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Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Dancer
Date: March 27, 2019 08:02AM
Some of this has had to happen to you guys. So get a machine air test it than go out hunting. Many times I come across a coin , not so deep and my machine seems to struggle with. Than Bam ole shallow bangs hard on a coin so deep , I'm thinking , Just how deep does this machine really reach ?
I'm just guessing, maybe the shallow coin was on edge a little or hasn't been lost very long. So maybe the hard hitting deeper coin had a halo effect due to being buried longer. Think , maybe.? I don't know. So I've had (more than a few Times) Vdi readings for fifty cents. Turned out to be 2quarter's.
Smart machine , huh ? Never had a machine call it a quarter, ended up 2dimes & a nickel. Just never happened to me. How come ?
You guys ever think about this stuff ?
Dancer

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: bklein
Date: March 27, 2019 09:20AM
I posted a question on geotech about how pi’s seem to test badly in air tests yet have a reputation for going deep in saltwater. My DF was actually worse in wet sand so I don’t know who or what to believe and am trying to determine if mine’s normal or something’s wrong with it. Maybe just halo coins are deep and recent drops are not.

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Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: dewcon4414
Date: March 29, 2019 07:58AM
Angle of the coin, near targets, not centering the target are some of the reasons why shallow targets may provide poor signals.



MDT 8000, EQ 800, CTX, Blue Xcals
Dew Meeker

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Nauti Neil
Date: March 29, 2019 11:48AM
Hi Dancer,
The only way to test how deep a detector will detect something in air is to be completely isolated from electrical interference.You often hear about how somebody's detector does poor in an air test then it is taken to a field and miraculously gains a good couple of inches in the ground.
If you watch video's of air tests,most are done with the detector lay on a table with the coil in the correct position to attract all the electrical interference in the house the test is conducted in.Electrical and magnetic interference will limit the efficiency of the detectors own magnetic field and that is why some detectors seem to do badly in air tests.
I've done this test myself with a lot of my own detectors...i no longer need to do it as i have proved to myself that........ a detectors maximum in air depth(tested in an area of no electrical interference),cannot be exceeded by a target in the ground.A detectors magnetic field is determined by the amount of power sent to the coil....it doesn't stretch or penetrate any deeper no matter what the soil conditions are,it is what it is.I can't remember who said it now but it was an electrical engineer who pointed out that if your detector air tests badly(in ideal conditions) it will do badly on depth in soil.
It can however work the opposite way around.......soil conditions can lessen the depth of a detectors magnetic field due to high mineral content and other factors.
So in summary,if you take your detector to a place where there is no electrical interference and air test it on different coins for example,these will be your detectors maximum depths on those objects....you will not get the same targets any deeper,in most cases you will loose varying amounts of depth depending on soil conditions......do the tests yourself and you will see what i am saying is accurate.
As for the halo effect,i'm not convinced to be honest......i just don't think that a targets reaction with the soil will allow you to detect a coin or artefact deeper......the metal object hasn't got any bigger or gained mass,it's just a chemical reaction to the soil......detectors don't pick up chemicals.Rusty objects may be detected slightly deeper as as they expand in size as they get heavily corroded but not non ferous items.
The above is also true of beach searching both fresh and saltwater.You don't have to take what i have said as Gospel but if you do some testing yourself i'm sure you will reach the same conclusions.



Edited 2 time(s). Last edit at 03/29/2019 12:11PM by Nauti Neil.

