Cody... question on surface blanking...

[Joe G]

In one of your posts you discussed how you could blank out shallow / surface targets by using the "learn / reject" function while lowering the sensitivity.

I've tried that and, even at the beach (where you know there's a lot of shallow trash), I don't pick up any targets when the sensitivity is set under 12... I tried manual sens and 4 or 5 as you suggested and the coil doesn't pick up a thing.

I tried it at a sens. of 12 and went though the learn / reject (and cleared out any upper right rejects in the edit mode) After that, I raised my sens to about 25 or so. Basically, the targets after that were in the 6" or better range, although there were some shallower junk targets... probably missed them during the intitial learn / reject.

I don't think it's my coil since I've gotten some crazy deep targets in the wet sand; even at a sens of 20. Any ideas why I can't find a target with the sens. below 10? I'm using the larger stock coil.

Thanks.

 

[Cody]

I set the sensitivity low and sweep and area to learn targets to be rejected. Once they are part of a program then turn the sensitivity back up to 28 or higher if you can. That this does is take advantage of the non-linear discrimination of the detector. The targets that are rejected from the surface down to about 6 inches will be detected at greater depth. As an example if I reject a 3/28 those targets are clearly rejected down to about 6 inches. However, the same 3/28 will be detected at greater depths.

What I do is reject the ferrous trash for the most part and then dig anything that is non-ferrous. When detecting if a non-ferrous digital combination pops in I know that a 7/25 that is hit again and again is not a gold ring or the area would be full of gold rings. I reject the 7/25, at time, and just go after the deeper ones.

All I am trying to do is improve my odds and yet not miss a good shallow target. The above is not something I do all the time but rather another way to detect in heavy trash. I don't like to reject anything that is on the right half of the display. By the same token I don't like the left half to be black. My solution is to learn the non-ferrous trash to be rejected and keep a threshold all the time. I hope this makes sense.

 

[Captain Kirk]

Cody,

I did not really study your surface blanking posts (probably because I could not understand it), but one thing has me confused. If you to set up to reject a 3/28 target with a low sensitivity, why will ever accept a deeper 3/28 target (while you reject a shallow 3/28 target) when you increase the sensitivity after the blanking process?

The source of my confusion is that the explorer rejects targets based upon the ID of the target and does not take depth into account (Or at least that is what I thought).

HH,
Glenn

 

[Cody]

I think a good way is to put a pulltab on the ground and set the detector to reject the pulltab using learn and a single pixel. Now sweep the coil over the pulltab and slowly raise the coil. The pulltab will be rejected very nice and solid but the higher the coil is above the pulltab the less it is rejected and then bingo it is not rejected at all. If we consider digital numbers my assumption is that is why there is a small, medium, and large box for learn. That tells me the digital numbers are in clusters and are what is called a notch window. A small notch is not going to reject a larger cluster of numbers for the pulltab. The further from the pulltab the coil is the larger the notch or cluster of numbers. All of this is based on the coil and detector not having linear discrimination form the bottom of the coil to depth in my opinion.

I really never thought that much about it other than a simply test demonstrates it works. I try to not promote this very much as I use it to reject surface ferrous trash on the left of the screen. I will use it on the right side at time if the trash is really heavy but like to keep the non-ferrous area open. If we use learn to see what all that stuff is in the ground we find that most of the 1024 pixels are never used. Why black them out if they are not going to be hit to cause a null. I scan an area and only kill the ferrous trash that is common to that site and then will take a close look at anything that is ultra deep since I get a digital number and hit anyway. We can take the EX and go to learn and hear targets that we can never reject or accept no matter how many times we sweep the coil over the hit. My assumption is the detector can never give the target a positive ID so it will not save the target. This is just one more indication to me that the discrimination is not at all linear.

It does make me wonder if other guys EXs work like mine? I am sure I saw a post by one of the guys that used this and found a pretty good coin but I may be dreaming.

 

[Joe G]

I understand how to set up for surface blanking, that's not my problem. My problem is pretty simple though. When I set the sensitivity to 4 or 5 in Learn / Reject, the coil doesn't pick up a thing (to reject); no targets. Keep in mind, I've done this in dry sand at the beach and it's loaded with shallow / surface trash as one would expect. However, I have to go to 10 or 12 before I start picking up anything.

So, I decided to try it anyway. Everything worked properly and I was able to pick up a lot of surface trash in Learn / Reject. I then went to Edit and cleaned up the upper right hand corner to ensure I wasn't disc'ing out coins / silver. When I went to detect (sens 25 manual), it certainly appeared that the targets I was hitting (even trash) were 6" and greater (except for items I didn't get in the initial Learn / Reject coil sweeps)

I'm in Ferrous, Gain 7, Deep on, Fast on, Audio 2, limits 10, variability 10 (I will try your 3 tone approach)

Any ideas why my machine won't detect targets under a sens of around 10? Seems strange.

