Presumably the coal seam is not saturated with water.  If it were, then a core will fill with water simply due to the pressure gradient.  For an unsaturated coal seam, using a double action pressure swing to first locally infuse the seam with water, then extract it, will obtain enough for later analysis.  A device will need to be invented for this purpose to lower into the core, however.  The alternative is to extract a core sample and then analyse in the laboratory.

Regards,

Jim R. Jones