According to Duane Bozarth :

As a FYI: I don't think they do horizontal loops here. For each ton
of heating, they drill a 100' deep 3" diameter vertical hole. IIRC,
they were 10' apart minimum. As it was explained, at peak loading,
you might "lose" a small amount of available operating time due to
freezing the ground.

I gather that it's not too hard to dig there...in TN that would be
bedrock and pretty expensive drilling...

It'd be bedrock here too. We're on the Canadian Shield ... Granite,
quartzite, with relatively shallow sedimentary overburden in places.
Probably very similar to TN - old hard rocks.

but I'd agree the deeper the
better...as noted, at the time I put in the system in TN, the only local
installer was almsot exclusively working in a development on Tellico
Lake and using the "loop in lake" ploy...

That's not geothermal. That's a water-source heat pump. There's also
ground source heat pump that operates by circulating an anti-freeze or
alcohol solution through pipes buried in the ground - either in trenches,
or in deep holes. Or, ones that pump water out of the ground (and either
back into the ground, or simply dumping it on the ground) for their heat
source.

Geothermal is slightly different. Geothermal is where you put the
exchange coils of the compressor _itself_ underground. Rather like
unravelling the plumbing on the back of your fridge and burying it.
These things use "real refrigerant" (often a freon of some sort)
in their buried loops. Not a heat exchange liquid.

These are more efficient than water-source heat pumps, because there's
only one loop rather than two. One heat exchange interface, not two.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.