Air Tests for PI/VLF machines

I just did an air and ground test of my machines. I posted this in another forum but since were talking about I thought I would post here too:

Since I just got a new excal I thought I would do a few air and ground tests. The results were a little surprising. To begin my tests I charged the battery to the excal and replaced the batteries to the other units. So all new batteries. Then I tested a place in my yard with no metal and no EMI. I have a high end emi detector that I used to determine the EMI. I used a nickel, and quarter clad via air and ground tests. My experience with these units - I have used the CZ21 for several years now, the Whites DF for about a year, and I literally got the excal yesterday. So yea I am a newbie with the excal. For each of tests I played with various settings, especially for the excal since I was new to it. I know how to get the most out of the DF and CZ21 already.

Ground test

For the ground test I used a quarter and taped it to a ruler. The ground at my house is a mixture of sand and soil. It was also damp from rain. It is a bit salty too. I live about 1 mile from the coast and about 300 yards from the intercoastal waterway.

Whites DF

12 inches - Nothing
10 inches - Nothing
8 inches - Break in threshold. I may of stopped but it would of been easy to miss.
6 inches - Yes

Excal II - 10 inch stock machine

12 inches - Nothing
10 inches - Nothing
8 inches - Maybe a very faint tone. Not sure I would of stopped but sometimes I pick these up. In Disc it would null and then ring but then I couldn't repeat the ring very easily.
6 inches - Yes

CZ21 - 10 inch stock machine

12 inches - Nothing
10 inches - Nothing
8 inches - Maybe a very faint tone. Not sure I would of stopped but sometimes I pick these up. It rang as iron in disc 0.
6 inches - Yes

Air tests

Whites DF

Nickel - 10-11
Quarter - 9-10

Excal II

Nickel - 10-11
Quarter - 11-12

CZ21

Nickel - 10
Quarter - 10-11

What did I learn? Overall all 3 detectors are about the same depth in air/low mineralized soil. Now, how they handle mineralized soil/water is a different story. My beaches have very little mineralization so I dont expect different measures at my beaches. In the water, I expect the CZ/Excal to begin to drop off but the DF should stay the same.

Interesting discussion.

A question or two to throw into the ring here. I am interested to read others thoughts.

IS a freshly buried nickel or quarter more difficult to detect than a nickel or quarter that has been in place in settled soil for several decades? (Same depth)

IF we answer NO. we don't need to go any further.

IF we answer YES, what is it about our coin and the surrounding ground matrix that makes it 'easier' for a VLF or PI type detector to detect it?

IF there is some effect that takes place between the coin and the surrounding ground matrix, does the effect INCREASE non-ending over time? Or does it reach a point and stabilize?

IF the effect does INCREASE over time, how BIG can the combined coin/ground matrix target become?

As a reference note; in NASA Tom's original DVD on Inland Metal Detecting, he discusses the building of his test garden and his deliberate inclusion of coins with carefully preserved cocoons of dirt around them (halo's) and the adding of a brine mixture to bond the coin/cocoon to his test gardens' soil matrix. ( I don't want to put words into Tom's mouth here, just making a reference on my perceptions of his information)

(Actually, Tom may have some test data applicable to the discussion. He has, in very detailed fashion, measured the performance of a number of CZ detectors. I do not know if his testing includes AIR tests and GROUND tests, but if it does perhaps he can give us some additional thoughts about his own observations to include in our thoughts about this.


Gonebeepin'

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Just one more good target before I go.



Edited 2 time(s). Last edit at 05/27/2016 06:29AM by Gonebeepin'.

@gonebeepin: the 'halo effect' has numerous threads on this forum, this being one recent one:
[www.dankowskidetectors.com]

Pimento Wrote:
-------------------------------------------------------
> @gonebeepin: the 'halo effect' has numerous
> threads on this forum, this being one recent one:
> [www.dankowskidetectors.com]
> .php?2,102496


And this one which is linked in that one has a good explanation by nasa tom:

[www.dankowskidetectors.com]

Here are Tom's quotes from that thread which I think sum it up perfectly:

Quote

The longer a metallic item is in the ground........... the more compacted the dirt....... in general...... around that object. This allows a much better 'electrical connect' with the ground. The metallic object is more 'grounded'. If the object leaches into the ground........... the 'electrical connect' with the ground is greater. It is not the oxides that provides a better/larger detection of the object............ rather............... it is the fact the metallic item is well grounded...... allowing better detection.

