Ground Balance Theory

[Davor]

I'm thinking of replicating viscosity measurements using "Visual analyser" software and a soundcard. In effect it is capable of measuring inductance at various frequencies. Perhaps I'll have problems with calibration, but I expect to have some idea on the samples I have.
Did anyone perform such measurements yet?

[Aziz]

I'm thinking of replicating viscosity measurements using "Visual analyser" software and a soundcard. In effect it is capable of measuring inductance at various frequencies. Perhaps I'll have problems with calibration, but I expect to have some idea on the samples I have.
Did anyone perform such measurements yet?

Hi Davor,

I'm using Visual Analyser (VA) regularly to measure inductances. But it isn't accurate and it isn't well implemented.
You can see the swinging in the FFT spectrum left and right during a measurement, which is showing, that the receive and transmit channels weren't synchronized. The reading is "noisy" hence.

One can implement a much more accurate inductance measurement system with the sound card. But you would need some kind of a "hardware controller".
Aziz

[Davor]

Thanks. The Visual Analyser is stuck in development but seem promising.
What you attribute to sync could also be a consequence of poor alias removal. I made a waterfall sweep of my PC soundcard and it revealed far too much intersecting lines indicating a lot of aliases.
Every quality sound card should be full duplex. I'm thinking that a solid sound card, being immensely cheaper than a grown up instrument, could do the trick.
Can you estimate precision of your measurements using Visual Analyser? Any hope for two frequency susceptibility measurement?

[Aziz]

Thanks. The Visual Analyser is stuck in development but seem promising.
What you attribute to sync could also be a consequence of poor alias removal. I made a waterfall sweep of my PC soundcard and it revealed far too much intersecting lines indicating a lot of aliases.
Every quality sound card should be full duplex. I'm thinking that a solid sound card, being immensely cheaper than a grown up instrument, could do the trick.
Can you estimate precision of your measurements using Visual Analyser? Any hope for two frequency susceptibility measurement?

Big errors for low inductance measurements (typical range for metal detectors).
I didn't compare it with the inductance meters. So I can't say anything.
Aziz

[Davor]

Too bad

Guess it is better with larger inductances then.

[Aziz]

Too bad

Guess it is better with larger inductances then.

Exactly. The sound card output has an high output impedance and can't deliver much current for the low inductance measurements. But you could use high inductance sensors (coils) and make your sample measurements with it. It should work.

Aziz

[Tepco]

Half-Titanic

Or more like Exxon Valdez, not completely sink.


One small update of one method I proposed here and some results I get with it. Yes, it works, this is effective GB mechanism, but as usual, having it's own good and bad sides.


Technically, method is similar to what ML call DVT, generating one relatively long (50uS in this case), and one relatively short (adjustable, but at least 10 times shorter, even down to 0.5uS) pulse of opposite polarity, releasing same amount of energy. Achieved by constant current control and two different voltages to produce required pulses without need for external timing, voltages are same polarity but two sections of the coil connected in opposite directions to get bipolar field, fairly simple circuit built around SG3525 chip. Idea is, long pulse around target TC will excite it and ground, short one will excite ground but target very little excited, then substract.


Result: it works, having no “holes” and rejection zones in response, less substraction is needed, only some practical aspects limit it's usability. Problem is with short\long pulse ratio and two effects they produce. First one is, despite no dead-zones, sensitivity to very small, low TC targets fall rapidly as it's TC approach short pulse width. Another is practical duration of this short pulse. It is possible to generate very short ones, uS or below, but with very short pulses another issue appear. Now total pulse width is comparable to flyback release time, this waveform produce different effect in soil compared to long pulse, alone cannot be used to establish reference. Limiting practical pulse ratio to say 10\1 , like 50\5uS or so (using longer “long” pulse will not help much, target TC consideration is for short one). Then this short pulse became comparable to target TC, reducing “contrast” it ment to achieve and sensitivity, again lot of substraction needed. Workable but not effective, loss of sensitivity, overally not better than GS approach, instead of response hole, now small TC sensitivity suffer. Alone, not a good solution. But was interesting to try. When time allow, I will modify this setup to try few different tricks. What I have to try is, first, true log-amp front end running in parallel with normal one, to see how it can be exploited best, and some playing with nonlinear inductance (saturable core) in the circuit, to get current ramp different from linear. By all chance, this may end up like nice DIY project, sort of bipolar pulsing (maybe logarithmic) GS incarnation. With time available, maybe finished before end of days. Any suggestion before I disassemble this prototype? I have something different, apparently workable and relatiely new on my workbench, will post more, at least idea.

[Tepco]

Update on Titanic

And, update on update, I just tested completely novel method, that seems to do the trick perfectly well, similar to, but free from ML stuff, with potential to outperform it. Technically, contrast is achieved with two, same energy release pulses after flyback, one normal linear ramp, another is bit strange F\2 resonant waveform using 2:3 duty cycle H bridge to drive coil+resonant capacitor circuit in series of asymmetric full sine cycle+ half sine, at half it's resonant frequency, so average current is ramped up to same energy as with normal pulse. Only normal pulse is used for target detection, ratio of two gives GB reference. Hardware is just rubbish for now, must be made far more precise, actually I have to get decent AWG (Arbitrary waveform generator) to play with, before any definitive circuit, hardware or timing is defined. Unfortunately, I have all equipment needed, on my current job, but I have to keep my MD hobby well away to keep that job, so this will be slow process. But this is way to go, workable concept promising really good performance. Will post everything here in details.

