Trying to understand wraparound better...

Here's Randy Horton's take on it. It sounds more logical and reasonable than any other that I've heard. He uses an Xterra for his example, but as he says, it's applicable to any VLF with notch.

My Circular Discrimination Theory​

My circular discrimination theory would likely apply to all VLF detectors with notch discrimination capabilities. But for the sake of explanation, I’ll use the 705 as my example.​

If I were to ask you what direction, in degrees, that straight North represented, what would you say? Is it 360 degrees? Or is it 0 degrees? Actually, it is both. Take a look at your compass. Many of us think of metal detector discrimination as being linear. In our minds we can visualize a straight line, running horizontally, with the most ferrous targets located on the left end. And the highest conductive targets clear over on the right. In the case of the X-TERRA 70 and 705, there are 28 target notches spaced evenly in between.​

The X-TERRA doesn’t give any indication that its discrimination arrangement is circular. In fact, the X-TERRA stops scrolling when you try to go lower than the lowest non-conductive number, or higher than the highest conductive number. I submit that is what leads us to think of it as being linear. But I believe, just like a compass, that the Discrimination Scale is circular, not linear.​

Follow me along on this... Take that flat line of notch segments and bend them into a circle, like the numbers on a clock. Instead of twelve numbers representing the hours, we have 28 separate notch segments, evenly spaced in a circular pattern. Starting at the top and moving clockwise, they range from a low of -8, to -6, -4, -2, +2, +4 ... on and on, all the way around the dial to the +48. The spacing between each notch segment is the same, including the space between -8 and +48. If you have a target that has a TID that bounces between 36 and 38, you know it is a target with a fairly high conductivity. With a TID of 36 to 38, it is most likely a dime. If you have a target that reads a -4, and jumps down to a -6, you might not know what it is. But it is definitely a target with a low conductivity (or high ferrous) content.​

Now, if you consider that piece of ferrite I mentioned, and think about those deep pieces of iron you dug this summer (anticipating a silver dollar because they read a +48 ), and toss in this theory of circular discrimination, you get an idea as to how it happened. You dug a piece of iron that provided a TID of +48. Did it read a negative number after it was dug up? Probably. Did the X-TERRA mislead you, making you think it was a highly conductive target when it was still in the ground? Yes, it can be misleading. But based on the information available, the X-TERRA made its “best guess” as to where to position that target in its field of “circular​

discrimination”. Just as a dime reading 36 can bounce to a 38 under given conditions, a piece of iron can bounce between -8 and +48. And, since the 705 we are discussing does not have any notch segments between the adjacent notches of +48 and -8, the iron will identify with the one it is most closely aligned to. Factor in the ground conditions, and you can see how that deep old iron can fool you into thinking it is a silver dollar.​

My theory is, regardless of how many notch segments could be added between the -8 and +48, there will always be targets that “fall” between the most negative TID and the most positive TID. The best solution I have to offer is to use the tips I’ve tried to provide throughout this eBook, and listen more intently to the audio response made by these wrap around signals. When working the coil over the target, iron tends to provide both high tone and low tone harmonic signals.​

From a strictly technical point of view, I don’t believe the wrapping of ferrous and non-ferrous is truly circular, or 360 degrees. I believe it to be semicircular at 180 degrees. However, in an attempt to not make my explanation more confusing, I used the example of a compass, and the terms circular and 360 degrees, instead.​

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That made sense to me. Although that may not be saying much.

Longhair said:

Here's Randy Horton's take on it. It sounds more logical and reasonable than any other that I've heard. He uses an Xterra for his example, but as he says, It's applicable to any VLF with notch.

Click to expand...

Thats a good informative read and inline with other things I have read on it. Im wondering though if there are ways to know when this is happening , is there something to look for or listen for , are there circumstances where it would happen more often , or is there any way of avoiding it , or is it simply an explanation of an unavoidable reality that happens randomly but nothing can be done to know whether its happening or not ?

I have to wonder if its more likely to happen while hunting in iron infested ground or along a fenceline , above a gas line , etc. Does the presence of iron in the ground cause it to happen or does the detector just decide to throw a false reading randomly. Sometimes it happens in reverse , instead of calling iron silver it calls silver iron.........from what I read anyway. Seems like there would be a root cause , maybe the size or depth or relation to a nearby object ?

I was hunting a sidewalk tear out where I could easily follow the gas line along the edge , if I got to close to it I could hear it.....of course it was marked with paint and flags by the work crew. In an area where there should have been old coins , nothing is guaranteed but this was as good a spot as any for old silver , but I was getting iron signals all over and no high tones to be found. There was likely some masking going on but I wonder if some of those ultra low tones might have been coins. Not knowing what actually causes wraparound for all I know there was just a bunch of nails out there.

Good explanation. It's still tough to wrap your head around it though!