Welcome to Pond Boss, Catdaddy!
Have you been observing this pond since you were a kid? (That will help with some of the answers.)
Why do you think your pond has a leak that needs to sealed?
How does the pond respond after significant rains when it gets full? Basically, if it rapidly drops X feet in a week and then slowly drops thereafter, you probably have a leak in the side of your dam or pond slopes. The water rapidly runs through a sizeable leak, and then once the level of the pond drops below the elevation of the leak the water loss slows to just your evaporation losses.
If your pond drains rapidly at first, and then drains to a very low level at a DECREASING RATE of loss, then you may have a bottom leak. The higher rate of water losses when the pond is full is due to the higher hydrostatic pressure more effectively pushing water through the leak. As the water level in the pond goes down, the hydrostatic pressure also goes down, so your water losses through the leak exhibit a decrease in rate.
Even when a pond has a "bottom leak" it frequently will NOT drain completely. Sometimes there is a "threshold pressure" for water to move through the leak. If that pressure is not exceeding with only 3 feet of water in the bottom, then the pond will quit leaking at that point.
I hope that gives you a little more "background" on what might be happening at your pond. If you can digest that and then post some more info about your pond, the forum does have some "leak fighting" experts that can give some more specific advice.
Finally, the view in the picture looks like a pretty shallow pond, that may have LOTS of organic material entering every year. Any chance the pond has just become filled with muck, and that has cost you the depth of your pond?
