Discrimination and Depth

Under actual field conditions, things are not as straight forward as air testing a desirable target. Two other factors you have to keep in mind in the context of this discussion.

First, (this applies to most detector types) when the detector goes over a target that is discriminated, the audio signal is turned off (ie. audio discrimination) by the discriminator circuit, and stays off during the recovery time of the detector. If, after passing over a discriminated target the detector then passes over a desirable one within the recovery time period, one of two things may occur.

A: Shallow desirable target.
The desirable target, if shallow and of sufficient size, may generate a strong enough signal to break through and sound off.

B: Deeper desirable target.
Signals from a deeper desirable target are not sufficiently strong to break through and are not detected.

The other problem is that the discriminator filter does not have "perfectly sharp corners", but instead have a transition skirt on either side. The detector's sensitivity to targets whose VDI falls on the skirts is impaired. In other words, the target is not fully discriminated in the skirt region, but it is attenuated somewhat with a consequential loss of depth.

Hope the above made sense.

I have never believed that less disc= more depth.

It doesn't. But if you use higher levels of discrimination it makes it more difficult for the detector to identify the good item from the background noise the ground is creating.

The problem is compounded by people thinking that a correctly ground balanced machine has balanced the ground and therefore nullified the effect.
In reality all that has happened is that you have moved the point of balance forward making it more positive so that the negative sounds move into the silent zone.

An air test cannot be used to test out theories on discrimination v depth as its so tied to the ground mineralisation.

For those who really just want to detect to the best effect they should ignore the theory and use the facts. The less discrimination used the better, except if its just shallow or more modern finds that are sort, where digging less rubbish allows more ground to be covered which could provide a greater return at the end of the day if your not worried about what your missing.

I'm certain someone will come in now and say what type of machines and in what conditions more discrimination provides more depth. If they don't I'll provide the answer on Monday (I'm away at the weekend if I can get my car through the snow).

An air test cannot be used to test out theories on discrimination v depth as its so tied to the ground mineralisation.

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I don't agree. Now a will agree that ground minerals will affect results, but not to the degree many have been led to believe. I believe this is another one of those theories with not much evidence to back it up. That is not to say that really bad ground won't result in a difference in air and ground results, but that those differences are not near as profound as many have been led to believe.

Here in southwest Kansas mineralization isn't a major issue, and I'm sure Kansas isn't the only area where this is the case. I've done lots of testing with various detectors with pretty much the same results. I can get roughly the same depth in an air test I can get in the ground when the detector is adjusted correctly. I've shown my video of depth air tests with my DFX getting about 9-10" and I get the same results in the ground. Adjusting the discrimination only gives me a better, or worse, response from a target not more depth.

Yes ground minerals will skew the results between the air and ground tests, but far less than many are led to believe.

Freshly buried targets are the exception. When a target has been in the ground over a period of time, undisturbed, the ground around it takes on the same matrix making the target easier to detect. For the sake of making this easier to understand allow me to simplify. Lets say the areas ground matrix/phase is -95. Your detector knows that -95 is the balance point, and looks for a change in that balance point(phase shift) to say there is something there. Simplified again, the amount of sensitivity tells your detector at what point a detected change in the balance point(-95) will be processed as a possible target.

When the ground matrix(-95) has been disrupted by resent digging like burying a target, the area no longer has the same matrix(-95). When your coil is moved over the recently disturbed soil, which now has a different matrix than the undisturbed soil, the difference between the target signal and the ground matrix is not so pronounced as it would be on undisturbed ground.

I am a firm believer that air tests depth results ARE a good indicator of depth ability one can expect under the best ground conditions. Of course this doesn't factor in the halo effect, which actually doesn't give better depth but makes the target easier to see, and very poor conditions, which would result in even less depth that air tests.

I agree Detector that its not depth as much as target response. I think Rudy explained it pretty well, but also threshold comes into play when dealing with those deep targets if the deep responses cant break the threshold it wont be heard. That really comes into play when targets are competing with EMI and minerals at depth and have to be filtered out.

I have spent quite a bit if time testing my X-50 in the ground and I have found that I can run the desc hi and clean out copper pennies and silver; when I go back with no desc I don't find deep targets I missed (not yet anyway) what I do find are slightly more consistent dig targets when trash is close.

Basically, as many people, I think have found with the trash signal un blocked I can located it and see something else good is there.

As far as my Bounty H, I do loose some depth on copper pennies but only when I desc up to about zinc.

Detector said:

I don't agree. Now a will agree that ground minerals will affect results, but not to the degree many have been led to believe. I believe this is another one of those theories with not much evidence to back it up. That is not to say that really bad ground won't result in a difference in air and ground results, but that those differences are not near as profound as many have been led to believe.

