The gravle in the square pane is 3" deep x8x8" wide and i put basalt stones on the sides to give it a real world situation , for the most part , the one basalt stone is very mineralized, and the outlaw in discriminate could not get thrue it . but what is nice about the out law is you can then use one of two all metal modes, and with the ground balance, I can get the taget in all metal SO NICE= gladd i got this machine , The DD coil can get thru the magnetic gravle and black sand in disc, so its not probly as badd as the magnetic basalt , but the concentric coils do not get thru it in eather mode, disc or all metal the concentrics dont like this , the DD sees 70% less ground [video]http://www.youtube.com/watch?v=gaCHvV2eAjw&list=UUjtTA1ZSEXhfWtQ12CPzXnA&index=1[/video]