Can someone who knows about gold mining give me a heads up about...

Can someone who knows about gold mining give me a heads up about trenching and the apparent low grades found? Am I right to assume these trenches are not very deep - say 4-5 metres - consequently low grades would not be unusual? Presumably trenching helps identify better drill sites so are the odds on finding better grades and more of it hugely improved by drilling?

I'm glad we got the ann even if I don't really understand it.