DD coils have the entire center strip active, and provide a wider sweep. They are not as tight on pinpointing. The frequency of the pulse is what affects depth. Lower frequencies tend to go deeper but miss smaller targets. Higher frequencies are shallower but are better for smaller stuff, thus the multi frequency and usually higher end machines. Concentric coils project their signal as a conical shape that comes to a point as depth increases, thus a very small search area as the depth increases.