GROUND BALANCE ( without the ground :-) )

[Carl-NC]

If you don't know what the loss angle of the ground is, I don't see how. Maybe you see something I don't.

[moodz]

So can we conventionally adjust the sine() to zero without a ground sample ..?

[Carl-NC]

And why multiply the "soil response" by the sine of the ground angle?

VLF detectors have quadrature demods, so they effectively take the sine and cosine of the incoming signal to create the resistive (R) and reactive (X) signals. for GB we want the R signal to be zero in the presence of ground, even if the strength of the ground varies. So we adjust the sine() term to be zero.

[Carl-NC]

E1 might correspond to the GR and E2 to the IB vector.
The result is E0 the resultant vector.

The White's XLT & DFX had a GB procedure whereby you lift the coil in the air and pull the trigger; then place the coil on the ground (away from metal) and pull trigger. With the coil in the air the detector would grab the X&R data which would represent E2. With the coil on the ground it would grab the X&R data which represents E0. From that it could calculate true ground phase. Most detectors don't do this, you can bob the coil over the ground and effectively get the same results. The null vector (E2) ideally will never change but can with temperature, but in a well-designed detector this will be minimized to the point that it is in the mud.

All the designs I see in VLF for instance aim to solve for E0 and calculate a VDI which is resultant of superimpose vectors E1,E2 .... En in a real system

VDIs are mostly taken from the 2nd derivative which will have removed all of E2 and most of the static effect of E1. You'd think that leaves just the target vector, except that mineralization can also rotate the target vector in a dynamic way. I don't know if any detectors try to correct for this, I expect that MF models attempt to do so.

[JoyJo]

The equation for a single frequency sinusoidal VLF is something like this:

f(GR,GB) = mag(GR)*sin(ph(GR) - ph(GB))

Let's say the phase of the ground is 2°, so you adjust the GB phase to 2° and the sin() term is 0 regardless of the magnitude of GR. But if the magnitude of GR is zero then f = 0 for any setting of ph(GB). Offhand, I don't see a way around this.

The same is true for PI subtractive GB which relies on the GR decay rate:

f(GR,GB) = GR(t0) - k(GB)*GR(t1)

If GR = 0 then the multiplier k(GB) doesn't matter and f = 0 always. But when GR != 0 then f = 0 only for the correct k(GB).

What do you see that I don't?

And why multiply the "soil response" by the sine of the ground angle?

[moodz]

Ha, ha you shot me.
If you say the principle, it will be very interesting.
But think about it - is it wisely to share it now?

PS: Oh, yes, you did it

LOL ... in short maths is not patentable ... methods are.

[boilcoil]

Ha, ha you shot me.
If you say the principle, it will be very interesting.
But think about it - is it wisely to share it now?

PS: Oh, yes, you did it

[moodz]

The equation for a single frequency sinusoidal VLF is something like this:

f(GR,GB) = mag(GR)*sin(ph(GR) - ph(GB))

Let's say the phase of the ground is 2°, so you adjust the GB phase to 2° and the sin() term is 0 regardless of the magnitude of GR. But if the magnitude of GR is zero then f = 0 for any setting of ph(GB). Offhand, I don't see a way around this.

The same is true for PI subtractive GB which relies on the GR decay rate:

f(GR,GB) = GR(t0) - k(GB)*GR(t1)

If GR = 0 then the multiplier k(GB) doesn't matter and f = 0 always. But when GR != 0 then f = 0 only for the correct k(GB).

What do you see that I don't?

Even though you see diagrams for discriminating metal detectors as a polar phase diagram with a single vector drawn on it the signal from the RX coil is actually a composite vector made up of multiple sinewaves with the same frequency but different amplitudes and phases.

For example a simple one for an RX coil on an IB .... with two phases that we might call IB ( tx imbalance ) and GR which is a ground response vector.

E1 might correspond to the GR and E2 to the IB vector.
The result is E0 the resultant vector.

So my solution rearranges ( using hardware) the below diagram to do the following things ..

1. Solve E1 and E2 phase and amplitude using only E0 and knowing how a ground signal will affect the result.
2. Use a method to solve for E2 such that any changes in E1 will have no impact on E2.

All the designs I see in VLF for instance aim to solve for E0 and calculate a VDI which is resultant of superimpose vectors E1,E2 .... En in a real system

The solution is to NOT use an FFT / Goetzel etc ... hint hint.

Below is just a general representation of the problem ....

[moodz]

Yes Moodz, but you will need to generate a whole family/library of curves, collected in a common point "air" and branched to different points "ground", so that when you bring the coil to the ground, the detector can choose a curve from the library to work on.

This could work if the detector quickly matches the ground to the curve but on the fly how will possible targets ( eg small / deep ) targets not confuse the system.... my solution does not use this.

[boilcoil]

Yes Moodz, but you will need to generate a whole family/library of curves, collected in a common point "air" and branched to different points "ground", so that when you bring the coil to the ground, the detector can choose a curve from the library to work on.