Here in southwest Kansas mineralization isn't a major issue, and I'm sure Kansas isn't the only area where this is the case. I've done lots of testing with various detectors with pretty much the same results. I can get roughly the same depth in an air test I can get in the ground when the detector is adjusted correctly. I've shown my video of depth air tests with my DFX getting about 9-10" and I get the same results in the ground. Adjusting the discrimination only gives me a better, or worse, response from a target not more depth.

Yes ground minerals will skew the results between the air and ground tests, but far less than many are led to believe.

Freshly buried targets are the exception. When a target has been in the ground over a period of time, undisturbed, the ground around it takes on the same matrix making the target easier to detect. For the sake of making this easier to understand allow me to simplify. Lets say the areas ground matrix/phase is -95. Your detector knows that -95 is the balance point, and looks for a change in that balance point(phase shift) to say there is something there. Simplified again, the amount of sensitivity tells your detector at what point a detected change in the balance point(-95) will be processed as a possible target.

When the ground matrix(-95) has been disrupted by resent digging like burying a target, the area no longer has the same matrix(-95). When your coil is moved over the recently disturbed soil, which now has a different matrix than the undisturbed soil, the difference between the target signal and the ground matrix is not so pronounced as it would be on undisturbed ground.

I am a firm believer that air tests depth results ARE a good indicator of depth ability one can expect under the best ground conditions. Of course this doesn't factor in the halo effect, which actually doesn't give better depth but makes the target easier to see, and very poor conditions, which would result in even less depth that air tests.

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In other words you say, if a possible coin is at 12" and you dug 10", the coin will be lost? Because the ground is disturbed? Time for a better detector I think.

Yes, there are some out there they do so, but but those I sold very fast.

Detector said:

I don't agree. Now a will agree that ground minerals will affect results, but not to the degree many have been led to believe. I believe this is another one of those theories with not much evidence to back it up. That is not to say that really bad ground won't result in a difference in air and ground results, but that those differences are not near as profound as many have been led to believe.

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You may not agree, but the fact is that highly mineralized ground, particularly when it is predominantly ferrous mineralization, has a large effect on target depth recognition. You are blessed that you are able to hunt in mildly mineralized ground. I've experienced it first hand and in his books, Jeff Foster, has tabulated experimental evidence of this.

Think of it this way, the ground mineralization responds to the transmitted field just like a target, except we consider it background noise. The more mineralization, the larger its response and the smaller the signal to noise ratio is present at the receiver. There comes a point where the target's signal is so small that it is overwhelmed by the ground noise and no amount of additional filtering is able to "pull it out" of the noise.

Ground balancing the detector is not the same as instantaneous ground noise canceling. Detector electronics can do the former but not the latter.

In other words you say, if a possible coin is at 12" and you dug 10", the coin will be lost? Because the ground is disturbed? Time for a better detector I think.

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No. What I am saying is find a coin at 12" and remove the top 10" AND then put that dirt back over the coin. I think you'll find it is gone now.

Rudy, what I'm saying is I have yet to see a detector that could detect a given target deeper in the ground than in the air. Yes the halo effect can improve the detection depth of a target, but it is the target that is given off a stronger response, not the detector going deeper. If you could reproduce the halo effect in the air you would get the same depth or more.

An Air test represents the best possible depth response a detector will produce.

Detector said:

No. What I am saying is find a coin at 12" and remove the top 10" AND then put that dirt back over the coin. I think you'll find it is gone now.

Rudy, what I'm saying is I have yet to see a detector that could detect a given target deeper in the ground than in the air. Yes the halo effect can improve the detection depth of a target, but it is the target that is given off a stronger response, not the detector going deeper. If you could reproduce the halo effect in the air you would get the same depth or more.

An Air test represents the best possible depth response a detector will produce.

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I guess I didn't understand your previous post. Yes, on most detector designs out there, the air test is as good as it gets.

I understand your thougth Detector. If there isnt anything but EMI affecting the signal then air tests are accurate gages of the maximum depth of your detector. Based on that then why not tweak for maximum depth in the air? Yes there are more factors affecting depth in the ground, minerals, EMI, salt, and mositure. However, ive swong over goffer holes before and dead air space just isnt good not even raising the coil at the end of your swing especially on the Explorer... so why would it be better air testing?

Perhaps I should direct you to the posts re the MXT detecting small coins at a greater depth in ground than in air ?
Or the detectors that require damp ground to conduct the signal ?
Or those that have auto sensitivity that in air or non mineralised ground have a marked drop in depth and sensitivity ?
There's also been tests on the effects of the concentration of the electromagnetic field pattern of mineralisation (to the good). You may be able to find some discussion of this by Bob Podhrasky, but what does he know, he was only only Garretts chief engineer for years.

To go back the "Detectors" original post...how many coins does he find in air ?
Find a weak signal out in the field or use an established test bed then try some tests.

I'm not sure where this is leading to but in my opinion if you don't have the ground figured in with the testing then the test is not complete. But in a lot of situations the ground will reduce the depth no doubt. Yes I knew Bob had recently retired from Garrett and he was there a long time with Mr.Garrett.

