Ground Balance Theory

[Ferric Toes]

In saying that Eric, do you think if Minelab were to change their TX waveform pulse shape that, while they might get a consistent ground signal, it could result in them losing the significant depth advantage they currently have in the mineralised ground regularly encountered in the Australian goldfields?

Are you able to compete on depth with the current generation of Minelab detectors in Oz mineralised ground? If so, when do you expect to have a detector in the market place to compete on depth with Minelab , and do you have any idea of the final price?

Is it possible that Minelab are trading off some consistency in the ground signal in some ground to achieve the depth advantage they obviously have?

These look like loaded questions and my answers will be brief and clear.

What ML do or not do is up to them. They no doubt have good reason to use the waveform they have. Em noise, powerline and ground noise do however limit the performance of ML detectors in many areas such that the maximum detection capability is not realisable.

I am not trying to compete with Minelab and have no plans to produce detectors for general sale. I just do research, development, consultancy and a few one-off specials mainly for the industrial market.

Eric.

[Ferric Toes]

Eric,

Picking up from your statement "There are different design philosophies for metal detectors and good reasons for doing something one way often has a tradeoff in other areas". Can you envision computer controlled metal detectors being developed and used to evaluate the ground conditions and variations within one sweep width to recommend an optimal (1) pulse width, (2) amplitude, (3) sweep speed and other TX or RX parameters for optimizing searching in that specific physical location for specified targets with preset TC ranges?

Thanks

bbsailor

Joseph Rogowski

Hi Joseph,

No reason why it should not happen. Mine detection technology is already moving in that direction. The tradeoff in other areas that I referred to was really a downside effect. Improve one aspect e.g. depth, and other performance aspects suffer.

Eric.

[Aziz]

Hey many thanks!, the interesting thread is continuing!
Looking forward to the outcome.
Aziz

PS: Ground balancing / mineralized ground always reduces the detection depth (compared to air test).

[Paul99]

There is of course the possibility that the TX waveform of the Minelab PI is not the best for generating a consistent ground signal.

In saying that Eric, do you think if Minelab were to change their TX waveform pulse shape that, while they might get a consistent ground signal, it could result in them losing the significant depth advantage they currently have in the mineralised ground regularly encountered in the Australian goldfields?

Theoretically the decay of the SPM non-conducting particles in mineralised ground obeys a certain law within very small limits. However, this can be adversely affected by a non-ideal pulse shape. My viscosity meter utilises as near to a true rectangular pulse as is possible of exactly the same amplitude whatever the pulse width. By comparison ML pulses are nearer sawtooth shape. Using the rectangular pulse waveform I see very small variations in decay in samples from different areas and countries, but nothing that would cause more than a minor adjustment in the ground balance setting, and this has been proved in the field in UK, Australia, and USA.

Are you able to compete on depth with the current generation of Minelab detectors in Oz mineralised ground? If so, when do you expect to have a detector in the market place to compete on depth with Minelab , and do you have any idea of the final price?

There are different design philosophies for metal detectors and good reasons for doing something one way often has a tradeoff in other areas.

Eric.

Is it possible that Minelab are trading off some consistency in the ground signal in some ground to achieve the depth advantage they obviously have?

[bbsailor]

Hi Chris,

There is of course the possibility that the TX waveform of the Minelab PI is not the best for generating a consistent ground signal. Theoretically the decay of the SPM non-conducting particles in mineralised ground obeys a certain law within very small limits. However, this can be adversely affected by a non-ideal pulse shape. My viscosity meter utilises as near to a true rectangular pulse as is possible of exactly the same amplitude whatever the pulse width. By comparison ML pulses are nearer sawtooth shape. Using the rectangular pulse waveform I see very small variations in decay in samples from different areas and countries, but nothing that would cause more than a minor adjustment in the ground balance setting, and this has been proved in the field in UK, Australia, and USA. There are different design philosophies for metal detectors and good reasons for doing something one way often has a tradeoff in other areas.

