I've done something similar, but could only be done at abandoned derelict spots, where no one would care (or notice) if you dug:
I take a flat shovel, pick the most likely area (to each side of the entrance stairs, for instance), and spend 10 minutes flat-shoveling out an area the size of a mattress or kitchen table, to about 4 or 5" deep.  Then stop, detect, and repeat as necessary.  Eg.: if I were still finding zinc and aluminum at that level, I might take out an additional 4", and  so forth.
But would only go to this level if it were something really old, or I was really ambitious.  Not gonna do that for a '20s home-site. 
heck, I know a few guys in a city I won't name, who've gone to an old-town park that is a BLANKET of wino caps, hypodermic needles, junk, zinc, etc...   They wait for a moist rainy winter day (middle of January, for example, since this is below the snow-zone).  Then at 6am on a Sunday am (at the first hint of day-light, yet when the streets are deserted) will go with flat shovels there.  The section out an area 10 ft. x 10 ft. square.  Make cut in large rectangular squares.   And litterally ROLL the turf aside (or put to the side in nice even square block-sections.  They do this down to 5 or 6", so that they're below all the modern coins and junk strata.   Then stop and detect 

   Once they've removed the old coins, they merely put back all the grass blocks/rolls, & stomp up and down on it.   You can't even tell they were there.  But that's for another topic/thread.  Doh!