I'm afraid you may confuse some newer users of the Explorer by using the term "ground balancing" for what is really "noise cancelling" . One of the nice aspects of the Explorer is that it CAN NOT be ground balanced- it constantly takes care of comparing metal object signals to the general ground conditions all by itself. While it is not "automatic ground balancing" and certainly not "preset ground balancing" as used in other detectors, it does the same job- but better. There is no control button on the Explorer for any sort of ground balancing. Steve explained this in his comment above on June 17; maybe I can make it even clearer. The button you are pushing while resting the coil on the ground scrolls through 11 different combinations of signals chosen from the 28(? I think this is the number from my feeble memory) different signals transmitted by the Explorer. I think I am correct when I say the Explorer does NOT use all 28 different signals at any given time; it uses a smaller selected group of them that still covers most of the spectrum. If you are hunting far from any sort of power lines or other electromagnetic sources, whichever group out of the 11 is chosen should make little or no difference in depth, ID, or how the detector works- each group ranges across the whole spectrum. If you pass over a patch of highly mineralized ground you will doubtless loose depth and may have ID problems, but it will affect each of the 11 frequency groups equally. Changing from one to another should not improve the performance of the detector, unless by chance one of the 11 signal groups just happens to have one of its signal frequencies that works particularly well on a very specific item that you have hit. However, the Explorer makes its identification decisions on how an object (and the surrounding soil matrix) react to a variety of different wavelelngth signals. Back to the "noise cancelling" button- most of us are not hunting in the wilderness, but are bombarded with all sorts of radio, TV, and other signals. How to check for outside "air" interference? Put sensitivity as high as it goes. If your detector is quiet and stable, you are in one of the rare spots that is interference free. This is OUTSIDE interference, in the air and possibly conducted thru the soil as well- rather like static on your radio or phone. Most likely, your Explorer will beep erratically and flash an erratic series of numbers on the screen even if you hold it way off the ground. Some of those stray electromagnetic signals are entering your coil at exactly the same "wavelength" as the signals being sent out by the detector and being returned when an underground coin is under the coil and is being detected, so the detector is fooled into thinking it has received a "good" signal- and it beeps and creates a digital number or cursor position. Now you can lower SENSITIVTY hoping these stray signals are weak, for that is what lowering sensitivity does- it cuts off the weak signals and ignores them. Obviously then it may cut off weak signals from a deep coin as well, so lowering sensitivty does cut depth. The other way to try and get rid of these unwanted signals from the air is to try out different combinations of signals (Minelab has 11) and find which one has by chance a grouping of signals that are NOT on the wavelengths being used by the static. Minelab is unique that it can do this- other detectors operate on one wavelength, and if static is bad on that wavelength all you can do is use another detector that is on a different wavelength. This whole process is rather like some modern detectors that have a switch that can shift their frequency just a fraction, so that you can detect (as in a competitive hunt) close to another guy with the same detector brand as you. He is making "static" with his machine, so you pick a frequency that is different from his and you no longer have a big problem. So Minelab lets you push the "noise reduction" button and you wait patiently while it switches from one "group" to another and listens for interference at each stop- and when it is over, it automatically picks the group that received the least interference/static. Then you start hunting. The mineralization in the ground has NOTHING to do with this. In fact, many Explorer users feel that you should keep the coil HIGH OFF the ground when doing noise reduction; you certainly couldn't do any ground balancing with the coil far away from the ground. I guess you could say the "noise cancel" is sort of an "air balance" as opposed to a "ground balance".
It is possible as you move away from power lines, or top a hill that you may suddenly receive a new or different batch of static/electromagnetic signals so it may then be a good idea to occasionally redo the noise canceling. Many Explorer users also feel that you should redo the noise cancelling if you change any internal settings (like sensitivity, fast, deep, ferrous/conductive etc). I suppose it is possible that changing these settings may affect the sensitivity/reaction of the detector more to some wavelengths of outside interference that others, so that a different group of the 11 may now give smoother operation. It can't hurt to do it again, plus it gives you an excuse to rest your arm for 30 seconds.
I hope this lengthy story makes things a bit clearer.
