First we experimented with two small human presence radars. We figured out that we can detect things through a sheet of paper but as we expected we couldn't detect things under ground.

Then we soldered a simple metal detector. It worked, but we couldn't get an analog value from it to use in the Rasberry Pi.

Lastly we tried a magnetometer. It is something like a digital compass. We could detect buried ferromagnetic coins like 2€, but it didn't detect coins such as 50c coins. For more reliable detection our little prototype searches for magnets. The final product could have more sensors at once including the georadar.