Temperature sensing...

Short wave length thermal camera.

Any hand-held thermometer over 1ft in length would do the job. Contact 'Stinky Pete' offline. He might have one in his skip that you could have.

Guest,

are you looking for a sensor that can detect temperature variations by a factor of 20 degree as input and given to controls or data loggers or just measuring instrument to read temperature 1 foot beneath , there are loads of instruments and sensors that are quiet cheap , even normal mercury thermometer can do the job

This is a bit blurry: you want to detect a diffrence of 20°C no problem, even your hands can feel the difference.

But this happens below 1' of soil.

A thermal imager can show you difference but only in surface temperature. it will take some time before the effect is seen on the surface.

If you need to trace a pipeline/object which should be 20° warmer than the surrounding soil: no problem. go for the thermal imager, or wait till it snows.