx on time that effects the targets less. I think the data makes sense using 3 stages of amplification to increase the resolution, getting the chart time out past 200usec(probably more than the target and GEB sample and delay times combined).

[green]

Charted some of the data. Took 40usec for the coil to reach 1 amp with constant current control so I haven't run the 40usec charts. The constant current is a little higher amplitude than constant rate. I think if peak current was increased for equal average current they would be close. 240usec is a little higher than 80usec. Not sure it's worth the lot higher average current. Needed a heat sink for the mosfet at 240usec. Probably wouldn't need one if the current wasn't under servo control. The clay slope reduced for the 240usec constant current run. Looks like the amplitude gain wouldn't help if using GEB.

[Funfinder]

Your tests here are great for further improvements of metal-detectors
and yesterday I have also tested the behaviour of metal-objects
under real high mineralized clay conditions.

Result:

We have to consider that the GEB will mask finds if its set to higher mineralized!
Because in that case the "energy" of the coverying clay makes the find invisible
to the coil if the GEB was set to: "ground mineraliziation completly deleted".

But of course therefore the detector really needs a ground-balance that even
is able to filter out extremly high mineralized ground, which most detectors don't have!

The Makro Racer has it, the GEB standard is at Level 90 but for high mineralized soil
it even can be reduced to zero so there will be no more fail-signals at all!!

But if set to zero the 10cm layer of clay is so strong that the 3cm rusty-steel shard
no longer was detectable while if set to Level 15 it still is - with only very few GEB distortions.
Those are reducable by moving the coil more slowly and 5cm away from the surface.

Facit:
For best results the GEB should be adjusted also to the wanted finds:

Finding silver coins at high mineralized locations by seting the GEB to max. mineralized
is no problem but little gold-nuggets easily can made invisible if the GEB is too high at
regions full of red soil, hot rocks or loamy river-beds.


And at low to very low mineralized soil, GEB is a question of detection depth!
Like as in air usually no GEB would be needed there at all but if set to high mineralized
the detector will be more sensitive to iron or deep relics while set to low mineralized or "off"
it will detect silver coins or other noble metal-stuff deeper, ca. 15-20%.

Almost the same applies to medium-mineralized soil but here of course
a compromise has to be found to make the GEB ground distortions vanishing
good enough while still not discriminating some deep iron and weak gold signals!

So for real good GEB tests with objects inside or below high mineralized materials
always the GEB value should be checked from where certain finds start to vanish if
the GEB was set to higher mineralized levels.


Very simple explained:
1kg of clay or heavy min. soil can have the same effect to the detector as 10grams of iron or 2grams of gold!

[green]

Funfinder, thanks for the reply. Don't know how to compare GEB with a PI vs VLF. I could null the sock with coins the other day. I tried the US quarter and a 1oz copper coin this mourning with GEB. Could null the US quarter but not the copper coin by rotating the coin. Need to experiment more with coin angle and location on coil. KR is experimenting with 40usec Tx on time, so I charted the 40usec to see what it looked like. The ground decay slope is steeper and the amplitudes are a little less. Look at the first 40usec of the 80usec constant current scope trace for current wave form. Almost constant rate. The clay and the US quarter were in the same locations for all of the runs.

[green]

Played with tilting the US quarter and the sock of coins. Adjust tilt angle of quarter with coin in center about 2 inches away from coil for a null with GEB on. Slide coin to a different location(same angle and distance) reads higher than with no GEB. Sock of coins, maybe 15% of no GEB. The previous runs were with quarter near the edge of the coil. Charted one with the coin in the center. Lower amplitude, similar decay slope.

[green]

Carl started a thread, (Cache Test) back in 2006 that I found interesting. The reason for a lot of the GEB testing. Wanted to determine the major problems in detecting the cache. I don't have a bunch of silver quarters. Maybe the coins I've been using don't give the same results. One more set of charts using US pennys. Coin orientation has a lot to do with it. Like I stated earlier the 1 oz copper coin doesn't show the effect. Maybe large silver or gold coins wouldn't either. The coins in the sock were bunched up and were in mixed orientation. Wasn't adjusted for a null, just set on spacer. The -1.48 slope is what I got for the clay at the Tx settings. Decay slopes for the clay and the sock of coins very close(hole).

