stacked coil

[Tinkerer]

Have you tried this?
I'm asking this question because I would expect a bucking coil above the TX to simply cancel the magnetic field in that region, plus reducing the field below the coil to a lesser extent. What mechanism could cause the TX magnetic field from the top of the coil to be deflected downwards? I doubt this is what's happening.

Think about it this way:
In a standard concentric coil (with the TX, RX and bucking coils in the same plane) and the bucking coil wound around the inner RX, the TX magnetic field is drastically reduced at the coil center. However, it does not push the TX magnetic field outwards, thus causing a wider field.

I think it would be great if it were possible to divert the unwanted magnetic field above the coil and make it point downwards where it is needed the most. Unfortunately I cannot see this happening with this particular configuration.

Do you have any experimental evidence to prove your assumption?

Yes, I have tried this. I made a few coils and many different configurations. My understanding comes from my observations.

I also tried different configurations of bundle bucking coils, outside, inside, on top and below the bundle RX coil and found differences in signal amplitude.

Take two magnets and push the two north poles together. Sprinkle iron filings on a sheet of paper and place it above it. You will see how the two magnetic fields deflect each other.
Even better, download the free MAXWELL software from Ansoft, that will give you visualization of the magnetic fields.

Tinkerer

[AUDuke]

Back in the Seventies Garret made a stacked coil they called "Co -Axial" and it was claimed that it did not detect on the top side. At least that's the way I remember it.

[hobbes_lives]

The Sniper coil for the ACE150/250 is very directional and very good near large metal objects. It may be a stacked coil arrangement, or perhaps the bucking coil is placed in such a way to make the coil directional.