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10ft below ground is enough to take advantage of geothermal heat. You don't have to go "very far" to reach warmer soil in winter because the soil PLUS the snow on top is pretty much just insulating the deeper ground from the cold air.

Start getting into permafrost though where the cold is more constant and that cold layer gets deeper.

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10ft is definitely not enough for practical use. In order to heat a rural house with a heatpump connected to geothermal you need in order of 200-300ft deep hole, at least here in Finland.

For ground source heat pumps you have two approaches. Either you have deep hole. Or you have a large field. In later case not as much depth is needed, but you do need much larger area.

Good point and very true!

At a certain depth the temperature curve is 6 months delayed from the surface. So it's getting to its warmest during the winter, and coldest during the summer. At a deeper depth it's pretty constant year round.

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