About iron

[Tinkerer]

Skippy's post:
Off-topic, I know, sorry:
The "iron see-through" label was one that usually got applied to VLF's with high operating frequency, such as the 100 kHz Compass range, like the Yukon. The idea was the high frequency only penetrated iron items to a tiny depth ( ferromagnetic materials have very small skin depths, and at 100K it's really small ) so the iron targets didn't in turn create a strong magnetic field of their own, and they didn't distort the detectors field in the ground so much, thus allowing the possibility of finding non-ferrous items nearby - items that would be hidden/masked by using a more conventional lower freq VLF.

This is an interesting post, that could be the beginning of a useful discussion.
We know that PI detectors "have an affinity" for magnetic targets.

I believe that magnetic targets have 2 distinct Tau's. An initial short decay and then a long decay.
How does this happen?
Lets look at it a bit closer.

[Monolith]

Skippy's post:
Off-topic, I know, sorry:
The "iron see-through" label was one that usually got applied to VLF's with high operating frequency, such as the 100 kHz Compass range, like the Yukon. The idea was the high frequency only penetrated iron items to a tiny depth ( ferromagnetic materials have very small skin depths, and at 100K it's really small ) so the iron targets didn't in turn create a strong magnetic field of their own, and they didn't distort the detectors field in the ground so much, thus allowing the possibility of finding non-ferrous items nearby - items that would be hidden/masked by using a more conventional lower freq VLF.

This is an interesting post, that could be the beginning of a useful discussion.
We know that PI detectors "have an affinity" for magnetic targets.

I believe that magnetic targets have 2 distinct Tau's. An initial short decay and then a long decay.
How does this happen?
Lets look at it a bit closer.

Pi is not using a continuous frequency, it is more like a step function, therefore different rules apply.

[green]

Skippy's post:
Off-topic, I know, sorry:
The "iron see-through" label was one that usually got applied to VLF's with high operating frequency, such as the 100 kHz Compass range, like the Yukon. The idea was the high frequency only penetrated iron items to a tiny depth ( ferromagnetic materials have very small skin depths, and at 100K it's really small ) so the iron targets didn't in turn create a strong magnetic field of their own, and they didn't distort the detectors field in the ground so much, thus allowing the possibility of finding non-ferrous items nearby - items that would be hidden/masked by using a more conventional lower freq VLF.

This is an interesting post, that could be the beginning of a useful discussion.
We know that PI detectors "have an affinity" for magnetic targets.

I believe that magnetic targets have 2 distinct Tau's. An initial short decay and then a long decay.
How does this happen?
Lets look at it a bit closer.

Picture I posted awhile back. Ferrous targets decay straight line log-log in the beginning. Size can effect how long straight line log-log. The targets in picture decayed close to same slope as ground for over 100us.

[Carl-NC]

Iron targets have 2 responses: a magnetic response and an eddy response. Thinking in terms of VLF phase angle, a purely magnetic target will always exhibit a phase shift of 0-90 degrees and a pure eddy target will have a phase shift of 90-180 degrees. This is why some iron targets (like nails) with little eddy response end up in the ferrous region while those with a substantial eddy response (like washers) end up looking non-ferrous.

In a PI, I suspect what you see early on is a magnetization effect akin to VRM. That is, the TX pulse magnetizes the target and during early RX the magnetization decays. The later slope would be the eddy decay.

[Monolith]

Picture I posted awhile back. Ferrous targets decay straight line log-log in the beginning. Size can effect how long straight line log-log. The targets in picture decayed close to same slope as ground for over 100us.

Ferrous targets give distinct response depending on shape and direction. This is more evident when using an induction balanced coil arrangement.
Presenting a "crown cork" in the flat position gives a response of somewhere near 180 degrees from the response in the vertical position. It gets even more interesting. A very rusty similar target behaves still different.
We ask ourselves, what does the rust have to do with it? Or is it the corroded uneven surface?
To what extent does the white metallic coating over the steel of the "crown cork" have an influence?
In the Australian gold fields they find places where the gold diggers, some 150 years ago littered the ground with the sardine and bean cans. After all these years only rusty slivers of the thin steel sheet metal are left over, but they greatly bother the detectorists.

[waltr]

I end up digging a lot of horse shoes and tack rings (large iron harness rings) the look good on my TGSL (14.5kHz, IB VLF).
These ID in the Sliver coin to brass button range.

[6666]

In the Australian gold fields they find places where the gold diggers, some 150 years ago littered the ground with the sardine and bean cans. After all these years only rusty slivers of the thin steel sheet metal are left over, but they greatly bother the detectorists.

Oh yes

[Carl-NC]

Presenting a "crown cork" in the flat position gives a response of somewhere near 180 degrees from the response in the vertical position. It gets even more interesting. A very rusty similar target behaves still different.

A flat bottle cap is primarily an eddy target, on edge it is primarily a magnetic target.

I end up digging a lot of horse shoes and tack rings (large iron harness rings) the look good on my TGSL (14.5kHz, IB VLF).
These ID in the Sliver coin to brass button range.

Iron rings are almost all eddy response which is why most any VLF detector will show them in the silver range.

[Melbeta]

Hmmm. With both magnetic responses, and eddy responses, how can the TID meter depict an accurate reading?

Second question. Is the magnetic response more or less an mineral response? And the eddy response a metal response? Trying to understand the definition here of magnetic and eddy responses. I was not aware there were two different responses. Can you explain?
Melbeta

[Carl-NC]

A magnetic response is due to magnetizing the target. Google "BH curve" for more info. The phase of a magnetic response is limited to 0 to 90 deg, where pure ferrite is around 0.
An eddy response is due to eddy currents induced. Phase response is limited to 90 to 180 deg.
As you can see, there is no phase overlap between a purely magnetic response and a pure eddy response. However, most iron targets have both. That's why nails (mostly magnetic) end up looking ferrous and washers/rings (mostly eddy) end up looking non-ferrous.

[dbanner]

I think iron has magnetic remenance when energised by magnetic field, the removal of which does not follow the same energizing curve, thus leaving the material somewhat magnetized ( the degree to which depends on the strength of the magnetic flux). Perhaps it is this which shows up in the early slope.I wonder if a symmetrical arrangement wouldn't minimize this effect , ie the field is equally energizing and de-energizing. But then again, that would only get in the way of a fast sample.