Speculum Observation and Trajectory Measurement in Gas Extraction Drilling: A Case Study of Changling Coal Mine

Table 1

Coal seam characteristics.

Coal seam	Coal seam thickness			Layers of gangue	Thickness of gangue (m)	Apparent density (t/m^3)	Recoverable range and reserve ratio
	Minimum	Maximum	Average
C1	0.8	1.05	0.89	-	-	1.70	Sporadic recoverable with 2% reserves
C5^a	0.8	1.42	1.09	-	-	1.60	The recoverable area is 23%, and the reserves account for 5.3%
C5^b	0.86	5.51	2.75				The whole area is recoverable, accounting for 69.7% of the total reserves
C6^a	0.8	2.48	1.28			1.65	Recoverable area 50%, reserves 13%
C6^c	0.8	1.56	0.97	-	-	1.65	Recoverable area 52%, reserves 10%