I finally got everything installed- such as a bunch of temperature sensors, and a raspberry pi mini computer that monitors the temperature and controls when the fan comes on.
And so i have pretty graphs!
Heat sink vs the same depth in the soil just outside of the insulation- it shows the temperature difference increasing as the spring sun takes effect
The little blue line at the bottom of the graph is when the fan comes on, it shows that this does indeed heat the mass up!
The air putput is on average 3C cooler than the input, with the highest difference being 6C-its transferring plenty of heat to the mass
I'm continually amazed that something I built works!