I'm not sure exactly what it would be at 20m, but I suspect not too much different from what it is at 2m, in most places. Once you start to go down a significant distance, it warms up again, but I don't think that would be the case at 20m, unless in a geothermal/volcanic region.
The easiest way to find out would be to find a water bore of suitable depth nearby and lower a temperature logger/thermometer down on a cable to measure it, if you need to know precisely.
Why do you need to go so deep? It isn't really geothermal if you are only going 10 or 20m down anyway for use with a heat pump, it's ground coupled. Geothermal heat is generally found at much deeper depths.
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