煤炭采空区
coal mining worked-out area
煤矿多层采空区
multi-layer worked-out areas of coal seam
煤矿采空区
gob
coal mine gob
coal mined-out area
goaf areas of coal mines
the depletion region of coal mine
depletion region of coal mine
goaf of goal mine
Goaf of Coal Mine
coal mine goaf
gob area of coal mine
GOAF
coalmine gob area
coal gob
mined out region
gob areas of coal mine
old coal mining area
Coal-mining area
exhausted area of mining
mine goafs
磷矿采空区
phosphate mined-out areas
空区稳定性
Stability of mined-out area
空区探测
cavity detection
Exhausted area prospecting