yes some places have too much trash to make it worth detecting. If I start a place like that, I will abandon it, and go to "greener grounds". Unless the spot holds some sort of super old promise of rewards. For example: In my town, in our "blighted" vice district, there are some vacant lots which have been homeless camps for decades. Like buildings had burned down or been torn out of there in the 1950s/60s, and they are now nothing more than blighted vacant lots. Simply impossible to detect them, since they are a sea of wino screw caps, foil, trash, hypodermic needles, blah blah blah. However, the lots are in a colorul and old part of our town, so it stands to reason that there MUST be some coins there. It was the "china town" district back in the 1880s to 1920s. So what I would do is take a shovel, and go along the sidewalk frontage of these lots, digging out a uniform scrape of perhaps 4 to 6" deep. I'd scale down a sample area about as big as a billiard table around. Then stop to detect just those dug down spots. This would reveal old coins. But a lot of work!
I know a park in my state, which is in a very bad side of the town. But in the 1800s, it was in the elite high-rent side of town. And today, there's so much junk, that the surface of this park is a blanket of foil, tabs, wino-caps, etc.. Even hypodermic needles (it was a drug-haven in the 1970s/80s). So some guys I knew employed this "scrape" method, by actually taking flat shovels, and cutting out sections of turf, to "roll back" (much like you could imagine turf rolls that you'd buy from a landscape store). And by doing so, they were rewarded with seateds, etc... Needless to say, they had to .... uh .... use careful timing in their choice of times to do this. Like from 5am to 8am on a Sunday AM, etc.... haha