You will need: a mountain bike, a go-pro, a pair of crutches, a box of assorted bike accessory parts, a few long bolts and a roll of gaffer tape for on-trail repairs.
Remove the arm brace from the crutch and put them aside, you will be using the main crutch pole for this project.
Select handlebar mounts to fit the crutch and your seat pole, joining the two together. Find a bold long enough to go through the frame of your bike and the crutch, and to go between two crutches. You can now bolt the crutch to the frame utilising existing holes in both. Now just bolt the two crutch poles together matching holes on each of the poles to set up your rink at the desired height. Attach your go-pro at the end of the top crutch, it may now be adjusted to record the ground directly behind your rear wheel.
Operating your rig
Set your go pro up; you may choose to record the rear wheel as it moves across the earth or you may choose to just record the earth itself. Should you wish to record your own breath as you pedal across the earth you will need to attach an auxiliary microphone, bear in mind that this will require a cable and an adapter. you will get tangled in the cable when you dismount and that the auxiliary microphone will compromise the water resistance of your camera. While recording you will have to ride very, very slowly if you hope to get clear footage. Because the camera is so close to the ground you will tend to get blurred footage even at 80 FPS. Stop and repair your rig with gaffer tape as required. For variation in footage, rather than repairing the rig allow it to drag behind you as you ride across the earth.
Later you can clean your bike, remove the rig and reassembled the crutches
As mentioned above, you have to go very slowly…. mostly I did not go slowly enough.
I also mentioned variation in footage due to system failure: in this of footage my seat-pole attachment failed causing the rig to drag behind me like a sled. it is the one section that does exactly what I was looking for, it traces a line through the skin of the soil.