Hi Muse, it largely depends on how much trash is in those pasture fields. If there isn't too much, I'd suggest using Relic mode because it is indeed the deepest. But if there's a lot of junk, Coin mode might be better, especially if you are hoping to primarily find coins anyway. Jewelry mode is a good compromise: deeper than Coin but faster recovery than Relic. It will also accept more low-conductor relics than Coin does.
As for sensitivity, I usually run it on my 540 at around 7 out of 10 in areas with EMI. I could see having to set it lower if the EMI is really bad.
Hope this helps!
-Ken