DEEPER PI DETECTION DEPTH

[Midas]

So what your sim needs now Aziz is a realistic non-constant and unpredictable ground effect component, then you can play around trying to remove it. From what I gather Minelab are doing this already by using some clever maths to combine the low frequency response with the high frequency response. They are actually use 28 frequencies which might be more to increase the wank factor than because its necessary or who knows perhaps it really does help.

Here's Minelabs consumer level explanation of their technology:

Find Every Target Type & Size with Every Sweep Generally, high transmit frequencies are more sensitive to small targets and low transmit frequencies give more depth on large deep targets. FBS simultaneously transmits and analyses a full band of multiple frequencies from 1.5 kHz to 100 kHz and is therefore sensitive to both very small and large deep targets at the same time. This means you only need to cover the ground once and can be confident you’re not leaving ANY valuable treasure behind.

If that's to be taken literally and isn't just sales patter then once your model is complete you should see some improved sensitivity to long TC targets by using a lower frequency that isn't captured in your current model. Perhaps as a result of the increased attenuation effect of the ground at high frequencies.

[simonbaker]

Hi SB,

good idea. I think 20-100 kHz would be enough too.

Regarding the phase change:
This will happen always due to coil coupling/inductivity change due to mineralisation of the ground/ferrous targets either. Hey, don't look at the f...ing discrimination at this stage. We should look at it, when we have tamed the ground balance first. We are light years away from it.

But if a target pops out of the background, maybe the info is readable?

Power efficiency (worth to look at it):
If you change the spice circuit I have provided and run a transient analysis (commenting the .ac analysis and uncommenting the .trans command) and would compare both frequencies (high and low), you will find, that the low frequency stimulation cost's enormous battery power compared to the high frequency stimulation.
This is really an eye opener!
That's the reason, why low frequency VLF detectors have high inductivity TX coils. Well, I have taken the standard PI coil (300µH / 0.4 Ohm) in the circuit simulation.

Good observation. Would be hard to have optimal efficiency at two frequencies, unless double resonant coils (messy).

Well the target stimulation response level tends to go to the same level for all targets (low, mid, high TC targets) if we increase our operating frequency. But the high frequency region is more power efficient (saturation of the TX coil current does not happen in the high frequency region).

Interesting...

Of course if we made the TX circuit resonant at some frequency(s), that relationship could be changed.

-SB

-SB

[Aziz]

Actually, looking at the graphs makes me want to do this:

Design a continuous wave MD (can't call it VLF, but same design) with two frequencies:

1. 200 kHz - this gives high amplitude response for all targets, but not much phase differentiation.

2. 5 kHz - this has differing amplitude responses, but good phase differentiation for discrimination.

If you ignore the longest time constant target, you could raise the second frequency to 50 to 100 kHz and get better discrimination depth. Or just compromise and use 100 kHz for both, but some trouble detecting short TC targets.

I'd still like to see some real data on how TC ("tau", time constant) is a function of target conductivity and size/shape.

-SB

Hi SB,

good idea. I think 20-100 kHz would be enough too.

Regarding the phase change:
This will happen always due to coil coupling/inductivity change due to mineralisation of the ground/ferrous targets either. Hey, don't look at the f...ing discrimination at this stage. We should look at it, when we have tamed the ground balance first. We are light years away from it.

Power efficiency (worth to look at it):
If you change the spice circuit I have provided and run a transient analysis (commenting the .ac analysis and uncommenting the .trans command) and would compare both frequencies (high and low), you will find, that the low frequency stimulation cost's enormous battery power compared to the high frequency stimulation.
This is really an eye opener!
That's the reason, why low frequency VLF detectors have high inductivity TX coils. Well, I have taken the standard PI coil (300µH / 0.4 Ohm) in the circuit simulation.

Well the target stimulation response level tends to go to the same level for all targets (low, mid, high TC targets) if we increase our operating frequency. But the high frequency region is more power efficient (saturation of the TX coil current does not happen in the high frequency region).


Aziz

[simonbaker]

Excellent Midas!


Why was it so much difficult to see the obvious behaviour?
We should interpret the findings further.
Any ideas?

Cheers,
Aziz

Actually, looking at the graphs makes me want to do this:

Design a continuous wave MD (can't call it VLF, but same design) with two frequencies:

1. 200 kHz - this gives high amplitude response for all targets, but not much phase differentiation.

2. 5 kHz - this has differing amplitude responses, but good phase differentiation for discrimination.

If you ignore the longest time constant target, you could raise the second frequency to 50 to 100 kHz and get better discrimination depth. Or just compromise and use 100 kHz for both, but some trouble detecting short TC targets.

I'd still like to see some real data on how TC ("tau", time constant) is a function of target conductivity and size/shape.

-SB

[Aziz]

Aziz,

Like everyone else I'm perplexed about exactly what the question is your asking but to keep the ball rolling I'll attempt to draw some (though quite probably not the ones your driving at) conclusions from your plot.
* Long time constant targets seem to respond maximally to a broader range of frequencies.
* With shorter time constants the maximum target response occurs at a higher frequency.
* As the frequency increases into the Mhz range target time constants starts to become irrelevant and all the target responses diminish together.

Midas

Excellent Midas!


