Hi Boston.
When you are using a Minelab threshold based gold VLF, I think one of the important things to concentrate on is the signals that do not fall in the category of clear signals like bips, beeps, zips, zaps and so on.
When a small to medium gold signal is under compact hardened ground and at a few inches and beyond, or larger and deeper but still within solid signal range, the Minelab units give a unique 'Dong' to the signal on gold. As long as the tone dial is set at the right level to match the height of the threshold.
Surface or 'in the dust' small or large gold can produce anything from zips to squeals, to the full blasts of blankouts on some models.
Some models like the XT18000 Version 1's and Dual Ace's had terrific audio. Some like the Version 2's had extremly low audio like the 2200d's. But regardless of a units audio range, a signal enhancer with the correct amount of added or built in suppression on signal height, is paramount in bringing the threshold way up into you're hearing. It is the only way to hear both the ground and where it's taking you, and repeatable threshold change.
Threshold based signals are far separate from signals that standout, and these Minelab units carry a far different type of threshold than on almost all the rest of other gold VLF's.
As the units were developed for usage on hot ground, they were made with what I call a 'true' threshold, almost like another dimension of sound that carries its own range of signals.
But the dropout I mentioned is not just limited to that type of ground, as it behaves in quiet ground the same way. Dropout is a complete and quick silent blank.
Only heard on the deepest part of the DD coil, the centre:
8" coil - you have a length of about 2".
11" coil - 3"
14.5 Coiltek - 4"
Give or take, round a bout.
These sizes are very important to take note of, they are just over the correct forward movement to make on each swing if you want to make sure the ground is covered to the full depth the unit and coil combination can manage.
Any aftermarket coil larger than this gains no more depth, 15" is the limit before dissapation occurs, and only causes bad broadening of signals.
It's very important also, to match the coil size to the frequency you want to use, both for the size of gold you think is in that spot and the ground type itself, regarding both mineralization and depth of penetration.
No good using 20khz for instance on the 14.5" all terrain coil. The coil is of a size that wants to carry a frequency deep, and the 20khz will not penetrate far enough to give a nice clean solid dropout in threshold, although the match will produce some. The 8khz and 6.4 kHz will give a signal much much deeper than the 20khz will on that coil.
Those lower frequency's are much more sensitive with the correct set up on a detector than I think even Minelab thought they'd be.
The 20khz on an 11", 8" or 10"x5" coil, will still give a noticable threshold change. As the 32khz and 60khz will on the smaller coils, but at a far lessering degree.
High frequency and small coils, even run with amplification, take us into a shallow miniature world, and we simply cannot hear those changes as much, they are too minute to really pick up on.
The threshold must be set to a nice rock solid and unwavering even point, where the hot ground noises cannot cause a quick dropout in the threshold at any time. Way past the often quoted mosquito buzz level, but nowhere near full.
If you are using an early tone modded model, or a later one with a tone dial, it is important not to have the tone set too low, as it broadens the signal response too much. The same goes for a tone set too high, as it can place a good signal in the same sound range as a shallow non-repeatable pip or pop on ground that is producing those.
As the tone affects both threshold and signal alike, it is important for noticing changes.
Remembering too, that every few feet to few yards, can produce the same signals at different depths, depending on quiet ground, mineralised to heavily mineralised ground changes, surface to shallow ironstone masking and so on.
There are 4 types of threshold signals that are made on the deeper gold targets.
1. Threshold Rise:
These are the signals that are just under a full signal response, depending on coil size and frequency used, so they make a slight rise in threshold, repeatable and excact from all angles, unlike clay domes.
2. Threshold Suck:
If on the same ground as above, but the piece is a bit deeper, it will draw the threshold down instead, giving the impression of trying to suck it down.
But the piece isn't deep enough yet to cause a break in the threshold ...a dropout.
3. "Dropout 'Dong' Dropout"
When the piece is again deeper than the above, it will produce a quick dropout, then a very slight feint 'dong' that only gold gives, then the other side of the dropout.
It is a nice even signal of it's own. Unmissable.
4. Full Dropout:
The piece is deeper again than above, but so much deeper that it cannot produce even a very slight dong, not even an iron type scratch.
A full dropout of threshold is simply a nice compleat dropout of threshold, pure blank, for the entire length of the deepest part of the coil, the centre of it.
These are the true deep signals. They come in clear, especially on the bigger coils. They are so clear that you start liking to size the piece by simply moving the coil left to right back from it then forward again.
Once you get used to a coil and frequency combination, the judgement of size and depth of a target gets easier.
I hope I explained it okay that it made a bit of sense Boston.
