The DFX's Signagraph COULD possibly be a reliable way to tell if the target is a Au $1or $2 coin over aluminium bottle caps.
BUT(and this COULD be due to the settings on my DFX), I found that the target ID is inaccurate and will vary with:
1: It's depth
2: the soil conditions around it.
So using signagraph for ME, I have NOT found the DFX to be THAT accurate when trying to determine what you are about to dig is when it comes to the Au$1and $2 coins. This detector DOES find stuff. To me it lacks or maybe I am lacking in using it....I'm not entirely sure. It's close to but only to about 70% for the Aussie $1 and $2's .
It's good enough to say that a majority of bottle caps can be determined over the Au $1 and $2 dollar coins BUT by no means ALL Aluminum bottle caps or aluminum contorted or melted to a similar mass of a bottle cap.
The default on board PRESET programs for coins on the DFX, actually excludes the Au$2 coins .
Easy enough to learn it. The DFX is also programmable. Similar to how Explorer does it. I really like the DFX and just wish I could tame it to my liking.
I could be nearly there.
Other detectors that have the ability to maintain more accurate digital id on the Au $1 and $2 coins, seems to be achieved with the MXT (used Raoul) and from all accounts the Xterra(used by pinpointa-Ray).
Personally, I have found the Explorer does this id'ing of the Au$1&$2 Au very well on the EX XS, EX 2 and EX SE. Must the the nature of the alloy used to make these "gold nuggets". This may also be because I am more familiar with the Explorers settings. Maybe try this( If you haven't already):
With a newly switched on to system defaults Explorer either with coil in air or a clean patch of ground( kept still) or better still done with the x-1 probe held perfectly still while NOISE CANCEL is occurring (make sure the x-1 is not near any metal objects):
If you LEARN the Explorer to accept(tick) only AU $1 and $2 in a BLACKENED screen discrimination pattern using the SMALLEST CURSER by firstly doing a:
NOISE CANCEL........
with the Explorer in:
1: IN SEMI-AUTO
2:
OPTIONS ADVANCED RESPONSE NORMAL
3:
OPTIONS ADVANCED [RECOVER FAST
4:
SENSITIVITY 1
5:
SOUNDS CONDUCT.
NOISE CANCEL.....AGAIN
Using the stock Explorer coil, re adjust the SEMI- AUTO to MANUAL(NOT TO HIGH) and INCREASE the SENSITIVITY UP to suit.
I found that with the SOUNDS CONDUCT that the Au $1 and $2 dollar coins have a higher pitch than MOST aluminium bottle caps though the curser seems to be in the same location as the Aluminum bottle caps.
Also try the AU $1 and $2 dollar coin together.(Not an unusual combo)
Try other Aussie coins near or put together with each other.
See what they read and sound like.
You may wish to include these others coins individually or in combo's in a search pattern.
Unfortunately, this pattern may mean including some areas where some bottle caps come in.
That's where I have found that TONE ID'ing of the Explorer can help to determine when and when not to dig.
Ah yes, it's all such good fun.
When searching for the most currency I always expect to encounter bottle caps BUT I also leave a lot and I'm pretty confident that is ALL I have left behind....

shrug:
I don't bother checking on lawns or grass BUT I do always check at the beach because I usually run a wide open all metal pattern(IRON MASK).
After doing this a while you can tell the difference MOST of the time and you can relay on what you hear and then see.
Let me know how you get on.
Wish me luck , I hope to be in possession of THE new 11" Explorer coil tomorrow.
While wright this , I have realized that maybe if I reduced the DFX's AC SENSITIVITY and PREAMP GAIN, I might be able to create a tighter target while trying to LEARN a target. I'll give it a try and see how it does.
I promise that if the DFX ever give me what I want it to I will report it honestly and accurately. NO BS.
Take care.
David Di