Misleading VDI/Depth readings?

[Mister7]

My wife got a nice strong 50cent reading w/3in pinpointed depth using her Prizm lll,after cutting her plug and digging down 3inches she checked the hole and the dirt on the towel with her Bullseye....NOTHING! She rechecked the hole with the Prizm and sure enough the target was still somewhere in hole...More than a little confused she called me over and I checked the hole with the MXT..... Sure enough,I got a full bar VDI (+83)reading with a 3in pinpointed depth showing there was a nice coin in that hole,or so I thought.....We dug down a good foot and found absolutely nothing.... I've had this happen several times with the exact same scenario,high plus VDI numbers,full bar displayed ,25 cent/50 cent displayed and when the area is checked(usually the pinpointed depth is in the 3in range) to the pinpointed depth nothing is found... The only reason I can come up with is that the machine is picking up a larger,much deeper object with similar alloy composition to a quarter or fifty cent piece......Outside of this theory I don't have a clue as to what is going on when this occurs.......As always,your collective thoughts are greatly appreciated...

[Elixir]

I think your thoughts are correct, very large, deep, and conductive targets will give shallow, higher, vdi readings.

[Rudy]

Your assessment is correct Mister, with the possible exception that the actual target need not be a very close match in alloy composition. That is because very deep targets are sometimes phase shifted by the ground mineralization thus changing the VDI reading.

Halves and cartwheel dollars are found rarely enough to arouse suspicion when you get such a VDI reading. Try this next time:

Say the depth indicated by the detector is 3", now raise the loop 9" above the ground and sweep again. Are you still picking up the target at 12" indicated depth? If you are still getting a healthy, repeatable signal, then it is not a coin sized target but it is a much larger target buried deeper.

Also, you should be able to get an idea of the target's size when pin pointing. A coin sized object should pin point pretty tight, unless it is ferrous. See also this thread http://metaldetectingforum.com/showp...86&postcount=1

[Mister7]

Thanks Rudy,that definitely helps clear things up and your explaination makes alot of sense.....The thread you forwarded looks like a great reference,much appreciated.

[Rudy]

You are welcome.

[CyberSage]

Something that might help in the future is something called the coin/can test. Raise the coil 12 to 15 inches of the ground and sweep. Do you get the target responding. If you do it's probably a deeper more conductive target (aluminum can) or URO (unidentified rusty object) wrapping around the VDI.

Keep Swing'in
Jack

Edit: Sorry, I just read Rudy's post and he has stated this test already.

[Ferrous]

Did you ever dig down far enough to see what it actually was?

[Mister7]

Most of the time it ends up being a mauled beer can,other times I've dug down as far as 14 inches and still haven't found the source...My luck,its probably a bag of silver dollars and I just got done getting disgusted,refilling the hole and walking away....Think I'll keep digging next time..LOL