Several geophysical tests have been done with the objective of selecting the most suitable geophysical method to detect the presence of the voids produced by an old underground coal mine, where the system known as "rooms and pillars" were used. The final aim is to ensure the security conditions for people and machinery during the excavation of a new open pit mine, made to recover the coal left by the old one; many of the galleries may be nowadays collapsed by falling of the roof and swelling of the clay, but some of them can constitute cavities. The coal seams have the geometric disposition shown in fig. 1; in the actual mining system, starting from a situation like the one indicated by the line AA of fig. 1, geophysical surveys are carried out on platforms like a-a, to try and know the presence of voids inside the shadowed zone, before going on digging to reach the new situation indicated by line BB. Due to the metroIogicaI requirements of most of the different geophysical methods, measurements have to be made in absence of mining activity; another conditioning for the geophysical works is the rough topography of the surrounding area and of the mine itself, which is continuously modified, allowing a very short time to make the diagnosis for the presence of cavities. So, the main characteristics of this surveys are the detection of a group of connected voids, overlaying at several depths, with the limitations which represent the mine labours.


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