Colloque

Geophysical Detection of Unsurveyed and Abandoned Coal Workings in Southern India

Maurice K. Seguin

N. Sen

Membre a labase

Maurice K. Seguin

Résumé du colloque

The objectives of the research are threefold: 1) outlining the boundaries of old workings, 2) estimating the volume of voids, 3) positioning of cavities in a three-dimensional space. This last problem is the most difficult to solve. In the last 20 years or so, old coal workings started to collapse. This led to social problems. Some visible damages include sinkholes in cultivated and inhabited areas, cracking and sinking of homes and buildings, tilting of high power transmission lines, underground fires difficult to control, water loss, decreased insect for farming and construction, downward ground use pattern, depletion of groundwater, inundation of working pits, loss of lives, etc. In the Ranjganj coalfield, 45 unsafe localities have been identified involving an estimated total volume of voids of 1.6 x 10^6 m^3. Test geophysical surveys consisted of microgravity, vertical gravity gradiometry, electrical resistivity, and underground in-seam reflection. Except for gradiometry, these surveys failed to detect the cavities. Gradiometry needs some improvement in terms of lower stability and topographic correction. In relatively dry, shallow, and non-conductive areas, georadar can be used. The most promising methods are electrical resistivity (cross hole and uphole), and seismic (surface, cross hole, and uphole) using a shear wave source.