John Tomlinson,CET
John's Detectors

I understand that ground conditions do affect the depth a detector can obtain, but for the most part, the depth you get in an air test is what you can expect in the ground under good conditions. I've seen posts that say they can only get 4-5" in an air test, but what I've also seen is they are looking for good solid signal, they're not listening for that faint signal you often get on a target 6" deep or more on some detectors.

As I said at the start of this thread I am asking a question more than making a statement.

I have done tests with quite a few detectors, White's, Garrett and Minelabs, and all air tested to the same depth I get in the ground in the best conditions. Never heard of a detector that "requires damp ground to conduct the signal." Doesn't sound very practical.

Detector said:

No. What I am saying is find a coin at 12" and remove the top 10" AND then put that dirt back over the coin. I think you'll find it is gone now.

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Sorry, I misread the info in the post, Detector. Yes, I'll do this on my next hunts. This is a phenomenon which I had to one detector.
This machine detected dug and refilled "holes", even if the target was gone. (checked back in a target free area).

All the Best.

Detector said:

I understand that ground conditions do affect the depth a detector can obtain, but for the most part, the depth you get in an air test is what you can expect in the ground under good conditions. I've seen posts that say they can only get 4-5" in an air test, but what I've also seen is they are looking for good solid signal, they're not listening for that faint signal you often get on a target 6" deep or more on some detectors.

As I said at the start of this thread I am asking a question more than making a statement.

I have done tests with quite a few detectors, White's, Garrett and Minelabs, and all air tested to the same depth I get in the ground in the best conditions. Never heard of a detector that "requires damp ground to conduct the signal." Doesn't sound very practical.

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This is my opinion, too. I totally agree. Otherwise it would be impossible to hunt fresh plowed fields.

i agree with u detector im new at detecting but found that with my bh ivi had no depth change but found out that with dis set past 7o clock i didn't pick up nickles iv been detecting since april o9 and just realized it makes me wonder how many i missed!!

Hmmmmm. Just for conversation, heres something to consider. Depth ( in the ground not air test ) does have an effect on conductivity of the target and the electric field generated by it , it may be minor but it can effect how most detectors "read" the signal. If anybody doubts this then why do very shallow targets usually ring out clearer and louder than the very deep ? So its only natural that as you turn up the disc. and block out lower conductive targets then you are losing a certain amount of the "functional" depth of the machine on deep targets even if theoretically your machine is still "seeing" the target but filtering it out because its distance from the coil or the amount of minerals ( dirt ) is making it appear the condictivity is different. The targets I am talking about are the deepest and at the very bottom of the signal cone. Seems that any way you put it your functional depth gets reduced by turning up the discrimination. Like a gold ring could atleast in sound theory drop a notch and be read as iron or atleast something other than gold range at the extreme edge of your detectors depth ability. That is still loss of depth. Does this make sense ?

ohiochris said:

Hmmmmm. Just for conversation, heres something to consider. Depth ( in the ground not air test ) does have an effect on conductivity of the target and the electric field generated by it , it may be minor but it can effect how most detectors "read" the signal. If anybody doubts this then why do very shallow targets usually ring out clearer and louder than the very deep ? So its only natural that as you turn up the disc. and block out lower conductive targets then you are losing a certain amount of the "functional" depth of the machine on deep targets even if theoretically your machine is still "seeing" the target but filtering it out because its distance from the coil or the amount of minerals ( dirt ) is making it appear the condictivity is different. The targets I am talking about are the deepest and at the very bottom of the signal cone. Seems that any way you put it your functional depth gets reduced by turning up the discrimination. Like a gold ring could atleast in sound theory drop a notch and be read as iron or atleast something other than gold range at the extreme edge of your detectors depth ability. That is still loss of depth. Does this make sense ?

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As to the question of why a shallower target rings out more clearly than a deep one, the answer is of course that there is much less distance between the target and the receive coil. Since the field generated by the target decays as the square of the distance (it is actually worse than this), the signal strength of deeper targets diminishes quite quickly. For example, take two dimes, bury one at 4 inches and the other one at 8 inches. The signal from the transmit coil to the dimes induce the fields that will be detected by the receive coil. The signal from the transmit coil to the dime at 8 inches is four times smaller than the dime at 4 inches. Similarly, the induced field from the dime is attenuated on its way to the receive coil. Again, the signal from the dime at 8 inches is attenuated by a factor of four compared to that of the shallower dime. The net effect is that the signal from the deeper dime is 1/16th that of the shallower dime.

If the targets are lower conductors, the signals are even smaller because the induced fields are smaller, but the proportionality due to distances remain the same.

As to why higher than needed discrimination costs you detection depth, you might be interested in what I wrote on this topic earlier. http://metaldetectingforum.com/showpost.php?p=256704&postcount=2