Electrical and gamma-ray logging in gondwana and tertiary coal fields of india

Abstract

Electrical and gamma-ray logging have been very useful for identification and accurate determination of depth and thickness of coal seams in Gondwana and Tertiary coal fields of India. The characteristic resistance/resistivity peaks of coal seams in a particular area have been correlated, thus providing a picture of the subsurface structure. Physico-chemical properties of layers or sections of coal seams are responsive to electrical logs. Gammaray logs are found to be very useful for correlation and have sometimes been the only logs used in cased and dry boreholes for detection of coal seams. Under favourable conditions a single-point resistance log reveals a detailed picture of the formations and picks up thin coal seams as well as thin shale bands within the coal seam. But in some cases it fails to differentiate between coal and sandstone beds in spite of high contrast in true resistivities. Multi-electrode long-normal and lateral logs are found to be more useful in differentiating such formations because of higher penetration in this system. Long-normal and lateral curves can also be used to determine true resistivity of the formation. But long normal logs cannot pick up thin coal bands and/or thin shale partings within the coal seam because of the “adjacent bed effect”. Gamma-ray logging can be done in both cased and uncased boreholes or even in a dry borehole but its resolution for shaly coal or thin coal is not sufficient. Combined study has been found to yield the best results.