Eric.

Eric,

Picking up from your statement "There are different design philosophies for metal detectors and good reasons for doing something one way often has a tradeoff in other areas". Can you envision computer controlled metal detectors being developed and used to evaluate the ground conditions and variations within one sweep width to recommend an optimal (1) pulse width, (2) amplitude, (3) sweep speed and other TX or RX parameters for optimizing searching in that specific physical location for specified targets with preset TC ranges?

Thanks

bbsailor

Joseph Rogowski

[Ferric Toes]

This experience is with a minelab PI. The problem with high variability in ground mineralization is one that is probably almost unique to gold prospecting, especially residual type placers. I have experienced this hunting gold in Australia, California and Alaska. In some places it is actually possible to tell how close you are to where the gold is by how chattery the signal from your detector is. The more noise and frequently re-ground balancing is required, the closer you are to the gold. This is an effect of the hot solutions that actually deposit the gold because they are often also strongly iron bearing. It also occurs sometimes as iron minerals are heavy and concentrated along bedrock as is the gold. It is not universal in all goldfields, but as Zed pointed out and I agree, it is more common than one might expect. Single samples from any particular location will not demonstrate this, but would pretty much require a series of samples taken at maybe 6 inch to 1 foot intervals over a length of 30 to 50 feet.

Hi Chris,

There is of course the possibility that the TX waveform of the Minelab PI is not the best for generating a consistent ground signal. Theoretically the decay of the SPM non-conducting particles in mineralised ground obeys a certain law within very small limits. However, this can be adversely affected by a non-ideal pulse shape. My viscosity meter utilises as near to a true rectangular pulse as is possible of exactly the same amplitude whatever the pulse width. By comparison ML pulses are nearer sawtooth shape. Using the rectangular pulse waveform I see very small variations in decay in samples from different areas and countries, but nothing that would cause more than a minor adjustment in the ground balance setting, and this has been proved in the field in UK, Australia, and USA. There are different design philosophies for metal detectors and good reasons for doing something one way often has a tradeoff in other areas.

Eric.

[Reno Chris]

I missed the comments by Zed and I wonder what detectors show this large variability in ground signal? Are they PI or CW? I have accumulated a lot of measurements on different soils and rocks and the decay is remarkably constant.

This experience is with a minelab PI. The problem with high variability in ground mineralization is one that is probably almost unique to gold prospecting, especially residual type placers. I have experienced this hunting gold in Australia, California and Alaska. In some places it is actually possible to tell how close you are to where the gold is by how chattery the signal from your detector is. The more noise and frequently re-ground balancing is required, the closer you are to the gold. This is an effect of the hot solutions that actually deposit the gold because they are often also strongly iron bearing. It also occurs sometimes as iron minerals are heavy and concentrated along bedrock as is the gold. It is not universal in all goldfields, but as Zed pointed out and I agree, it is more common than one might expect. Single samples from any particular location will not demonstrate this, but would pretty much require a series of samples taken at maybe 6 inch to 1 foot intervals over a length of 30 to 50 feet.

[Ferric Toes]

In the quest for higher accuracy in the measurement of magnetic viscosity in soil and rock samples, a new type of sensor coil was constructed. Instead of the usual monocoil type solenoid winding a bifilar centre tapped winding was used. The potential advantages of this type of coil winding having been discussed previously on this forum. Effectively half of the winding is used as a TX and the total winding as RX in a PI type circuit.

