Sentence:  Prior to this investigation, it was thought that there might be a correlation between the coal gassiness and the average methane concentration above the coal surface. Higher methane concentrations might be observed for <WSD>gassier</WSD> coals. However, figure 5 shows no obvious correlation, probably because the <WSD>gassier</WSD> coals were stored in open-top silos where better air circulation reduces the methane concentration. 
Meaning of the word marked with <WSD> tags:  Having the nature of, or containing, gas.