Unfortunately, we are in an odd-hobby that "catches the eye" when someone sees the guy swinging the wierd geiger-counter-thingy. And once that happens, people ask themselves "just what is he doing?" "will he harm anything?" blah blah. And no matter that you will leave no trace, "image is everything". So even if you have so much as a screwdriver just to probe for shallow clad, you will probably still loose the issue, as powers-that-be just don't want to baby-sit and over-see debates on probe, vs pry, vs dig, vs re-pack, etc... I mean, ask yourself: why should they say "ok"?

And this can be true no matter HOW many "yes's" you get from city hall: Those "yes's" will just as quickly be turned to "no's", the instant you mention digging, holes, etc... I have heard of many people who proudly show their "permission", only to have it just as quickly revoked, when some busy-body gardener or cop-in-the-field gets on the phone to city hall and says "
well I don't like it" blah blah blah.
My solution to this is: 1) like yourself: if there is nothing specifically disallowing detecting, I just go. I do not consider "defacement" type rules to apply to me/us, because we will [ultimately] leave no trace of our presence. 2) If someone has an issue with that, they are welcome to tell me. In which case, I avoid just that one person, or just that one park, in the future [till I know he's not around]. 3) I go at off-times to avoid such busy-bodies, to begin with. Ie.: at dusk on deserted nights, early AM's, holidays when police are on skeleton staff, and city non-essential staff is all off. And even night-hunting (so peaceful! no kids or busy-bodies

)
I know the "avoid busy-bodies" and "just go in the absence of specific prohibitions" will rile some people. But I can assure you: if you, or any of us, thinks they can do this hobby in any public park, school, or beach, while not riling
anyone, at
any level, they are mistaken. I gaurantee you that if you look long enough and hard enough, you can and will find someone to tell you "no". If not for the digging/defacement bolgona, then for something about cultural heritage. If not for that, then something about "prohibitions of collecting". Therefore: just like how you can never avoid a nut who might flip you off in traffic, so too is there going to be an occasional busy-body in the field whom you can not please
You can certainly fight it, but what may end up happening, is that you will merely get an actual rule created in your town, where none currently exists (ie.: "to address your pressing issue"). And then, instead of it just being one arbitrary cop, and one or two isolated incidents to avoid, you will get un-wanted attention to even the most innocuous of places, where perhaps no one ever cared before.
