Virtually all of the rings, gold and silver that I detect and recover are just inches from the surface of the sand. I'm talking about the DRY SAND part of the beaches. I would say most are between 1inch to 5 inches. Although these targets may have been dropped from a few days to weeks and even months, I would categorize all of them as 'fresh drops'.
Most of the beaches where I live in Southern California, have been visited by literally millions of beach goers from all over the world and over many decades and there must be many thousands of gold and silver rings and articles of jewellery deeper down and beyond the reach of the best detectors on the market. I say that because metal detectors only started to become popular in the mid 60's and nothing like the interest which is being shown in this hobby these days! Also the metal detecting technology was not like it is today, although my original detector, a 'blue box' CoinMaster GEB! did a heck of a job detecting articles for me!!
The only way to get these deeper targets would be to take some heavy equipment/tractor/backhoe with a big bucket! and dig down a few feet. I believe the rains, the weight of the article, gravity, tide/wave action, 'renourishing' the beaches i.e. bringing truck loads of sand to replenish storm ravaged beaches are just some of the reasons why they sink or are buried so far down.
Now the WET SAND is a little different. It has been my personal experience that targets are generally deeper due to the action and turbulence of the surf constantly washing over the targets. A simple example would be like the action that you see in the flow of water in a prospectors riffle where the gold sinks to the bottom of the riffle or Long Tom.
There is some science attached to this question but I don't claim to know all of the 'mechanics' and theories of your original question but it's a really interesting topic of discussion and thanks for asking!! I hope others chime in and contribute their expertise on this subject.