Heat v depth

I note FTR that geothermal heating (ie mostly radioactively-generated heat from the hot core of the earth) should be about 1 degree at 39m (of course this does not mean that geothermal is a significant contributor to the heat pump - it is very small - about 40W for the house and pump area compared with 15kW peak for the pump). The temperature change due to heat pump heat extraction over a year might be similar.

But the daily temperature fluctuations reach down a few cm, annual fluctuations 2-3m and on the same basis at 39m we are talking 400 years or so! (quadratic with depth). Though groundwater movements might reduce this a good deal I imagine.