Ah, ha! My specialty. I have the 9" elliptical coil for me Deus. Since I got it late last summer I almost exclusively use it. They will pull tiny items out very deep, and big non ferrous items even deeper than what you want to dig! In a foundation and around it, I would use the 28.8 frequency or the 74 frequency. They punch deeper. I got some small buck shot from a colonial field today at 5 to 7". One "bad" signal was half an aluminum can at 18". I was getting tired of digging that hole. In your situation a copper at 6" will blow your ears off with the HF coil. It excels in deep items. For a better idea, check out my thread on the colonial relics from the other day. That was run at 74 hkz with the Deus Fast mode. And go slow. No need to be in a rush. Better to do a small area slowly than to cover a lot of ground fast.