Hi argyle, I couldn't get to the goldfields today but did get a few hours to practice in the vacant lot across the road. The ground in not mineralized at all, but is full of trash.
I started by turning the threshold up to about 3 o'clock and balancing as you said. Full volume and sensitivity, audio on normal, all metal mode. First thing I tested was a small vial of flour gold from Sovereign Hill (maybe 0.05g). At 60kHz I was getting a small threshold rise with the vial directly under the coil, but only down the centre from the nose to the heel. The signal was a little stronger if I ran the vial across the top of the coil. To be honest I couldn't really pick any difference between the 1" sweet spot in the middle and the rest of the centre of the coil.
Next I tried the same test at 20kHz. I could just discern a slight threshold rise underneath the coil, which again was a little stronger over the top of the coil.
Next test was an air rifle pellet, firstly at about 1". Different frequencies gave different signals as you mentioned in your last post. The I pushed the air rifle pellet into the ground at bout 4". 60kHz barely recognised it, 20kHz gave me a threshold rise and 6.4kHz gave me a "dropout, dong, dropout".
I moved on to a small piece of ground (about a square metre) that I'd been over before and has previously cleared some of the trash out when I first got the detector. I started out at 20kHz and pulled out a few screamers that were pieces of iron, roofing screws, 3" nails etc near the surface. Then I went over the same ground again and started pulling out small pieces of Al foil and some small pieces of nails. At this point I could really concentrate on any threshold changes and continued to dig out tiny pieces of wire or little pieces of iron smaller than a pea at depths up to about 4" or so.
For the heck of it I then tried 6.4kHz over the same ground again and could hear some very tiny changes in threshold. I was amazed at how small some of these targets were when I dug them out. There was even a couple where I would dig them out of the hole and then lose them in the dirt pile. But a quick flick into 60kHz would find them easy again.
I'm feeling a lot more confident with the Eureka after trying some of your tips. In the lower frequencies it now seems a lot easier to discern between the tracking noises and the targets. At 60kHz I still need more practice, as you said it's like being in another dimension but I'll get there.
I need to get an amplifier, as currently I'm using Apple earbuds - but since I'm pretty handy at electronics I'm going to build one of these:
http://tangentsoft.net/audio/cmoy-tutorial/ That should help a lot.
Thanks again for your help. I was almost at the stage where I was going to sell the Eureka and get a 2nd hand SD, but now I will persevere with it.
Au