Good question. My soil is moderately mineralized and I also hunt old farmsites for old coins. And, I rarely hunt modern parks. An occassional yard now and then. But I usually hunt 100+ year old sites that are now just farm fields. If I don't know where (in the 80 acre field) the house may have been located, I will start out with the 9-inch concentric at 3 kHz. Once I start getting multiple signals in each swath, (and risk target masking) I switch to the 6-inch DD coil at 18.75 kHz for better target separation. So if I had to chose one coil as my "first choice"..... considering you probably already have the stock 9-inch at 7.5 kHz....... yep, I'd opt for the 6-inch DD at 18.75. Now, if you get up enough money for a second accessory coil, don't overlook that 3 kHz concentric. It won't go any deeper than the other two 9-inch concentrics. But in all metal, multiple tone mode, it provides a distinct audio tone on that old, deep iron. JMHO HH Randy