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Ok, maybe more of a geology question than physics, but maybe somebody has been involved in modeling these heat flows?

Essentially I'm asking if we know what sort of depth the heat source becomes primarily from below rather than from above. I've heard that the fully solar domain is down to about 5m, and that temperatures below that are fairly stable, but crucially the temperature at the bottom of a 100m borehole is supposed to be around the mean temperature form that latitude, which suggests to me that solar is at least a large input even to that depth.

I'm struggling to find any reference to this exact point in the literature.

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To clarify your question a bit, the average heat flow is towards the surface (if you average over a long enough period) so I guess you're asking to what depth does the heat flow, i.e. temperature, vary with the time of day or season of the year. In that case Google for something like "diurnal variation temperature depth" and "annual variation temperature depth". You'll get hits like this one that states the diurnal variation is limited to 1 metre and the annual fluctuation to 9 to 12 metres.

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The point came up when discussing whether a borehole ground source heat pump was getting energy from mostly solar or mostly geothermal sources. I always assumed geothermal since it descends below the region where solar input can be detected, but the fact that the temperature at the bottom of a 100m borehole is related to latitude makes me question this. –  Seret Dec 28 '12 at 14:15
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