irrigation valves use stainless steel metal parts, ferro-magnetic locators won't work. Tresure locators which locate non-ferrous metals usually don't work do to the extremely small footprint of the parts and the depth/soils mass.
if you choose to a metal detector you fall into the "even a blind pig gets a peach now and then" catagory. it's more luck than not.
that's what my original post should have said.