[Davor]

I'm quite curious where it will lead you. What you explain are a sort of iambic pulses (dee-daah-dee-daah...) and as I imagine it, they'd eliminate ground directly. Also the EF can be removed by this method in a more time saving manner, thus improving S/N.
We already discussed such pulsing before, and I mentioned "iambic" at some occasions ...
http://www.geotech1.com/forums/showt...759#post161759
http://www.geotech1.com/forums/showt...920#post168920

It seem to me that short pulse can be controlled by applied voltage much easier than by applying varied timing.

[Aziz]

Hi guys,

what's happend? Why the standstill?
Aziz

[Davor]

Maybe just a food for thought, or a reason for opening a new topic.

I recently acquired some hot rocks by stumbling upon them on a beach. As it turns out they are mostly jaspers, of which some are saturated with iron, hence fairly lively under MD. And now the fun part - many other gemstones are magnetically susceptible.

So far I learned about some tricks for finding gold in alluvium as following the trace of black sands to the higher concentration of gold etc. Also that people feeding their sluices with gravel being clever enough to seek for gemstones too, as the sluices tend to aggregate these as well, but the fundamental property of certain gems to have magnetic properties was beyond me. Say garnets - they stick to a magnet!

Perhaps it wouldn't be a bad idea to process the ground variation information as indicative of mineral content?

You know, the discriminating rigs tend to omit iron from indication so you pass happily over a valuable meteorite, perfectly oblivious to it's being there waiting to be picked. The information is there, but we chosen to ignore it.

So, how about using the ground variation information selectively? I know the argument about the gold fields of Australia, hot ground etc., but what about everywhere else? While in-lands, I could use ground information as indication of minerals being there (at long last), but I could discard the ground indication while at terra rossa hot grounds.

How to do this is still out of my grasp, but knowing that something is out there is tempting.

[Ferric Toes]

Hi guys,

what's happend? Why the standstill?
Aziz

The subject has not lost its interest by any means. However, I have many other things to do, such as refurbish, modify, and test the High Power Aquapulse and get it sent back to the owner before the end of June for its next treasure search. Others also likely have more pressing things that have priority at the moment. Be patient .

Eric.

[Reno Chris]

This thread is of great interest to me. Dealing with the response of ground mineralization in all its forms is one of the most difficult aspects of designing a detector - especially those suitable for hunting gold. In reading through this thread, one of the most interesting statements is this one by Zed (and it received very little attention):

Ground types with high mineralization are relatively easy to discriminate out as long as its time constant remains constant...but ! this is generally
not the case,with high mineralization you usually get high variability resulting in bigger and faster shifts in the grounds time constant and this is where
most GB systems come to grief.

This so very true! An important concept underlying many GB designs is that the TC of the ground is functionally just that - a fixed constant. While the various magnetic properties of a soil may very significantly from location to location, at a single, individual location, it is assumed it will change only slightly as one detects around that certain location. This allows an adjustment to subtract out or otherwise ignore soil of certain properties. While that may be true sometimes, its often not true as Zed points out. The settings to adjust out (by whatever electronic means) are often narrow and comparatively limited in their scope. The problem comes in environments where the TC of the ground changes rapidly over short distances.

Geologically, it is common that gold will form at rock contacts because the faults associated with these contacts are the conduits for fluids bearing gold, iron and other minerals. The mineralized fault zone is not a single break, but a complex zone of broken or fractured rock that has been moved around and variably mineralized by the addition of iron, gold and sometimes other minerals. It is not terribly unusual that one can ground balance over a spot, but at the end of a single swing of the coil (say 2+ meters away) the detector is significantly out of GB. Other times it may take 2 or 3 swings to be out of GB. Places like this are present in the US, Australia and gold bearing districts in many places. Some detectors have an automated GB to deal with such situations, but these are a big compromise. Slow auto GB adjustments leave the detector always out of GB and working to catch up, but fast auto GB will easily make faint targets disappear as they are GB'd out.

[green]

I've been trying to follow the thread. Looking at the ground plots I thought ground plotted a straight line on a log log plot with little slope variation and targets had a TC and plotted a straight line on a log linear plot with a large variation in slope with different targets.

[green]

Didn't mean to kill the thread. Looking at Eric's plots the obvious way to cancel ground is. Sum two samples into an integrator. Preamp out at a delay of say 4 to 8 and inverted out with a adjustabe gain around -1 at a delay of maybe 64 to 128. It can't be that easy or everyone would be doing it. Is the problem with ground response not being predictable, with hardware or some other variables? Received a copy of ITMD a couple weeks ago. Maybe I'll get smarter and ask better questions.