Earth tube calcs for heating/cooling?

earthtubes, the geothermal air temp exchange system.  has anyone seen calcs for how many feet of underground tubing is required to change the temp inside a small cabin?  obviously it depends on the insulation in the cabin but i was wondering how many 4'' tubes would theoretically be required for keeping the inside of a 600sqft heavily insulated cabin at a reasonable 75deg temp year round (or close to it)  Winter temps here are in teens sometimes, summertime can be up to 100.

I would be interested in installing a couple tubes maybe 10ft apart as once they heat or cool the surrounding soil in the heat exchange then they dont work anymore until the earth re-establishes the ground temp around the tube.

Most of the installations that I have seen have been for new builds where the earth tube are installed under the foundation.

If your cabin is already build you could attach a greenhouse and leave the door open so the cabin benefits from the earth tubes.

About a 100 ft of earth tubes installed 5 or 6 ft below the frost line.

I hope by bumping this topic other might chime in.

nothing built yet, just planning and wanted some idea on sizing.

100 linear ft is what i had read somewhere but 100ft per what, per 500sq ft of living area/greenhouse?  

at some point you exhaust the ability for heat transfer between the tube and the air once the soil around the tube hits close to the same temp as the incoming air.  I imagine this depends on the temp differential and the length of tube - I just dont know what the actual calcs would be.

These tubes would not be under the house, just in a run up to the house.