immac

I'm no expert, but my understanding is that 100 feet is all that is required for the bore-hole type. The alternative is a ground loop which is run in a shallow trench, not deep into the ground. Another option, is to run it through water - perhaps a lake. It is not the heat of the ground, but the difference in the temperature between the house and the ground that is important for the heat exchanger to work.
In summer, the coolant (water or anti-freeze solution) dumps the heat into the ground, in winter it collects the heat from the ground.
Ian