Cob evaluating road gravel slab

[Keith Breuker]

I'm looking for three or four temperature sensors that I could bury at
different locations in my compacted clay and gravel slab.  I expect this
will help me evaluate its performance, particularly if I am losing heat
to the earth.  

Does anyone have knowledge of such things that could make a
recomendation?  I don't want to spend much if it can be avoided.

Keith