I have very limited coal drilling experience so there may be other techniques I'm not familar with. Basicaly you can drill to the target using a air flushed rotary drill. In hard rock exploration you see them referred to as reverse circulation drill (they come in air and water flush versions). To preserve the integrity of the coal you do dry/air flush drilling.

When you hit the coal horizon (as evidenced by the return chips) you have the choice of switching over to coring or continue using reverse circulation drilling. Either method yields more or less the same assay results but with rotary you log and assay the chips that are flushed out of the drill and with core you log it. Some machines can do both, in other systems you move one drill off the hole and move a coring system on. I don't know what technique will be used by GXS.

As with all coring, the lubricant (whether water or air) flushes out the end of the bit around the core tube (vs from the core tube) via slots in the bit. The diameter of the drill bit is slightly larger than the diameter of the drill rod so removed material flows back between the wall of the hole and the outer wall of the drill rod.

In air flush system the pressures are very high and a collar is placed on the top of the casing to direct the air flow to capture the return materials (in a device similar to a dust collector or home central vacuum system) for assay.

Hope this helps.

... Rob