Man, I've dreamed about scraping off say about the first 6" of soil at some of my sites that I know have the potential for some super deep coins out of the range of even the best of machines like mine. I'm confident I can get perhaps 9 to 12" in wet conditons at some of my sites. I can get 9 easily at some sites in dry conditions and maybe 10 here and there, but many of these sites range from milder to heavier mineralization so it can be a stuggle for even the best of machines to break the 9" barrier sometimes. Sites like that I've managed 11" on a coin with good perfect/strong IDs and audio but in ideal wet conditions in order to overcome the mineralization more. But if 6" was scraped off then I'd be reaching down even further with better ability on stuff that were in that 9 to 12" range prior to the scraping and be going beyond that to who knows how deep.
One time we hunted an empty lot in a small town. I was the last to arrive there about a year after a circle of friends beat the place hard. The soil was very mineralized with black sand and iron. I didn't pull but I think a few wheats out of there, and at the time I didn't have the machine I have now that is known for penetrating the worst of soils. Either way, they had already got all the "easy" shallower stuff in that rough soil and made some really good finds using better machines then I had at the time that were known for handling harsh ground conditions like this well, and that was before I got there a year later.
Anyway, one day I drove by and saw they had scraped about 12" of soil off the site and I was excited. I called my friend who originaly brought me to that site (only common courtesy to do that of course, and I don't like to hunt alone anyway) and we met up later that day to hunt the spot. We got a bunch of good coins out of there that were out of reach before due to the bad soil.
Bummer was much of the area was scraped down to hard limestone clay, and thus the coins couldn't sink deeper over much of it and were probably taken away with the 12" of topsoil. I even found a few indians laying right on top of the clay by the naked eye. We saw the dirt all in a huge pile and tried to find out where that soil was going but we never could. I bet there were tons of good silvers in that huge pile of dirt. Luckily at least parts of the site still had deeper exposed soil and it all wasn't just a hard clay layer like that.
Just goes to show you that there are plenty of sites where even the best of machines will struggle to find stuff that others in other states would consider well within range in their soil, so never consider a site hunted out until you've dug all the super deep "junk" hits, as those could be coins just out or range or masked badly by the mineralization or something.