i have found that once you pick up on the tones the machine produces, you can easily discern between good targets and garbage. Depths vary between surface and about 10 inches. the conductivity number on the machine can be deceiving i.e. 37 or 38 for a copper penny while zincs will be 33 or 34. however; the tones don't lie and if you can pick up on it there is a difference in sound.