how does ground temperature vary with depth down to 2-3 metres?

Question

and what is the seasonal correlation between daily max and min ambient temperatures and ground temperature?

Answer 1

All depends where you are. For instance, way up north or on the antarctic where you have permafrost (frozen ground all year round) the temperature is going to be below zero up to 3 below ground, though, probably at 3 m you're looking at the maximum depth.. I'm not exactly sure about that, but if you can find mamoth you can dig it up and eat it as the meat is supposed to be fresh.
However, at this depth the average temp is going to be around 15 deg C higher than the ambient ground level temp.
In hot places, the reverse is true and temperatures around 3 m below ground level are going to be around 15deg Cooler.
The main factor working here is the specific heat of dirt.. Not easy to work out exactly but has a general average which works.
Seasonal changes do vary but if we go back to the permafrost situation we can see that the ground is frozen constantly to this depth, even though on the surface we may get a bit of thaw and things get damp and sticky.
The same is true for warmer places.. Ground temperatures as with sea temperatures at depth, do not change that much from season to season, and with the ocean we can talk about from decade to decade.