Welcome Chris.

I think that the geothermal pump would keep the pond full based on an average of 1/4 inch of evaporation loss per day and a half acre pond.

325828 Gallons/acre foot divided by 12 inches/ foot x .25 inch= 6788 gallons of evaporation loss per day

The pump would add 6gpm x 60 minutes x 24 hours= 8640 gallons/ day

I have never heard of the "Double" clay method. As long as you have two foot of compacted clay in the pond bottom that should be enough.