One coal seam is near the surface close to our house. The data recorded from our water borehole gives it as 43 feet down and 3 feet thick with shale layers above.
This is on a hillside and close to the house. There have been sink holes further down the hill between 4 and 10 feet deep where the old workings have collapsed, the seam being closer to the surface.
There is a recent collapse in the next field uphill of where the track to our house runs, at a level that seems to indicate that the seam is only a couple of feet below the surface and the other day I noticed bubbles appearing in a puddle on the track so presumably we have methane escaping to the air already!
Our soil is about 6" to 15" thick with clay beneath, about 6" of pure clay and then with increasing amounts of stone and grit included followed by rocks.
I should probably report the gas bubbles to the Coal Authority.