Thermal imaging and GPR (ground penetrating radar) are two different things.
Obviously, but what I said is about thermal imaging, not ground penetrating radar.
“Only the surface temperature is actually detected, though thermal imaging may be used to detect burials to a certain extent. For example, a decomposing body buried at about 1m is a few degrees warmer than surrounding soil, but greater depths may make detection more difficult.”
from http://forensicsciencecentral.co.uk/archaeology.shtml