Can somebody please explain?

[BarberBill]

We've all read on the forums and sometimes elsewhere that air testing a detector is not a good measure of its actual performance as some machines test poorly in the air, but do much better on buried targets. I'd like to see a logical or tested explanation of how that works. Many targets do not leach into the soil as does rusty iron, so the so called "halo effect" can't account for every instance, especially with silver or gold that remains inert in the soil. Also, how can a machine recognize a target through soil or another medium, yet do poorly thru the air which would seem to have little or no blocking effect at all? If anyone can pass on a solid, logical explanation to this, I'd appreciate it. Always trying to learn more.
HH
BB

 

[markg]

n/t

 

[akille68]

I don't think that a machine can perform better on buried targets than on air.
How can ground amplify a magnetic field?
Also, It's true that a good air performer not always punch so deep in the ground.
My two cents...

 

[Adrian SS]

HI BB,
Your question has been asked many times in the past and I still have not heard a good explanation.
From my experience over 45 years with metal detecting this is what I have noted in regards to air test and in ground testing.
The only time I have noted an increase in depth over air testing for any type of hand held metal detector has been when the ground is wet and has caused a leaching of minerals from the target which seems to present a larger target to the detector and also minerals can be attracted from the surrounding matrix towards the target resulting in the same effect.In dry ground these effects are less likely to occur.

All of the targets I have in my test patch produce a weaker response from all of my detectors when the ground is dry than what they produce in air testing. The targets are sealed in plastic bags to eliminate any corrosion and leaching effects. The targets have been buried for longer than 12 months.

When they were first buried they produced a weaker signal than when air tested and after 12 months with the ground compacted hard through natural settling they produce a signal the same as the air test figures when the ground is bone dry. I test the ground with a moisture meter probe.
When the ground is wet the response is slightly stronger than the air test figure.

I guess if you want a scientific explanation you will have to bring a geological scientist on board .
Cheers,
Adrian

 

[BarberBill]

Adrian SS - Check the post in the metal detecting forum on this subject. The answer made sense to me.
BB

 

[Malox]

Has anyone ever air tested say, a quarter, and it tests at 10" exactly. Now bury the quarter ten inch's and see if you can still pick it up. No halo effect either way. Seems that would tell you if the dirt kills your depth or not.

Don't mind me I'm just nuts......

Jim

 

[Adrian SS]

The magnetic field from a metal detector extends out to an enormous distance from the coil not just a foot or two both into the ground and through the atmosphere. The ability of the detector to respond to a target signal or EMF generated magnetic field depends on the detectors receiver sensitivity and selectivity. Sensitivity is the detectors ability to detect and amplify the target signal, Selectivity is the detectors ability to pick out one signal from a maze of other unwanted signals. Iron mineralization in the soil can cause the magnetic field from the detector to squash out or concentrate beneath the coil and prevent the field from penetrating very deep into the ground but at the same time this concentrating of the field can produce a stronger signal from a buried target which can be at a greater depth than the air distance signal because the concentrated magnetic field produces stronger eddy current flow in the buried target. Conductive alkali salts will produce the same effect and therefore if the detector has good ground canceling ability and selectivity the in ground detection distances can be greater than the in air distance. irrespective of whether the target has a halo effect. When the concentration of iron and or salt mineralization becomes so high that the detector cannot tune it out then the signal from the mineralization overrides the target signal.

In theory we do not need a transmitter in our detectors because eddy currents are flowing through the earth all of the time generated by cosmic radiation and the earths own changing magnetic field; Plus we are pumping buckets of electromagnetic fields into the ground from all of our electronics and transmit ion lines; All we need is a receiver that is sensitive and selective enough that it can tune into these eddy currents which are all in different phase relationship to each other depending what it is that they are flowing through.

Adrian.