If you are searching for quart jars full of gold coins in saturated wet ground, unless ground is very hard clay or rock, etc., like any high specific gravity objects, gold nuggets for example, they would have continued to settle deeper until reaching something that would stop the downward settling, like a big flat rock, bedrock or hard clay layer. If it is soggy wet there much of the time, I would hate to think how deep they might have sunk. You may be out of luck with a detector because most will not detect a quart size object over about 3 feet deep or so, depending on mineralization, and gold is difficult to detect compared to copper or silver. The lid may or may not be rusted, depending if it was an old zinc type lid, but probably would not help the signal you would receive by much. A cache hunting detector with a really large round coil may detect a little deeper, but 2-box units lose depth of detection on smaller objects at greater depths. It may detect a quart jar at 4 feet, but so would a really large coil if conditions are perfect. Wet ground is also best for deeper detection for some reason, maybe better conductivity. A long probe and a lot of effort might work if there are no rocks and the soil is reasonably soft. Now that it is wet would be a great time for probing. A backhoe still might be the only solution. Good luck.
