Rather than just silver coins, it's "period" coins you look for. If you're digging wheaties at 3" and there's plenty of them, then there should be silver in that strata too. Same goes for nickels. You can argue they have differing mass but that difference is neglible in terms of overall population and distribution schemes. I've worked a lot of pure scrapes in parks and yards where you will find this seems to fit very well.
Some nearby parks have dramatic differences where that hard pack under the topsoil really comes into play. If you find you are digging all coins whether new or old no deeper that say 5" because the ground gets much harder at that spot, then that's probably the extent of the depth of coins and if there's no silver lining it's because, as in some of these nearby parks, they have been hunted for decades and those shallow goodies are nearly gone. So even the existence of a park improvement, like a bike path where the dirt may be cut or scraped down 5-6" will not find anything more of the old stuff than hunting prior to that cut in those areas. Seen this first hand several times.
In the cases like near me, where the parks have been hunted for decades, those shallow areas have been the most often visited naturally because they produced the best return for hunters, trash recoveries not withstanding. In areas where soft dirt is deeper and the movement of dirt through casual flooding or natural forces has allowed for deeper targets, trash is likewise deeper and so these areas have better remaining populations and ratios of old coinage/trash.
One spot in one park where a 6-8" cut was made for some parking spaces revealed a virtual bed of coins to me once. Many eyeball finds but all dated from the park's opening around 1900 through about 1920. They were everywhere and every denomination had gone undetected because of the huge volume of trash masking them in just that top 6-8" of diirt. I'm betting a few of the larger coins probably got picked from this area over the years since they'd be easier to pick out among that trash. This despite that ground being hunted by literally hundreds of coils over the years. While people would find the occasional coin down deep in that area, the amount not found until that scrape was clearly an indicator of the true effect of trash masking. To this day, you can hunt the area immediately adjoining that parking area and you will almost certainly not find much deeper than 5-6" unless you tear every bit of junk out of the ground between you and that strata. Takes a little more digging and ground moving than most parks I'm aware of would mind you doing.
Sand again has many variables. On the east side of Lake Michigan here near Chicago, the sand can hold them coins and jewelry near the top just fine, it's a bit more coarse and depth is dependent on water, wind, traffic and time. I've also hunted on the west side of Michigan, across the lake and didn't last too long. The much finer wind-blown sand can hide anything almost immediately. Walking on much of it puts you shin to knee deep in places so it's much harder to get anywhere near that level where the density of the matrix slows the simple effect of gravity. Probably a much better place to hunt after one of those strong storms from the north where straight-line winds have gone due south raking the beaches with large waves.
