I agree with D-holland: Unless there is something on the books saying "no detecting", then why should you think of yourself any different than someone going to fly frisbees in the park? (eg.: would you "ask permission" to fly frisbees?). You could go asking all sorts of desk-bound city folk if you want, but you might find a "no", simply because you asked, and simply because of someone's whims, when in fact, no one might ever have cared (unless you are being a total nuisance or eyesore or something). There's too many stories of cities and parks where no one ever had a problem. But lo & behold someone takes it upon themselves to walk into city hall and "ask permission". Their "pressing question" gets passed back and forth between multiple desks, and finally a "no" emerges. Then the old-timers are left scratching their heads saying to themselves "since when??" See how that psychology works? You can get a case of "no one cared, till you asked". So look it up for yourself, don't go asking someone's feelings or personal whims or moods or to find someone to morph something un-related (harming earthworms, etc...).
Now of course our hobby has connotations and draws lookie-lou busy-bodies, so a little "discretion" is in order. Ie.: just because there may not be any rules on the muni-codes there about detecting, doesn't mean that you can or should go at high noon waltzing over people's beach blankets. Go at off-times/low traffic times of day (dusk, or early morning, or whatever). If you see them tending the lawn or whatever at that particular park, then choose another park at that time, and so forth.