i found this on a search maybe it will help
Why are some older coins very shallow when newer coins are deep at the same sites? How can a new coin sink deeper in a few years than an old one in over 80 years?
The Density of Soil
The density of inorganic soil is from 2.6 to 2.8 and any object of greater density, including coins, would eventually sink until the density of the soil equaled the density of the object.
The Sink Rate
The sink rate is determined by the difference in density, the greater the density the faster the sink rate. Contributing factors are vibration, rain, frozen soil, grass buildup, leaves and a few others.
How often the ground gets saturated can be a much bigger driver of coin depth than any minor differences in soil density. Until the ground directly beneath the coin becomes saturated to the point where the dirt becomes suspended in the water, and can move to the sides of the coin due to the coin weight, then little depth due to sinking can occur.
That's why many coins seem to end up in the 6-8 inch range - it takes a real soaker to move them deeper. So maybe the discrepancies in coin depth can be attributed to minor differences in the local drainage. The finer the soil particles, the easier they get suspended and the faster the sink rate.
Chart of Densities
Here is a chart of the densities of some of the common metals we find with metal detectors, also the differences in the density of different metals and a major difference between most of them and soil.
Looking at the chart below, the dime should sink a lot farther than the penny, because the gravity is twice as high on silver as copper is? I know I've found silver just under the grass and then dug 6" or 7" for a clad penny. Nothing worse than getting a deep signal, dig it and it's a clad penny!
The good stuff is sinking faster than the trash. No wonder not many gold coins are being found!
Density of Precious Metals
Platinum 21.45
Gold 19.3
Silver 10.5
Copper 9.0
Densities of Some Common Metals
Aluminum 2.7
Lead 11.4
Magnesium 1.8
Steel 7.8
Tin 7.3
Zinc 7.1
Iron 7.87
another book writer has this theory
There are moving targets in treasure hunting. I have never been a believer that coins, rings and other treasures in the ground have a sinking rate based on weight, shape or size. However, I am convinced that both the actions of man and natural phenomenas can cause items buried to either sink or come closer to the surface. A good example of the latter is older, deeper buried coins that are out of range of most metal detectors, are found after the thawing of a hard winter freeze. They are pushed-upwards by this natural action. Erosion of the soil will produce coins that once were out of reach. Naturally urban renewal projects will often lead to the removal of pavement and soil bringing many great targets to the surface. I found nearly 300 coins predating 1940 when a city block was lowered nearly four feet over a three month period. That lot produced my oldest coin find in the US, a Hebrew 1/2 shekel 69 A.D. pictured here.
Urban renewal projects are my second favorite land sites to find treasure. My first is another site where the action by man causes deeper coins to come closer to the surface. This is resodding of schools, ball parks, and playgrounds. I have dug-up several thousands coins from these processes and my biggest prizes are the silver dimes and smaller fractional coins like half dimes, three cent silver coins and many small foreign coins too. One school resodding produced 36 Mercury Dimes and over 200 coins in a four hour hunt. There are many more ways that natural events can cause movement of treasure. My favorite water hunts occur after a severe winter storm hits the beach areas pulling tons of sand away for short periods of time. Three friends of mine dug more than 2500 coins in a one day period. Numerous gold items, including more than 30 gold rings, also were the result of that Northeaster hitting a popular east coast Daytona Beach area. I dug three treasure coins after a storm hit my Tarpon Springs, FL area. The pictured authentic counterfiet Spanish coin, that I just sold on ebay for a good price, is one of those coins.
i have NO idea ...
