Get ground pipes (plastic/drainage) have suitable holes drilled with an earth auger (these can be hired) drill to depth required, install pipes made up almost like a bucket which can then be filled with water, insert a thermometer on a string.
Sounds so simple, but getting a right will perhaps prove arduous, only trial and error will prove this.
The idea behind this is that the auger boring can be done to different depths, which I think is what you want, the bucket/pipes can then be inserted and filled with water, enough and it will spill over and fill any void giving contact between ground and pipe/bucket. You should be able to get good readings from this method, just stuff the top of the hole with sponges or rags or something easy to remove pull up the thermometer and take readings.
I would love to know what you are attempting...bosshogg
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Albert Einstein