Bontje, The GPX4500, no doubt in my mind the greatest detector released by Minelab. Sit down and read the manual, read it again and again.
Here are a couple of handy PDF's.
Good luck, with patience you will be rewarded.
It has taken me years to get my head around my 4500, and when you find settings that work in a particular place, write them down, as, if like me you only go out every few months, you'll forget what the settings were.
Following is one of my experiences, followed by an article written by Pat Cullinen
Wet day at Clermont: 12" Nugget finder coil:
Custom, Enhance, Fixed
Special Sens Extra
Auto tuned to 103
Hi-Trash
Motion Med
RX 9
Audio quiet
Tone 40
Stab 8
Signal 8
Target Volume 6
Response Inv
Tracking Slow
Iron reject 7
Pat Cullinan
soil timing; use enhanced for mineralise groundt
RX gain amplifies signal, if you can hear it it is working
Auto tune removes EMI Hold in AIR and find noisy spot. Can repeat several times if still noisy. Ground balance: on clear ground and not on target, sometimes might need to do every three or 4 m metres.
Threshold carries the signal and has to be balanced i.e. quiet hum. use fixed tracking, if changing ground balance again. When switching on machine auto balance and GB again.
Motion: use slow or very slow and swing slow
Stabiliser: keep high from the start, lower the RX before The stabiliser, the lower the stabiliser number the more it squeezes the waves down together.
Patch hunting: faster swing turn down RX If too noisy.
Custom deep and general setting nothing to do with the ground, if using Deep swing slower.
Special settings just change the pulse width.
Use Specific Ground Balance in very hot ground. When moving to new ground - Hold coil 30-45 cm above ground, change switch from fixed to tracking, when low pitch tone starts, pump coil no ground contact, when noise subsides, change switch back to fixed.
Custom mode has Hi-Mineral mode
Keep threshold as low as possible but still audible
Use Quick Track button when pin pointing
If still noisey, select Special extra, Salt-Coarse. Or Sens Smooth for small, shallow targets in severe soil and can iron out hot rocks.
Auto tune first and then manual tune turning dial slowly, coil vertical, 10cm above ground. Before tuning, turn 360 degrees locating noisiest direction.