Thanks.

 

[Captain Kirk]

Cody,

I take it from what you are saying that a 3/28 reading covers multiple pixels. It is therefore possible to reject some targets that give a 3/28 reading while accepting other targets that also give a 3/28 reading. Is that about correct?

HH,
Glenn

 

[Cody]

I am saying a 3/28 is an averaged reading. Let me use the DFX as an example as I think we can apply this where the Explorer is concerned. Also, the DFX will allow the data to be averaged or indicated on the graph as "raw data" When the coil is swept over a target there is a cluster of VDI numbers. If we hit average then we see a single or one or two VDI numbers depending on accurately we swing the coil over the center of the targets, heights, and sweep speed.

My assumption from what I see is that we have a detection window with the Explorer. Glenn it is like loading data into a register and shifting it out to be decoded. What the detector "sees" is a signal that is "decoded" as a ferrous/conductive reading such as 3/28, 3/27, 4/26, 6/15, 3/28. and the like. The wiggle is one way to improve the average by taking short sweep over the center of the target. That is what I am calling a detection window. Data is fed into the register that I will name the "detection window". We need to feed the data in as accurately as we can with sweep speed, height of the coil, and the coil centered over the target. Ground noise, mostly stray RF, cosmic noises, geophysical noises, earth magnetic field, all are considered. A single pixel represents a 3/28 of interest. This is fairly easy to hit on a consistent basis the closer the target is to the coil. If a 3/28 is a copper coin then we don't have any problems with a consistent averaged 3/28 for that coin.

It appears to me that the medium square, notch, is used and suggested. This grabs a cluster of numbers to compensate for the above. A major problem is the distance the target is from the coil we we a wider notch window to reject a 3/28 at depth. An averaged 3/28 at 6 inches is not an averaged 3/28 at 8 inches which is one way to put it. We kill the solid averaged 3/28 at 6 inches but At 8 inches we see a 3/28 peep through but the average is not 3/28 and may be something like 3/25. Since we are set to reject an averaged 3/28 we do so at 6 inches. However at 8 inches the 3/28 is now an averaged 3/25.

What I hear is the rejected 3/28 starts to come in past 6" or so. I know I have rejected a 3/28 down to 6 inches and sure enough the 3/29 is very deep. If I dig the coin it is an old Indian Head and when I sweep the coin on the ground or in the air it is a 3/28.

That is how I search for the ultra deep targets. There at times when I only dig the great shallow hits but for the most part only go after the ultra deep hits when using surface blanking. Also, ultra deep targets cannot be learned. No matter how many times we sweep the coil over the target it is never rejected. My assumption is the cluster of readings is so wide that the electronics defaults and what I hear is a very low tone that sounds like faint iron. Believe me some of those signals are coins. What I look for are good readings and icons peeping through the cluster of number and the tune played.

I am doing a lot of assumption here based on my understanding of tests I have conducted with the DFX and Explorer. I can test some things with the DFX that I cannot with the Explorer. However, I know they were cut from the same mold so compare the two heads up.

 

[Cody]

Joe, I tested my EX in air on a penny. With manual at 1 I get about 4 to 4 1/2 inches. What does your do on a penny in air?

 

[Captain Kirk]

Cody,

If a 3/28 represents a rejected target, then you should never get a 3/28 displayed on the screen. I seems what you are saying is that what reads as 3/28 for shallow targets will read something slightly different when it is a deep target. That is what allows deeper targets to be detected and displayed on the digital screen. Is that correct.

HH,
Glenn

 

[Cody]

Yes, exactly if 3/28 is rejected then we should never see a 3/28. A copper coin has a cluster of digital reading of which 3/28 is the dominant reading. At first I thought that was a problem but it works out to the good. I wondered why the Owner's Manual stated that Learn/Edit might in time become our move valued function. We can use learn, the size of the notch, and edit to peck around on the cluster. A 7/25 will be the dominant pair for my class ring. I can learn reject the ring and not detect it or pulltabs that hit as 7/25. I then reject the shallow pulltabs at 7/25 and can still detect my ring at a greater distance because the dominant pair is no longer 7/25. Since they skew depending on the soil I like to learn and reject on site using actual targets in the soil. To be on the safe side I keep the ferrous area clean at the start then add shallow non-ferrous to the pattern. As an example when I see a number such as 7/22 hit again and again at the same depth then I assume they are not all gold rings but pulltabs so I reject them. True I can miss a shallow ring but I play the odds knowing I will go back again some other time.

 

[Joe G]

I'll check that out tonight to see what I get in the air test and let you know.

Thanks,

 

[Joe G]

I tried an air test with the sensitivity at 1. I only got about an inch??? As I raised the sens, it got progressively better.