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Dancer
Date: March 30, 2019 05:04AM
Quote
Nauti Neil
Hi Dancer,
The only way to test how deep a detector will detect something in air is to be completely isolated from electrical interference.You often hear about how somebody's detector does poor in an air test then it is taken to a field and miraculously gains a good couple of inches in the ground.
If you watch video's of air tests,most are done with the detector lay on a table with the coil in the correct position to attract all the electrical interference in the house the test is conducted in.Electrical and magnetic interference will limit the efficiency of the detectors own magnetic field and that is why some detectors seem to do badly in air tests.
I've done this test myself with a lot of my own detectors...i no longer need to do it as i have proved to myself that........ a detectors maximum in air depth(tested in an area of no electrical interference),cannot be exceeded by a target in the ground.A detectors magnetic field is determined by the amount of power sent to the coil....it doesn't stretch or penetrate any deeper no matter what the soil conditions are,it is what it is.I can't remember who said it now but it was an electrical engineer who pointed out that if your detector air tests badly(in ideal conditions) it will do badly on depth in soil.
It can however work the opposite way around.......soil conditions can lessen the depth of a detectors magnetic field due to high mineral content and other factors.
So in summary,if you take your detector to a place where there is no electrical interference and air test it on different coins for example,these will be your detectors maximum depths on those objects....you will not get the same targets any deeper,in most cases you will loose varying amounts of depth depending on soil conditions......do the tests yourself and you will see what i am saying is accurate.
As for the halo effect,i'm not convinced to be honest......i just don't think that a targets reaction with the soil will allow you to detect a coin or artefact deeper......the metal object hasn't got any bigger or gained mass,it's just a chemical reaction to the soil......detectors don't pick up chemicals.Rusty objects may be detected slightly deeper as as they expand in size as they get heavily corroded but not non ferous items.
The above is also true of beach searching both fresh and saltwater.You don't have to take what i have said as Gospel but if you do some testing yourself i'm sure you will reach the same conclusions.

TheRover. I'm going to give you the nod on your expertise. Way out of my lane on this. So what do you think of a couple of other things, I think sometimes happen.
The Ghost signal. Quite common in my hunts. Happens right after I dig a coin. Get one beep over a empty hole, than proof it's gone. The sigh of the Ghost, huh.
So if not a halo, what ? As Dew said above, lots of reasons for faint shallow targets. But I cannot explain why a machine will sometimes hit hard on a target that usually is beyond it's reach. Could be my imagination ?

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Dancer
Date: March 30, 2019 05:28AM
Quote
Dancer
Quote
Nauti Neil
Hi Dancer,
The only way to test how deep a detector will detect something in air is to be completely isolated from electrical interference.You often hear about how somebody's detector does poor in an air test then it is taken to a field and miraculously gains a good couple of inches in the ground.
If you watch video's of air tests,most are done with the detector lay on a table with the coil in the correct position to attract all the electrical interference in the house the test is conducted in.Electrical and magnetic interference will limit the efficiency of the detectors own magnetic field and that is why some detectors seem to do badly in air tests.
I've done this test myself with a lot of my own detectors...i no longer need to do it as i have proved to myself that........ a detectors maximum in air depth(tested in an area of no electrical interference),cannot be exceeded by a target in the ground.A detectors magnetic field is determined by the amount of power sent to the coil....it doesn't stretch or penetrate any deeper no matter what the soil conditions are,it is what it is.I can't remember who said it now but it was an electrical engineer who pointed out that if your detector air tests badly(in ideal conditions) it will do badly on depth in soil.
It can however work the opposite way around.......soil conditions can lessen the depth of a detectors magnetic field due to high mineral content and other factors.
So in summary,if you take your detector to a place where there is no electrical interference and air test it on different coins for example,these will be your detectors maximum depths on those objects....you will not get the same targets any deeper,in most cases you will loose varying amounts of depth depending on soil conditions......do the tests yourself and you will see what i am saying is accurate.
As for the halo effect,i'm not convinced to be honest......i just don't think that a targets reaction with the soil will allow you to detect a coin or artefact deeper......the metal object hasn't got any bigger or gained mass,it's just a chemical reaction to the soil......detectors don't pick up chemicals.Rusty objects may be detected slightly deeper as as they expand in size as they get heavily corroded but not non ferous items.
The above is also true of beach searching both fresh and saltwater.You don't have to take what i have said as Gospel but if you do some testing yourself i'm sure you will reach the same conclusions.

