For picking through rusty nail/trash hell sites what you want to ask Minelab to build you is a concentric, coplanar coil. That coil has a cone shaped detection field, it's like using a 3" coil with the depth of a 9" coil.
Not a theory, I built one of these coils for a Minelab Explorer. The first target I dug with it, textbook signal LOCKED Indian head penny. Dug a 6 inch diameter plug, at the bottom was the Indian head penny. It was what else was in that plug that was shocking, multiple pieces of trash. I have dug many a plug with 10-11 inch Minelab DD coils, never one like that filled with a bunch of trash.
So what's the catch? concentric coplanar coils are difficult to construct, not impossible but annoying. Second they gulp a larger cylinder of soil so soil mineralization is more of a problem vs DD coils which concentrate the transmit field more down the center of the coil. But for mild to moderate mineralized soil the concentric coplanar can be deadly in heavy iron/trash.