Why was it so much difficult to see the obvious behaviour?
We should interpret the findings further.
Any ideas?

Cheers,
Aziz

[simonbaker]

Of course, things in nature come with obstacles not present in simulations. Also with oxidation and rugged surfaces, difficult soils, odd orientations etc. However, even with a flattened target response distribution curve, you'll get taus neatly grouped at distribution maximum for a given metal.

Given a choice between discrimination and no discrimination, I'd go with discrimination.

Ok. Maybe Tinkerer et al will be able to show us some data on real targets that will illustrate this.

-SB

[Davor]

Of course, things in nature come with obstacles not present in simulations. Also with oxidation and rugged surfaces, difficult soils, odd orientations etc. However, even with a flattened target response distribution curve, you'll get taus neatly grouped at distribution maximum for a given metal.

Given a choice between discrimination and no discrimination, I'd go with discrimination.

[simonbaker]

It would relate only in cases of thin structures where thickness is close to skin depth. E.g. foil. In such cases you'll get response as if it was a target of lower conductivity. Target mass is irrelevant for phase.

You may extend this idea to Tx coil wire gauge. Inductance will be the same for any coil of similar number of turns on a same size form. However, the coil Q will degrade if wire gauge is too thin.

By little mind stretch it is very same for PI as well.

That was what I previously assumed, but Qiaozhi and Carl seemed to indicate that target size is very important for "tau". This seems backed up by the huge range of "tau"s mentioned, much larger than the range of conductivities of typical targets.

So I'm still wondering....

-SB

[Davor]

It would relate only in cases of thin structures where thickness is close to skin depth. E.g. foil. In such cases you'll get response as if it was a target of lower conductivity. Target mass is irrelevant for phase.

You may extend this idea to Tx coil wire gauge. Inductance will be the same for any coil of similar number of turns on a same size form. However, the coil Q will degrade if wire gauge is too thin.

By little mind stretch it is very same for PI as well.

[simonbaker]

Target tau is directly related to VLF phase.

Then is it also true that VLF phase is subject to both target metal (conductivity) and target size/shape?

If so, how do we interpret our discriminators when they say "foil" or "penny" or "nickel" or "gold" or "silver"? Is it only accurate for coins of known size? Will a large zinc plate have the same phase/tau as a small silver coin, or something like that?

Regards,

-SB

[Carl-NC]

In general, I still have not heard a good explanation of how estimating the "tau" of a target tells me whether to dig or not. Anyone have some more data on how to interpret a particular "tau" once we have it in our little hot hands?

Target tau is directly related to VLF phase.

[simonbaker]

Yeah, it is a bit frustrating not to see it as an animated powerpoint presentation .

Aziz put a nice LTSpice model with a quoted post, where all the components had regular values. By first glance you'll see nothing much going on, but there is. You may easily play with values and see how the whole shebang gets affected. Everything is in AC (mag/phase) domain.

Now, with receiver signal having a 0 DC, it is obvious that whatever happens in time domain right after a PI pulse, must go through zero, and a whole signal integrated from pulse to pulse is zero. If target's tau is not affected by Tx coil's resistance (you may observe it as a transformer), you'll get a Rx signal that passes through zero with the same delay from PI pulse, regardless of depth/size, hence discrimination.

Attached is a LTSpice simulation of a White's oscillator working as a PI exciter, and a step voltage excitation I was meditating upon. There are ".step param" options to play with, but these should work as they are. Please note how clean the signal gets with stepped exciter.

It is just a thought ...

Ok -- I gather your point is to consider designs using zero-crossing events as a way to discriminate, sounds good. However, it seems these schemes are really looking at ways to estimate the "time constant" (TC or "tau") of a target (assuming a simple L/R model for a target).

In general, I still have not heard a good explanation of how estimating the "tau" of a target tells me whether to dig or not. Anyone have some more data on how to interpret a particular "tau" once we have it in our little hot hands?

-SB

[Midas]

Hi all,

anyone thought of what the AC response says to you?
I'm talking about the bode plot in the following post (right side, no modifications):

http://www.geotech1.com/forums/showpost.php?p=143373&postcount=221

Again, don't look at the phase response yet. Don't look at the discrimination ability. Forget the discrimination at all.

We want to focus to the fundamentals only.
Go ahead guys.

Cheers,
Aziz

Aziz,

Like everyone else I'm perplexed about exactly what the question is your asking but to keep the ball rolling I'll attempt to draw some (though quite probably not the ones your driving at) conclusions from your plot.
* Long time constant targets seem to respond maximally to a broader range of frequencies.
* With shorter time constants the maximum target response occurs at a higher frequency.
* As the frequency increases into the Mhz range target time constants starts to become irrelevant and all the target responses diminish together.

Midas

[Davor]

Duh ...

It says to me that with the same coupling, amplitude (target size and/or distance) remains the same ... within reasonable frequency range. In case I'm terribly wrong, could you please provide some hints?

[Aziz]

Aziz, I like what you do, and it is in many ways fundamental. But if you really wish to drive attention, and also keep people on the same track, you could provide some hints as well

Thank you

-You all are driving me mad! -


Where is the mental doctor for me?


You shouldn't try to guess, what I'm thinking. The question is plain straight forward.

What can you extract basic fundamentals on the mentioned AC response?


Aziz