Coin in a hole phenomenon

[ziphius]

Here's a question I've been wondering about...

I can detect and dig dimes to 8 inches with the F75, probably deeper. My hand is 7.5 inches long, so I've got good depth calibration. However, if I dig an 8 inch hole and bury my own dime, then pack the dirt down, I typically can't detect the target. Same thing happens even if I don't fill the hole. Is there some "target shadowing" going on here, where the geometry of the hole itself is preventing the EM signal from reaching the coin? Is this due to a lack of a 'halo' effect because the coin has not been buried for a long time? A bit of both?

Jim

 

[jimmyk]

Newly buried coins aren't detected as easily as a coin that's been lost and at the same depth. Some say "halo" effect. I suspect there's something about the ground being disturbed that reduces the readablility of a newly buried object. Just guessing. I've heard from some "experts" the "halo" effect is a myth. For whatever the reason, the fact is lost coins are easier to detect over buried coins.

jimmyk in Missouri

 

[RonNH]

It seems strange that you can't detect a newly buried coin, dirt or not, but you can do an air test at 10, 11, 12" with no problems.

 

[BarnacleBill]

It has to do with the consistency of the medium. The air is very homogeneous with little loss. The disrupted soil is no longer cohesive with the surrounding soil and has loss. A very weak analogy follows. Take a confectionery frozen ice pop, tap against the side with a spoon, a sharp sound is heard and travels through the solid frozen ice surface. Now crush the ice pop up, same material but the crystalline structure is now disrupted. Tap with spoon, very weak non-transported sound.

HH
BarnacleBill

 

[daryllpugh]

Are you trying this in disc. mode? If so try it in all metal and crank the sensitivty. I think you'll see a difference. The F-75 is an awesome machine in all metal. Good luck dig'in!!!!

 

[ziphius]

The ice pop analogy is very good, thanks. In fact, I did try this in all-metal mode and got a weak signal, with no visual ID. But it was superior to discrim mode, which has been my experience in the field. Thanks again for the comments. - Jim

 

[king-ghidorah]

It has to do with the consistency of the medium. The air is very homogeneous with little loss. The disrupted soil is no longer cohesive with the surrounding soil and has loss. A very weak analogy follows. Take a confectionery frozen ice pop, tap against the side with a spoon, a sharp sound is heard and travels through the solid frozen ice surface. Now crush the ice pop up, same material but the crystalline structure is now disrupted. Tap with spoon, very weak non-transported sound.

HH
BarnacleBill

I think that's a great analogy. Made perfect sense to me.

 

[ringfinder]

Here is something else I found out about my F75. When you get a signal on a coin and it's deep and in very dry soil, a lot
of times after you dig the hole, you loose the signal. I have found this to be true many, many times this year, here in Ohio
with very dry soil. I highly recommend the use of a probe. I use the SunRay Probe on my detector and really like it, in
fact, I never leave home with out it. Once you dig the hole, and can't hear the signal, use the probe. Nine times out of ten,
the signal is still there.

HH, Ringfinder

 

[ziphius]

Thanks Ringfinder. I bought myself a Pistol Probe and problem solved. That probe gets 6 inches on coin-sized targets, no kidding.

Jim

 

[jimmyk]

Is that in both disc. AND all metal? Sometimes I lose a signal with disc. mode and switch to pinpoint mode and can get it again.

keep on diggin'

jimmyk in Missouri

 

[ziphius]

jimmyk,

That's been my experience....all metal is much more forgiving. - Jim

 

[ringfinder]

Not sure if I ever tried it in all metal, but now I use AM most of the time. But besides all that,
I don't like picking up the detector all the time, trying to pinpoint a coin. I like using a probe,
much easier.

HH,Ringfinder