naive questions re: ground balance thread

[bklein]

My questions probably divert discussion or won't get answered in the GB thread so I'll make a separate thread for them.
1. I don't understand the motivation and goal. The scope shots and plots look too similar for me so see how to differentiate or remove any of the features seen in a real world of varing coils, coil height, and movement. How then would the PI detector change in design to somehow take advantage of the slight differences you guys are pointing out?
2. This magnetic viscosity effect is new to me. I guess that is what I experienced with the baby oil and steel wool particles - all clumping together after exposure to a neodynium magnet. Still clumped together after days. It was supposed to make a cool visualizer of magnetic fields and instead it is back to a clump of steel wool. What particles should be used then that would show the magnetic fields but not be magnetized by them so permanently?
3. Would such a visualizer show fields around targets influenced by a detector signal?
4. Why can't the PI be a good detector of black sand patches? In beach detecting they seem to find them ok - you just have to experience it and then realize what it is. So couldn't coil design, sample timing, or differentiation be revised to find these patches easier? A magnetometer seems overkill. I'm just wanting to see concentrations within a couple feet of the surface. More often less than 6". Seems that in the thread you are describing all these ground effects but now you say a magnetometer is best to map this one.(?)

[Ferric Toes]

My questions probably divert discussion or won't get answered in the GB thread so I'll make a separate thread for them.
1. I don't understand the motivation and goal. The scope shots and plots look too similar for me so see how to differentiate or remove any of the features seen in a real world of varing coils, coil height, and movement. How then would the PI detector change in design to somehow take advantage of the slight differences you guys are pointing out?
2. This magnetic viscosity effect is new to me. I guess that is what I experienced with the baby oil and steel wool particles - all clumping together after exposure to a neodynium magnet. Still clumped together after days. It was supposed to make a cool visualizer of magnetic fields and instead it is back to a clump of steel wool. What particles should be used then that would show the magnetic fields but not be magnetized by them so permanently?
3. Would such a visualizer show fields around targets influenced by a detector signal?
4. Why can't the PI be a good detector of black sand patches? In beach detecting they seem to find them ok - you just have to experience it and then realize what it is. So couldn't coil design, sample timing, or differentiation be revised to find these patches easier? A magnetometer seems overkill. I'm just wanting to see concentrations within a couple feet of the surface. More often less than 6". Seems that in the thread you are describing all these ground effects but now you say a magnetometer is best to map this one.(?)

Scope shots often look similar, or only very small differences are visible. However, if you take a good PI and pass a piece of metal in front of the coil so that you can just hear it as a definite target, you will not see any change on the scope even if you amplify it 500x with a preamplifier. You will of course if you bring the target nearer. The reason is that the target signal at maximum detectable range is buried in the noise - either pickup or internally generated. The point is that a good detector is looking for those very small changes that you wont see in the front end circuitry. Much of the circuitry after the preamp is to do with noise reduction filtering, and if you look there the original decay waveform would be lost.

Magnetic viscosity is a result of certain minerals in the soil that are energised by the transmitter field. These signals can have a great effect on the useability of a detector and can even make it impossible in certain parts of the world without a feature called ground balance, or GB. Again, if you looked at viscosity signals on a detector in the field, you would hardly see them, but their audible effect could not be ignored. What we are doing in investigating the decay curves of metal and soil targets is trying to determine what physical and mathematical laws they obey. My targets are in a small solenoid type coil 30mm diameter so that they are closely coupled to the TX and RX circuits. This pulls the waveform out of the noise so that it can be properly examined. You can then do linear, log/linear or log/log plots to determine the decay law. The shape you see in the plots will hardly change as you take the target further and further away, but its amplitude will uniformly reduce until you can't see it for noise.

Magnetic viscosity is a good case for this type of investigation, and personally I have spent a large amount of time on it. As you can see on the log/log plots I have posted, the negative slope is always ~1.0. This is useful knowledge in that it enables a filter to be designed that will tune out the ground from almost anywhere in the world, whether weakly or strongly mineralised.

PI detectors are generally regarded as being fairly insensitive to black sand. I have a few pounds of the stuff here, sent to me by a colleague in California. It has slight viscosity and you could use a PI, such as the Surfmaster, which does not have ground balance. Black sand has another magnetic property called susceptibility, which means that it will conduct or concentrate a magnetic field without any spurious viscosity effects. The black sand I have has very large susceptibility values, which will cause a local distortion in the earth's magnetic field, hence the suggestion of using a magnetometer. Years ago some manufacturers, Whites included, made induction balance detectors that had two signal meters. One labelled "metal", and the other "mineral". The mineral meter was there for the purpose of detecting materials such as black sand. The RX circuits had two channels, the metal one displaying 180deg phase, and the mineral 90deg components of the received signal. Perhaps someone else will come in and say if modern IB's can be adjusted as a BS detector.

Hope these brief explanations help.

Eric.

[Carl-NC]

Barry, to answer each question:

1. Eric covered this nicely. To expand a little, I often use grossly oversized targets on the bench so I can easily see the response at least at the preamp output. Ferinstance, an array of nickels (not touching) on cardboard, instead of a single nickel.

2. You've permanently magnetized the steel particles, probably got the magnet too close. Black sand will work better.

3. No, detectors & target responses rely on eddy currents which only exist in the AC domain, and visualizers don't work on AC.

4. To expand on Eric's reply, non-viscous black sand decays very quickly, so you probably need to sample below 10us to see it. You will then have a decent overall ground response and will have to take care to listen for changes in strength. It probably can be done but may be tedious. As Eric noted, an IB detector can work too, if the black sand phase deviates enough from the normal ground. The White's V3i has an "alluvial tracking" screen which scrolls real-time ground phase, so you can see when you cross a deposit. Even a good ol' BFO can find black sand. But, yes, a mag beats them all.

[bklein]

Thanks to your both for your explanations. Very good stuff. It would be cool to see photos of each of your test stations!

Eric: "Perhaps someone else will come in and say if modern IB's can be adjusted as a BS detector." Pun intended?? :-)

I have used both Surfmaster and GP Extreme at the beach. I didn't think of the ground balance difference between the two - now I have something to check out. If I hit black sand areas usually coins are nearby. (no gold rings found yet...). Sometimes there are some pretty thick layers.

I don't have any black sand saved up. As the makeup of black sand can differ sourced place to place, is there a purchase source for the best material for the baby oil gadget?

I'm a bit confused by the single nickel vs several nickels example. I'm thinking about the black sand again. You say we need a relatively fast sample time for the particles. But wouldn't a concentration of the black sand respond more like a large single body and be more identifiable in more delayed sample times? (this would be a no GB design)

What do you think about this: use two spaced coils and detection setups and then combine the received signals in a binaural audio fashion. The question would be how best to give the signals better audio qualities that our brains could work with. Perhaps the target response waveforms you are getting could be downsampled into audio range and be repeated at audio rate.