Funny you should say that - WD2XDW in Okie  did a ground heating experiement with the LF loop on 137 back in 2005 or so.
 
In winter we shoved on the Decca at full tilt for 24 hours at around 1.3Kw or so output - and using an infracamera and temp measuring source looked for any residual increase in "coming to surface" heating due to losses under the loop. Safe to say  there was lot of heat sinking I didnt see any increase in the ground temp which isnt suprising given area/depth.
 
Even our encumbent Ground hog "Boss" who was holded up under the loop stayed with us...
 
The hottest part of the array was the  3/8 inch threaded Brass in Ceramic feed thrus coming from the paralled loops (which Ive now bypassed in the current XPQ design)
 
Laurence KL 1 X