Its not the depth, its the speed of cycles and the speed of delay. Cycles is pulses of electricity, and delay is the switching from TX to RX, transmit and receive. In fact a slower pulse will actually go deeper, given the same power, but mainly find large iron objects, but wont find tiny things, or gold jewelry, same as a J W Fishers PI does, they have a slow pulse and wont find gold jewelry. That is because a slow pulse will hit a deep large object, and then find it, eliminating or not picking up tiny objects that may be around it. They are no good on gold jewelry. The faster those both are, the better it is on gold jewelry-not very conductive. Gold jewelry is the first thing that is discriminated out, because it is not very conductive, especially lower carat gold. Pure gold is very conductive, about up there with pure silver, but alloys like gold jewelry, good stainless steel, are not very conductive. On a stock old type White's surfmaster PI, the rate of delay was too slow to find all but large gold jewelry at very close range, if at all. The lower conductive metals loose the pulse too fast and it is very weak. But with the newer PI Pro, the delay is faster/shorter to catch the transmitted pulse with the receiving cycle before the pulse is gone. Some newer PI units also have 2 coils - TX and RX , instead of one to do both, to allow a faster delay, and even more sensitivity. White's Dual Field, TDI, Minelab's GP and GPX series and several other UK made PIs have this.
say you are looking for the heavies along a beach - I like Surfdiggers W pattern in likely areas - you may find an iron bolt that gives you a clue that other heavies like chunky gold are nearby, if you were discriminating you may have missed the opportunity - 
