A little tutorial and knowledge concerning concentric coils for your enlightenment and edification and why no area is ever hunted out...

[Uncle Willy]

Since most coins, rings, etc. will fit into an area of one-square inch I will explain the sheer logistics of coinshooting and why no area is ever completely hunted out unless evry square inch of soil is dug up to a given depth and sifted.

This applies to concentric coils only due to their winding configuration and how they transmit a signal into the ground and receive same. The signal enters the ground the exact same size as the diameter of the coil but morphs into an inverted cone the deeper it goes and draws nearly to a point at peak depth.

The old standard states that if you overlap your sweep 50% that you will pretty well cover it all but this just isn't so. As you can see in the rough sketch, at peak depth, you will be missing a piece of real estate roughly half the length of your coil, or in the case of a twelve inch coil - six inches. Now if you make a four foot sweep each time ( using the one inch square equation ) that equates to 288 possible targets you have missed with each sweep. Scary ain't it?

The only way to come close to full coverage takes time and patience but pays off. You must scan in a straight line ( not an arc ) and move the coil ahead only about two inches with each new sweep. It sounds gruesome but pays off in the long run especially if you're looking for old deep coins. At peak depth your signal is only covering a piece of real estate about the size of a quarter so fractions equate to missed coins.

Bil

 

[NM5K]

Good post. It's a good example of why I think I'll
eventually pass 500 coins out of my own yard. I'm
nearly to 400 now...About a 100 to go to hit that mark.
I've hit my back yard much harder than my front, and
it's starting to get kinda like that example. I've
hit most of the shallow targets, and now all I get is
the trash iron hits, and the few deep coins that are
there. I notice at this point, you really have to
sweep it careful, as the deep coins are the most
finicky hitters on the machines... IE: some might
be sporadic, or "one way hitters" etc.. Being the
front yard hasn't been hit as hard, I suspect most
of the next 100 will be from it. But I still prowl
the backyard for deep ones... Haven't had much time
to 'tect lately though, and I'm dragging behind my
pace.. I notice the front yard around the driveways
could use a sniper coil... But the large coil might
could get a few deepies from the clean areas..
I might oughta buy those before the price cranks up
more...They just had an increase for most Garrett stuff.
IE: all the places are now at 212.50 for the ace 250,
instead of the usual 199.99 before March. I think the
coil prices went up a little too... I'm just glad I
got my pinpointer when I did...It's 40 bucks more now,
with the detector, than I paid a couple of months ago.
I had heard there was gonna be an increase.. I guess
it's kicked in..
MK

 

[Uncle Willy]

Yeah everything is going up. What was originally situated on your yard. That's an inordinate amount of coins to pull out of your yard. Wish my yard was loaded with that many coins. I'd never have to leave home.

Course our whole neighborhood sits on top of an ancient riverbed laid down 15,000 years ago. If I could dig down 400 feet to bedrock I'd be rich. HA.

Bill

 

[Paul-Maine]

Great post.

Always learning from this forum,looking forward for the next one. HH Paul

 

[Spanky]

Nice pictures ...they would go great in your first book...

 

[NM5K]

They are all just coins that have been lost in the
45 years since the house was built. Before that, I
think it was just prairie land. Most of those are
pennies, so it's not like I'm making a killing.
But it has been a good training ground to get used
to the machine. I've plucked most of the shallow/midrange
coins from the backyard. So it's all down to the deeper
stuff now. But the front yard still has a bit of shallow
coins. I'm actually kind of curious what some of the iron
targets are, and may dig a few once I run the coins down.
If there were any real old stuff from when the area
was farm/ranch, would likely be iron and deep. Also..
Could be some construction trash too, like rebar, etc..
Using the pinpointer, I noticed one in the middle of
the back yard that was long but thin, like a pipe,
angle iron, etc..
MK

 

[Uncle Willy]

Have you found any rings or jewelry? First time out in my back yard my wife found a Black Hills gold ring. Really made her day. Which reminds me, I buried a quarter out in my front yard back in the seventies. I should go dig it up. Can't remember if it was silver or not.

Do you live on streets with parkways. They are a good place to hunt.

Bill

 

[NM5K]

No rings yet. The only thing along those lines
would be a "miraculous medal" I found in the back
at about 5 inches. It was in pretty decent shape,
but I think plated, not pure metal. Not sure about
the parkways, but there are a few parks I've been
scoping out. Most have kiddie sand lots. But ,
haven't had much time to check them out yet. When
I get time, it's usually dark, and there are no lights
out in those parks, except by the vollyball courts.
But I'll get to them sooner or later. Some are real
close.
MK

 

[NM5K]

BTW, I was messing with my antenna programs trying
to model a search coil. The modeling was kind of so-so,
due to the real low frequency used by the coils, but I
snapped a pretty good 3d picture of the usual loop plot.
I made a graph from it to show how the depth can effect
the area size being searched. I color coded the range
from 4 inches to 8 inches. The graph on the bottom shows
the appx width you would get at a certain depth. Pretty
slim at 8 inches... Course, this plot could vary a bit
from the real world, but should be fairly close. Hope
it's clear enough to read. I had to compress the quality
a bit in order to fit the 120k limit.
MK

 

[Jim Pa]

Would that mean that the big foot coils give you superior detecting because their signals go straight down?

 

[Uncle Willy]

If they are DD basically yes as they shoot a wedge-shaped signal into the ground the full length of the coil. They are only superior in high mineralization and the fact they have much better coverage per sweep.

Bill