I realize some on this thread have limited their maximum depth arguments to "coin-size" targets.
However I've also seen it stated or inferred that the "field" of the coil is set and cannot change. At the risk of staring another flame war, I just wish to point out a few other variables.
First of all, if the coil's diameter was the only thing that affected how deep the detector would pick up a target, I could walk up to my car and be sure that if I held my coil 12 inches away from it (having a 10" coil), I wouldn't pick it up. Now we've all run our machines close to large targets like this and have probably observed that we can pick them up at a depth of several feet (lets say, in all metal mode)! So, the field the coil is generating therefore must be substantially larger than a depth equivalent to the diameter of the coil.
Since that now opens the door for other variables (size of target being the most obvious) then it may be possible, under some circumstances, for the machine to pick up a desirable target at depths exceeding the coils diameter, since we've already proved it can pick up at least LARGE targets at a three feet plus. Using gain, and sensitivity, and adjusting to the area's soil conditions, we allow smaller conductive targets to now show up as a signal. So, yes, we haven't really increased the outreaching depth of the field, but since it may extend out to several feet, so what?! That's plenty for most digging I want to do.
Another factor is what some refer to as the "halo effect". Either you believe that coins buried in some soil matrix's leach out oxides that enchance their target EM field or you don't. I happen to believe SOMETHING is happening based on 31 years of detecting. I've dug too many deep coins whose signals disappear in the hole once they're partially dug (and this "halo" is broken) to think otherwise. The target is at the same depth, before actual recovery. If anything, removing mineralized topsoil from above a conductive target should ENHANCE the target's signal strength. Lastly, of course, there is the coil's actual diameter.
So, to me, the bottom line is, tweaking the gain and sensitivity controls definately allow the operator to increase the depth AT WHICH HE/SHE can HEAR the target being detected under some conditions. The size and conductivity of the target is also a factor, as is how long the target has been buried, and the mineralization content of the soil. The Minelab detectors give the operator the ability to adjust to the optimum settings for any particular site, and therefore INCREASE the depths at which they can hear the targets, within the rather large field the detector is actually generating.
Knipper
