Hi Oxyygon
I think that you have missed one thing here to get the depth you need the coil to be large and a large coil can not detect a small object. It's a vicious circle if you want to detect lets say a ring you will require a coil no bigger than 280mm but if that ring is at 3m you will never detect it with your small coil, but worst of all if you now use a 1000mm coil you will have the depth but you still wont find the ring because of to small a signal (eddy currents) but if there is an ammo box down there you will find it with your large coil. I do not know what your soil is like but I can tell you that I find objects from the Boer war (1902) at 100mm so don't despair your coins could be a lot closer to the surface that you think.
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Xavier