How to detect minute magnetic field changes ?

Doing any of these by looking for changes in static magnetic fields will be exquisitely difficult. There's a lot of noise, a lot of other magnetic stuff on the ground/seabed/underground, etc.

Radar or ground-penetrating radar will do the job in some circumstances, but for your 4 different circumstances you need 4 different systems. (And I'm not sure if any of 'em will help with #3).

Tim.

Reply to
Tim Shoppa
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SQUID's your best bet:

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Reply to
Paul Burke

A fluxgate magnetometer might do the trick but there is so much background noise from fluctuations in the Earth's magnetosphere. Caused by the solar wind. One of their applications is monitoring this activity.

Reply to
Kryten

No, a flux-gate isn't even close to good enough.

The field from an object falls off as 1/R^3.

At 1000 feet, one ton of iron would make about 1pT of field change.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

No, a SQUID is hopeless. It is a vector device. If you install it in an aircraft, it will change its angle vs. the earths field. We are looking for variations in the 1pT range here so I'll leave it to you to work out just how small of an angle we are talking about.

BTW: It would have to be a low temp SQUID to be able compete with a Cesium magnetometer anyway. I'm not sure I want to be on that aircraft.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

Fly at 200 feet AGL.

Almost any good quality magnetometer would work if it is a large iron pipe. If you can get to the ends of it, the usual "utility locator" will perhaps do it. If not use the "Binford-5000 as modified by Tim Alan" locator.

I assume this is an aluminum boat. Give up and go buy a new one. If the water is fresh, you may be able to make an EM system do it but it will cost more than the boat did in the 1st place.

A Cesium or Helium magnetometer would be the best for this. The Helium would have a slight power advantage and the Cesium a modest performance advantage.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

How to detect (open to friendly speculations) minute magnetic field changes ?

In application such as; * Detecting a crashed small aircraft in a very dense forest from another small aircraft fliying at a hight of 1000-4000 feet AGL. * Detecting metal pipe 10-20m below in ground. * Detecting a small metal boat sinked to depth of 00-200m from the surface of the sea. * Detecting earth magnetic field variations from a low orbit atellite or from a high altitude flying baloon.

Reply to
<D.N. Scalbile>

What is a *Cesium* magnetometer, and how is it different from a garden variety magnetometer?

Reply to
Robert Baer
[.. a bunch of stuff with "Cesium magnetometer" in it ...]

My garden has tomatoes in it but no magnetometers so I'm not sure what you mean by "garden variety" but I'll give a short run-down of the types.

Coil of wire on a core:

No description needed. This is really only sensitive to changes in the field but at 1Hz, 100fT RMS seems to be about the practical noise limit. This is vector magnetometer.

Flux gate: If you can somehome make the core inside the coil appear and disappear, you could make the above work down to DC. The flux gate uses a core with a winding on it to saturate the core at some regular rate. This makes a vector magnetometer with a practical noise limit near 10pT/sqrt(Hz).

Magneto-inductive:

This works on the fact that the core is not linear and thus the inductance changes due to the field. You can make about 50nT/sqrt(Hz) this way.

Hall effect and GMR etc:

The all work because the electron has a charge and when it is in motion a magnetic field can change the motion. GMRs involve the spin and work on the fact that the energy in the electron would have to suddenly change for it to tunnel from one metal stip into another. These too are vector devices. They seem to be limited to about 1-1000nt/sqrt(Hz)

Proton magnetometers:

Protons have a charge and an spin and as a result a small magnetic field. This field lines up with the external field. If you somehow missaling it, because it is spinning, it can't just snap back to the local field. Instead, it behaves more like a top falling over. It precesses around the external field at a rate determined by the strength of that field. The direction of the filed doesn't matter to the frequency produced so this is a total field device. I've never seen one do better than about

50pT/sqrt(Hz).

He and Cs:

An electron also has a charge and a spin. For the helium, the classical description still works but for the Cesium you need some quantum physics. It is still the case that the frequency depends on the strength and not the angle of the external field. The practical lower limit on noise performance seems to be somewhere near 50fT.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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