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heres my rough idea - I'm sure it could take some tweaking - see what you think:
The main idea is to let air pressure force you way to the end of the hole -
Imagine a bullet shaped piece of metal, say 1-1/2 to 1-3/4 in diameter (a little smaller than the smallest diameter of the hole).
there’s a ½” diam hole drilled right through the middle
There’s a series of 8 or 10 ¼” diameter holes drilled around the perimeter (say 1/4” in from the edge)
A real thin drilling mud is pumped out of the perimeter holes – this mud flows through the bullet head and out to the walls of the hole, and fills the space between the bullet and the hole wall, creating a smooth, airtight surface for the bullet to seal against.
The central hole is a relief hole for the air ahead of the bullet – the back end of the bullet is attached to a flexible tube that feeds out to the beginning of the hole.
Seal off the openings, and apply air pressure behind the bullet while pumping thin mud through the bullet to seal the walls as you go – the air pressure behind the bullet is greater than the pressure in front of the bullet (because of the central relief tube), and the bullet proceeds as fast as you can unreal the air relief line and the mud supply line.
Water would probably work better than air since it's not compressible - the key is the two tubes - one supplying the mud and the other relieveing the pressure in front of the bullet - one of the challenges would be feeding the tubes through the seal at the mouth of the hole while still maintaining the pressure seal.
I was also wondering if you couldn't use sime kind of closed-cell foam-rubbber for the bullett - to allow for minor expansions, contractions? Also, you would want the back end of the bullet to be cone shaped (to facilitate pulling it out of the hole), with an annular manifold to distribute the mud to the perimieter holes.
I actually like the bore-crawler idea as well, but we'd need something that's as compact as 2" but can tow about 5km of cabling (8x600m+cushion). Half of that is about as much as a reasonably strong person would want to carry over a short distance. If you're wondering why 8, that's because we'd need one wire for each data channel.