In your pictures it shows your pond drained down pro over 2/3rd of the way, my first mode of action would be to keep a log of your water level at that point, if the water keeps going down, or leaking out, even at that level it is a huge chance that your core trench and or the sand stone layer is compromised, which will be a much bigger problem to address, along with, you guessed it, much bigger expense.
If the water level, where it's at right now seems to be holding reasonably steady, that would be good news, from there on up should be a lot more economical to rectify.
I have sealed off several small bodies of water that were leaking in the top half of a dam, and even down to a rock layer, by digging a cross trench, much like have pictured, and recompacting it back in with a quality clay, mixed with a generous amount of bentonite.
But you may be able to seal it off with the product that Teehjaeh has without having to do the excavation, that would be wonderful as we all know, excavating costs these days are astronomical.
Good Luck!