TheRover. I'm going to give you the nod on your expertise. Way out of my lane on this. So what do you think of a couple of other things, I think sometimes happen.
The Ghost signal. Quite common in my hunts. Happens right after I dig a coin. Get one beep over a empty hole, than proof it's gone. The sigh of the Ghost, huh.
So if not a halo, what ? As Dew said above, lots of reasons for faint shallow targets. But I cannot explain why a machine will sometimes hit hard on a target that usually is beyond it's reach. Could be my imagination ?
Sorry Neil about confusing your name in the above.

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Nauti Neil
Date: March 30, 2019 08:32AM
I don't know about ghost signals even though i've experienced them myself.As for halo effect,i have often dug targets,particularly brass or copper targets that have "leached" into the surrounding soil causing a prominent green reaction with the ground around the target.I have tested this contaminated soil many times under my coil and have never got a signal from it.
I can only think that some targets that are normally out of reach may be surrounded by natural ground minerals that can cause a signal,making the target easier for your detector to see and making you think that a coin target is deeper than usual....it may not be the target your detector see's but the mass of ground minerals surrounding a target.....it may just be pure luck that a good target is located in such a situation.When you dig your target such as a coin and go over the hole you may still get a signal because the minerals are still being detected.Dig a hit more and you can disturb the ground enough to limit the effect of the ground......causing that disappearing signal.

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Dancer
Date: March 30, 2019 04:43PM
So has anyone experienced Fifty cents vid on their screen and dug 2quarter's. I might have this happen 2-3 times a year while using one of my Garretts.
I'm not making this up so someone has had to have something like this happen to them.

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Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: dewcon4414
Date: March 31, 2019 07:15AM
The halo affect is much more likely to happen with the more magnetic the target. Targets like iron once they rust are seen as large SIZE in the soil. Not nearly so on say the copper end...the response on the receive is much shorter...degrade faster. Angle seems one of the greatest factors.



MDT 8000, EQ 800, CTX, Blue Xcals
Dew Meeker

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Nauti Neil
Date: March 31, 2019 09:48AM
Hi Dancer,
Vdi readings are nothing more than innacurate at best.The responses to a certain coin etc is set up under certain conditions by the manufacture,not in real life detecting scenario's.You can also test this yourself......wave a certain coin under the coil (in air) and you will find that you probably get half decent accuracy on your detector's screen.
Take a number of the same coin,10 if you want and then bury them in soil at different depths,angles and in different types of soil and see what readings they give you.Go back to them after they have been in the soil for a while too........this test will show you just how innacurate vdi's are........a certain object can give many different readings depending on the conditions in which they are buried.
It is not at all wise to dig or not dig objects on the strength of what a vdi is telling you.

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: bklein
Date: March 31, 2019 11:54AM
https://youtu.be/BbWQhcXovr0
Vertical coil, in a house, not suggested conditions but seems to show how they compare. Now try a gold ring in wet sand at the beach on the df...4”, 8”, 16”?

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: bklein
Date: April 06, 2019 09:50PM
I have to say that after retesting, I see that the surf pi pro and DF are having trouble with the ring on edge - way more-so than the CTX and GP Extreme I was comparing them to. If the ring is put horizontal it is sensed 10-12” on top of or in the sand.

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Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: dewcon4414
Date: April 08, 2019 05:49AM
Surface and angle matters and affects machines differently. Put a ring on edge...... hit it ...... then hit it from the other angle. Less surface...... it sees it as a stright object rather than a round one. There can be a huge difference in depth in these three angles no matter the machine.



MDT 8000, EQ 800, CTX, Blue Xcals
Dew Meeker

Re: Air testing. The halo effect & Vdi. Wet sand / wading
Posted by: Dancer
Date: April 08, 2019 06:03AM
Great reminder you guys are bringing up. I used to call them "Cleated coins" Coins put on edge by being stepped on by a sports shoe with cleats.
Due to the "Astro turfing " in my area , another thing of the past. Might have been my imagination but I believe the 12" coil on my Gti 1500 Garrett was pretty good at rooting these edgy coins out.

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