[green]

Wasn't thinking about coin position on the coil effecting what happens when it is rotated. Charted integrator out with the coin spaced 150mm from coil, flat and on edge over coil center and coil edge. With GEB the copper coin didn't reverse polarity from flat to edge when the coins were over center. None of the coins reversed polarity when coins were over edge. GEB had a higher reading than no GEB with some of the coins but not the sock of pennys. Looks like if there is a hole it is at one location as coil is swept over coin. GEB could reduce signal but looks like there isn't a hole over the whole coil when looking for coins.

[Funfinder]

Hi,

looks like a very complex task ahead of you - comparing ultra-weak highspeed-signals.
Perhaps with best chances at the decay beginning phase - later it looks like all the same.

The problem with PI and ground or discrimination is the missing reference value.
There should be a second coil or second circuit with other frequency that measures all
the time the "broad" GEB value only but with slower pulses so the detector
may get some hints about what changes this usually pretty steady GEB value.
In this case its no induction-balance but some detectable EM-field-strenght-inbalance
while moving the PI-coil over the ground.



btw. there is a problem with coins, balls and especially rings using as test-objects:

They show much higher and stronger values as if the metal would have some other shape.

Coin caches still "work together" seen from the metal-mass even if the coins have a
rusty surface because of the eddy-currents-inductivity that concerns the nearby coins.


What really counts for maximum depth of course ist to amplify the
counting contrast-factor-window of the difference when there is a find vs. no find.
But at high mineralized soil regions also the field-values of the huge mass of the
ground then gets amplified by the same multiplicators.
And the more mixed mineralic the area, the more disturbing it will be.

Some other approach is seeing the ground as reflector like GPRs with fq. from 1 to 500 MHz do.
If a PI would be combined with it, it also receives additional conclusions about the ground conditions.

btw. a VLF IB motion-circuit simply could scan all sort of GEB values within a super fast time
to find out if the find-signal starts to vanish or not and at what value.


For real deep finds still the sensitivity and contrast-factor in combination with a large enough coil counts.
A strong enough and repeatable change of the EM-field-feedback no matter if no or heavy mineralized ground.
And as long as special mineralic stones with the weight of 1kg create the same distortions as small nails
it will be hard to find out the difference without X-ray-eyes.

btw. I hope you have some testing-area of at least 2x2m in size and filled 30cm with
mineralic material because just holding some metal-object behind a brick-stone would
distort the results extremly compared to real ground conditions.

At least ca. 50 mineralized brick-stones are needed for a mineralized carpet on the floor
for some simple tests. The best of course would be some 30cm thick layer of mineralized gravel
or crushed bricks mixed with sand, loamy stuff and even nonmineralic stones.

However:
My personal opinion is that PI "overloads" high mineralic areas too fast
thats why its pretty hard to use as very complicated and sensitive tool.

Depending on the coil size and mineralization-level the pulse-lenght
and energy should be adjusted to reach best contrast and depth.

But with our modern always better and faster becoming signal-processing,
also with PI detectors someone will be able to collect more and more
important and all kind of metal-detection related infos from the signals.

Good luck.

[green]

Charted 1 oz copper coin out to 1000usec.

[Chet]

Hi Green
Could you add an explanation of each of the color traces to your charts?
Thank you,
Chet

[green]

Hi Green
Could you add an explanation of each of the color traces to your charts?
Thank you,
Chet

Added a 2 stage amplifier(x9 each stage) after the preamplifier. Recorded each stage_x275,x2475,x22275 with and without the target. Subtracted the no target recording from the target recording for each gain stage and charted. Reply #32 I changed the scope gain to get the added resolution. When the scope input over scales it messes with the signal. I thought charting the difference of target no target would be alright. It's NOT [reply #34]

Hi Chet, When I charted data in the past I would try to use a coil that would give a large enough signal with the scope gain at 500mv/div. Amplifier out is +-2.5 volts full scale. At first I was trying to chart the target time constants and my interest was in how early do I need to sample. Didn't need to chart decay out past 50usec. I started looking at GEB and depending how long the GEB sample is I need to chart out to at least 200usec. My scope doesn't have enough resolution so I tried using three scope gain settings for added resolution. The data didn't make sense so I added two more stages of amplification and don't change the scope setting. When I chart I divide the x9 amplifier output by 9 and the x81 output by 81. Seems to make sense now. The higher gain amplifiers saturate until the signal decays enough to decay on the slope of the lower gain amplifier. Most of the longer decay targets I've charted don't decay on a exponential slope. Longer GEB sample times don't effect small nuggets. It's picking a GEB sample time that reduces the targets signal the least. Hope I can find one between 50usec and 150usec that works for most targets and adjust GEB delay to cancel ground.