The depth at which the target ID starts jumping around will vary depending on the site conditions and is normal. While the ID may jump around at least you get something on the deeper targets.
Mineralization, iron, and trash aside moist soil is generally better than bone dry or sopping wet soil. Sopping wet soil can be worse than bone dry soil if rusty nails are plentiful.
Here's a deep target tip...lets say a target at a given site 5 inches deep gives you a rock solid ID, 100% coin response. At 7 inches deep the ID begins jumping around, 80% coin response and 20% other. At 9 inches deep things are getting iffy, 50% coin response and 50% other. At 11 inches deep it sounds like crap, 20% coin response and 80% other or worse.
The question of the day is, what is this "other"? Where does "other" ID and more importantly do you have "other" discriminated out? Absent any rusty nails or trash other = ground mineralization. Mix 8 parts ground mineralization with 2 parts deep coin and what do you get? A bouncing cursor that hangs mostly over in the iron area of the screen with an occasional bounce towards the coin area. Sometimes you don't even get that, the worst of the worst stay in the iron area around iron mask -14 range. How many people have dug a crappy signal that sounded mostly like iron, recover a coin then sweep the hole with the probe and big coil and find there was no iron nearby? I have many times.
This behavior seems to hold true across a wide range of soil types, only the depth at which targets start to get iffy varies. In some parts of the US things get iffy at only 6 inches, in other parts of the country they can get a solid ID at 12. Most of the sites I hunt things get iffy starting around 8 inches for a small cent sized target.
So if you are hunting deep targets in the iffy detecting range running little or no iron discrimination can help. Nulls are sticky and stretchy, the machine doesn't like to let go of a null. If the weak coin signal is being pulled over to the iron side of the screen and the machine is nulling how will you hear the coin?. Nearby iron only makes matters worse. Conductive tones is of course counterproductive to this type of deep hunting. Iron sounds high, so do the coins and the cursor is bouncing all over the place. That doesn't give you much information to go on. With ferrous tones what you hear is the 20% high coin tones peeking and fluting through the low 80% iron tones. I mean lacking a target ID all you really have to go on is the tones, the pitch and shape of the signal as you sweep it.
I'll let some more air out of the digital screen approach, worn silver dimes can ID down in the indian head range and worse. Half dimes and 3 cent pieces also ID lower on the screen. Indian head cents can ID all over the dang place depending on how corroded they are. Even the mighty barber half dollar can get yanked well off its mark by a rusty crown cap directly overhead. The worse silver coin I dug was a worn smooth half reale which the cursor locked on steady to around iron mask -14 and stayed there. Yet I was hearing mostly a high silver tone in ferrous (another vote for ferrous over conductive)which freaked me out a little as I thought at the time that should not happen. That only goes to show you that tones rule and do not always agree with where the cursor is landing.
Lastly, I know it can be frustrating not finding deep coins when you think they are down there. But if coins are down there deep there will also be trash items down there, lead toys, junk rings, brass shell casings, and the like. I highly recommend that you go out and forget all about what the target is and just concentrate on how deep it is. Hunt using your depth meter. If you get a deep target at the depth you think an old coin should be, if its not a nail dig it up. A couple hunts like this can really expand your understanding of the machine.
Charles
