Interaction of magnetic fields

[Tinkerer]

We are surrounded by the earth's magnetic field. When we look for buried treasure, we can use the earth's magnetic field to give us some indication where to look.
A magnetometer is a passive sensor of the earth's magnetic field. It tells us about distortions of the field.
Such distortions can be caused by iron or steel objects.

A metal detector is an active sensor of magnetic fields.
The coil sends a continuously varying magnetic field into the ground in the case of a VLF detector. Or a pulsed magnetic field for a PI detector.

These time varying fields generate eddy currents in conductors. These conductors can be of the metallic kind, but they can also be in the form of conductive solutions, like seawater that gives a considerable response to the metal detector.

I would like to learn more about the interaction of the various magnetic field that come into action when we expose a target to the magnetic fields from the metal detector.

For example:

The eddy currents in the target generate a magnetic field around the target. This magnetic field is within the earth's magnetic field and causes a distortion in it.

Question:

Is the response that we sense with the coil, the actual magnetic field of the target or is it the distortion that this field causes in the earth's magnetic field?

Question:

What about a target that has it's own magnetic field? Now we have one more magnetic field that interacts.
How do they interact?

Tinkerer

[Fred]

Also:
Could the interaction of the (variating) magnetic field and the object (or the halo) cause a detectable electric field or anomaly in the natural electric field?
Fred.

[Tinkerer]

Interaction of magnetic fields

Also:
Could the interaction of the (variating) magnetic field and the object (or the halo) cause a detectable electric field or anomaly in the natural electric field?
Fred.

For the time being I am only speculating, but what if:

Lets say the target's own field distorts the earth's magnetic field. Now we apply an external magnetic field that distorts both these fields. Now the external field collapses.
What will happen?
How far distant will the magnetic fields readjust their alignment?

Tinkerer

[bbsailor]

Tinkerer,

You asked some interesting questions. I'll add another perspective.

Question:

Is the response that we sense with the coil, the actual magnetic field of the target or is it the distortion that this field causes in the earth's magnetic field?


The manetic field of the earth is weak compared to the local signal being emitted by the metal detector coil. However, as the distance between the coil and the target is doubled, the target signal seen at the coil is 64 times less. As you increase the RX gain in an attempt to receive deep, small targets, you get into the area where the very weak Earth magnetic field starts to generate a voltage in the moving search coil. This is why the Hammerhead has a second delay that samples the signal long after the target signal has decayed. This second sample is 180 degress out of phase with the target signal (entering the integrator, IC8A) to cancel or neutralize the Earth's magnet field signal.

When you have high gain and a moving coil, you need to find ways to ignore the effects of the Earth's magnetic field. It is not so much that the Earth's magnetic field distorts the target signal but, in the process of detecting a target, the Earth's magnetic field gets in the way.


Question:

What about a target that has it's own magnetic field? Now we have one more magnetic field that interacts.
How do they interact?


Ferrous metals may be slightly magnetized by the Earth's magnetic field but it is the TX energy of the coil that primarely stimulates the target. In order to do some experiments you would need to find two places on Earth with the following characteristics.
1. A strong magnetic field.
2. A very weak magnetic field

Then you would compare the results in these two locations to see if the Earth's magnetic field has any effect on the target's detection dynamics.

I would speculate that 98% of the detection capability is done through the energy supplied by the TX energy in the coil. If the earth's magnetic field has any effect, it is in the interference with weak target signals due to the moving coil generating a voltage rather than the residual effects of the Earth's magnetism on the ability to detect a target.

I hope this addresses the issues that you want to discuss?

bbsailor

[Tinkerer]

Magnetic field interaction

Tinkerer,

You asked some interesting questions. I'll add another perspective.

Question:

Is the response that we sense with the coil, the actual magnetic field of the target or is it the distortion that this field causes in the earth's magnetic field?


The manetic field of the earth is weak compared to the local signal being emitted by the metal detector coil. However, as the distance between the coil and the target is doubled, the target signal seen at the coil is 64 times less. As you increase the RX gain in an attempt to receive deep, small targets, you get into the area where the very weak Earth magnetic field starts to generate a voltage in the moving search coil. This is why the Hammerhead has a second delay that samples the signal long after the target signal has decayed. This second sample is 180 degress out of phase with the target signal (entering the integrator, IC8A) to cancel or neutralize the Earth's magnet field signal.

When you have high gain and a moving coil, you need to find ways to ignore the effects of the Earth's magnetic field. It is not so much that the Earth's magnetic field distorts the target signal but, in the process of detecting a target, the Earth's magnetic field gets in the way.


Question:

What about a target that has it's own magnetic field? Now we have one more magnetic field that interacts.
How do they interact?


Ferrous metals may be slightly magnetized by the Earth's magnetic field but it is the TX energy of the coil that primarely stimulates the target. In order to do some experiments you would need to find two places on Earth with the following characteristics.
1. A strong magnetic field.
2. A very weak magnetic field

Then you would compare the results in these two locations to see if the Earth's magnetic field has any effect on the target's detection dynamics.

I would speculate that 98% of the detection capability is done through the energy supplied by the TX energy in the coil. If the earth's magnetic field has any effect, it is in the interference with weak target signals due to the moving coil generating a voltage rather than the residual effects of the Earth's magnetism on the ability to detect a target.

I hope this addresses the issues that you want to discuss?

bbsailor

bbsailor,

thank you for your perspective. There are obviously many different angles to look at the problem.

Let me try to show one more.

I will go back to the magnetometer that passively senses distortions in the earth's magnetic field.

A Proton Magnetometer has a coil surrounding a proton rich fluid (it can be distilled water). The protons spin, lined up in the earths magnetic field.
When we create a strong magnetic field within the coil, the protons are forced to line up in the new field.
When we release our field, the protons re-align themselves to the earth's magnetic field.

This takes some time, up to seconds.

During this re-alignment, the moving protons create a waxing and waning magnetic field of their own that generates a sine wave in the coil.
This sine wave has a low frequency of 2000 to 5000 Hz, that is proportional the the earth's magnetic field at the spot where the measuring took place.

Now, looking at a target with its own surrounding magnetic field, either produced by galvanic currents or by magnetism, sitting in the earth's magnetic field.

The target's field distorts the earths field but both fields are static in a balanced way.

At the moment we apply the external field of the metal detector that is much stronger, the balance between the fields is upset and when the metal detector field is changing like the VLF or is removed like the PI, the original fields will go back to the original balance.

How long does it take for the original balance to be restored?

Is this a linear process that creates a decay curve?

Or is it a pendulum process that creates a sine wave?

If we could know the answers to the above questions, we could try to design a detector that could read the information that could be gleaned.

Among others, it could possibly tell us if the target has been buried a long time or is magnetic.

Tinkerer