There is an EXCELLENT article written by somebody on masking, and how a staple even stops the machine from seeing the coin underneath it. Based on his testing he estimates that the VAST majority of silver coins are still left at a "worked out" site and have never been recovered. He even says that many of these iron masking targets, small like a staple, won't even give you an iron response if they are say 3 or 4" deep, but they are still masking the coin below them. Your machine just hums along and doesn't even null because a tiny bit of iron is deep enough at 3 or 4" to not even respond with a null, but you just happily hunt along not even knowing that you just passed up a silver coin that never gave you the slightest peep.
Now, if the staple was laying right on top of the coin then your detection field with proper coil control can see the coin and not the iron if you sniff around at the edges. That's why people find coins with nails laying right on top of them in the hole. If you hit the nail first you won't see the coin because the nail is higher, but if you nip at the edges you'll see the coin and not the nail.
Google "Beneath The Mask, Staple" and those key words will pop the article up, and go ahead and read this excellent article. Also there is a part 2 but you might have to do a bit more searching for that. It will blow your mind reading the stuff contained in this article, and will show you that the vast majority of old coins are still out there waiting to be found. It's not just about the staple test, either. It has more information than that that is very enlightening.
Thus far, there doesn't appear to be any form of technology to change this, so you still need to dig the trash to find the coins if they are deeper than the trash and for the most part under them, although they can be fairly far off to the side of the shallower trash and still be severly masked. But why can the coin be further off to the side of shallower trash and you can't see it, but you can see a coin with a nail laying right on top of it? Because the coil's field is more broad the shallower it is, so it will first hit the shallower trash. But if the nail is right on top of the coin you've got enough "edge" to the detection field to see one and not the other.
That's where a sharp detection field is a must, and so is wiggling between targets to try to "sniff" out a deeper coin at the edge of iron and other junk. Even with those kinds of efforts, some coins simply aren't going to be seen by today's limits in technology unless you dig up the junk first. The detection fields have to follow certain laws of physics that can't be changed, and that means they can't go around corners or hit something shallower and still see a deeper coin. The key is the detection field. You want a sharp tool and not a sledge hammer when trying to "pluck" out those coins hiding among the trash. That means either a coil that further compresses the width of the detection field such as an SEF, or a small round coil to help with the legnth of the detection field.
I know these small odd sized coils that are longer than they are wide are popular to try to have great separation and still get good depth. The way I feel that's a trade off when going to a small coil. I want the most separation possible with depth being totally secondary. That's why I prefer a 5 or 6" round coil for heavy trash, and not one that has a increased tip to toe length to try to increase the depth. On the other hand, in a large coil I prefer a coil shaped like this like the 12x10. It gives you the depth but still compresses the width for excellent left/right separation. Not knocking the small odd shaped coils because I know they get excellent separation and some incredible depths for people.
I know, rambled on WAY too much....So I apologize, but hope you've found something useful out of this.
