[Aurificus]

Thanks for the reply J_P,
I concentrated on a changing thermal gradient in the theory because it is pretty easy for anyone to "accept". Things heat up & cool down every day...
It suits the idea of an IR detection device, natural fit.
Plus I think Esteban said works best between 10am & 2pm. Greatest transfer of solar
shortwave radiation into the ground.

BUT.....Heat is only one form of Energy and interchangable with any other energy
so detection of other changing energy is quite possible.

Re: distance I think Esteban says up to 10m with IR , so detection area would be smaller. Perhaps LED in pistol barrell would constrain it further.

Cheers,
Aurificus

Aurificus