Too much emphasis is placed upon the 'halo'. More focus needs to be placed on the enhanced 'electrical connect'.

Bad = Freshly buried or 'turned' soil. Target not electrically 'connected'.
Better = Target in compacted dirt for many years.....in natural setting..... and in good 'contact' with dirt.
Best = Airtest. No dirt minerals. Many variables removed.

I wonder, does this apply to both PI and VLF? And why do so many people claim their machine digs much deeper than it air tests?



Edited 3 time(s). Last edit at 05/27/2016 04:56PM by adamBomb.

Quote

And why do so many people claim their machine digs much deeper than it air tests?

My take:

1. Depth measurements are not reliable, usually overestimated due to assumptions from eye-ball estimates. In-air tests are not reliable, usually underestimated due to EMI. Together overestimation in the field and underestimation in the house yields occasional reports of greater in-field depth.

2. There is also bias in measurements done under field conditions. There is a strong inclination to deceive oneself that their particular detector choice is superior (i.e., the ego wants its actions/choices to be justified). The bias leads to deeper estimates of recovered target depths rather than shallower estimates.

3. Occasionally, in rare cases, a target's electromagnetic signature is enhanced, either by 1) broader/greater eddy circulation due to conductive soil salts (usually wet) or even non-conductive oxides when firmly pressed against the metallic target's surface such that electron flow density increases (and detection depth increases by a strongly generated secondary field) and/or 2) ferromagnetic ground minerals and/or magnetic trash in the vicinity of a conductive target bends/deforms the secondary field generated by the conductive target and by this channels field lines upward to the receive coil or alternatively bends the primary field downward to the conductive target. If we assume there is a degree of randomness in trash/mineral oxide layers where at times the action of ground mineral strata (usually in horizontal layers) will not be to shunt field strength laterally (parallel) to the surface (decreasing depth) but rather to deflect the field energy upwards to a greater degree, reaching the receive coil and thereby increasing detectable depth in that location. In either case, bending the primary field downward or the secondary field upward - detectable depth would be increased beyond the maximum air-test value.

For instance, a paramagnetic ferrite rod when illuminated by a magnetic field would produce a very strong field of its own (due to magnetic domain shifting) but also due to its higher permeability the rod would direct the primary field through itself. If the rod where horizontal then overall depth under that spot would be decreased (shunted laterally). If the rod were more vertical the primary field would reach deeper into the ground illuminating deeper targets. While ferrite rods are lacking in real-world scenarios, dense layers of magnetic minerals (or perhaps dense layers of conductive minerals which repel the field lines but also increase strength/density of field lines by concentrating them around itself) would, under the right conditions, act in a similar manner to the vertical rod.

I didn't mention "halo" because to degrade a non-ferrous object through oxidation/corrosion such that electrically active metals are leached away into the surrounding soil (and replaced by less active molecules from the surrounding soil matrix) seems counter-productive to increasing eddy currents in the target's surface. Eddy formation in the surrounding soil matrix would be highly unlikely. Rather, the more the surface structure of a coin is ruined via attack from the soil matrix (e.g., sulfates, dissolved salts, high soil acidity) the weaker the target's secondary field becomes as resistivity increases. At some point, the non-ferrous target would be undetectable at normal depths. The green/black patina/corrosion on a copper penny in less conductive, increasing surface resistance - meaning, electron flow is inhibited so that the penny increasingly 'looks' like low-conductive trash. As metal is lost and conductivity decreases the detectable depth would decrease, just the opposite effect claimed by "halo" aficionados. It is also less likely due to the need for the anode and cathode to be immersed in an electrolyte fluid for corrosion to begin (an electrical potential must be created). Under most soil conditions this state is not provided as water drains away, or is present for only for brief periods.

On the other hand, wet iron flakes around a decaying (oxidizing) ferrous target might increase detectability for two reasons 1) a wet surface would increase electron surface flow increasing the secondary eddy field (whereas dry would not) 2) some decayed iron flakes (spreading outward in all directions) would be found leaching upward toward the surface, gathered in sufficient density over time, it might produce a signal (due to magnetic domain shifting) becoming weakly detectable at the surface, whereas (perhaps) the original decaying iron target would be too deep/oxidized to produce a recognizable return signal.

Just some possibilities ... but I'd go with overestimated/underestimated field-depths/air-tests and detector bias as the main culprits.

john