According to 'Gary' on his website:
'Initally, don't worry too much about the history of the particular field
your friendly local farmer lets you go onto - just see what comes up. This
is a valuable part of the learning curve. Once you've tried a few different
sites you can get more fussy and do some research. But initially, just get
out there and detect.'
Sounds great fun! It sounds like carte blanche for anything!