Gravity Surveys for Lignite in Parts of Soutb Arcot and Tiruchchirappalli Districts, Tamil Nadu

Lignite occurs within the Cuddalore sandstones of the Tertiary Group. The Bouguer gravity map has demarcated the basin with the axis extending from north to south passing close to Andirnatam, with steep gradients towards east and south. The residual gravity map has revealed three broad negative zones, comprising several gravity closures with magnitudes varying from -0.2 to as much as -1.7 milligals. Only a fraction of the anomaly could have been caused by the lignite body. It is likely that these zones indicate possible local depressions forming troughs or sub-basins where lignite deposits are likely to be confined. The boundaries of the lignite-bearing zones are fairly well marked. Drilling in two zones has proved the presence of lignite seams of 3 to 14 metres thickness at depths varying from 67 to 135 metres. The surveys have proved that the gravity method could advantageously be employed for delineating zones favourable for lignite deposits in similar environs.