It is important that the soil or rock sample is measured in a uniform field, hence the solenoid type coil is roughly 2x the depth of the 10cc sample pot, with the sample centrally placed. Previously the single winding covered a length of 50mm with a diameter of 40mm. To achieve the same uniform field a bifilar winding was used which was so connected as to form a centre tapped inductor with one end grounded; the TX drive to the centre tap; and the RX forming the total winding from ground to the far end. The original single winding had a inductance of 200uH and this was also the case for the total winding of the Bifilar CT coil. Inductance from ground to the centre tap is therefore 50uH which is more favourable for a fast rise of current when the TX pulse is applied. The TX coil circuit resistance totals at 30 ohms, so the turn on TC is 50/30 = 1.66uS. The TX pulse is then flat from about 8uS to whenever the switchoff point occurs. In contrast, the single coil solenoid has a TC of 6.66uS and flat after 33uS. The small tradoff is that the ampere turns is halved for the BCT coil, and to compensate for this the TX pulse amplitude was doubled.

Simple resistive damping was applied to each half winding and consisted of a fixed resistor in series with a cermet preset. This arrangement would normally give trouble from the high voltage spike, but the half winding only generated 30V peak and did not come close to the avalanche voltage of the Mosfet used. Damping can be finely adjusted to give the waveform shown. The lower pulse is the delay interval which triggers the scope on its positive edge which coincides with TXoff. The delay is set at 10uS, but as can be seen, you could easily sample as early as 6uS, and with an improved preamp, even 5uS is possible.

A couple of viscosity decay curves have been plotted so far with this BCT coil and results are a bit closer to the theoretical t^-1.0xx but an interesting thing is beginning to consistently occur. For delays <20uS the decay appears to have another faster curve superimposed on it. This should become more apparent when I run a modified timer to observe signals at 5, 10, and 15uS where the discrepancies can be seen. Till now, I thought that it could be an instrumentation error, but as I see it with the faster coil, it may well be real. This has implications for those trying sub 10uS sampling, as the amplitude will rise even faster than it does with the fundamental decay.

Eric

[Tepco]

There is nothing about patent applications or anything similar, just hobby and some hardware hacking, best way to learn about something is to try to actually build it. I never sold even a single hand made detector, put aside anything else, not interested in this for any commercial reason. I supposed all this is already done before, just tried on my own. Never figured out that similar approach can be used to actually separate different sized objects.


Method I described still can be rescued, only not in it's original form. I tried to achieve GB contrast using two very different pulse widths alone, but this is wrong approach. Using two different pulses, one longer than ground TC and one shorter, maybe 10:1 ratio (no need for 500nS TX and similar extreme solutions) works. Then processed in two independent channels, each GB-ed separately using two sample method. Now, each channel will have it's own “hole” in response, but for metallic object holes will fall in different positions. Adding outputs together, this is almost “hole free”, well, some variation in sensitivity but no notch in response.


Another version, with resonant waveform, is also promising, but on prototype ended up with 6 Mosfets, 3 H-drivers and extremely messy timing to command them high, low or open... Time needed to produce something really workable out of this unfortunately by far exceed my available resources.


Best regard

[Prospector_Al]

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.

Hi Tepco,

The method you describe works. It has been used in industrial "tramp metal" detectors for about ten years.

There are no "holes" in the response and in this application, the loss of sensitivity to small targets is immaterial since only targets that are large enoough to damage the crusher need to be detected.

Good thinking, but a bit late for patent applications...

All the best,

Allan

[mickstv]

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.

The other problem you have in relation to GB is that some targets depending on size and TC can fall into whats called an electronic hole, depth on these targets will be reduced or not detectable at all.

[Ferric Toes]

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.

I've been out of the loop for a bit, working on a high power boat towed detector. That's now finished and delivered so lets think about GB again. Yes Green, in principle it is that easy. Not perfect, but an effective GB for most soils nonetheless. You don't want to go that far out with the GB delay though as you will need considerable amplification to achieve balance, with resulting increase in noise.

I missed the comments by Zed and I wonder what detectors show this large variability in ground signal? Are they PI or CW? I have accumulated a lot of measurements on different soils and rocks and the decay is remarkably constant. I could set the GB control to cancel a housebrick in England and it would only require a minor adjustment in Australia. This is of course with PI.

Eric.

[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.

[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.

[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.