Wireline logging boreholes can provide varied information useful in the assessment of coal deposits; their thickness, true vertical depth and inclusive layers within the seam.
Locating the Coal
Coal’s distinctive chemical and physical properties compared with other sedimentary rocks allow it to be identified quite easily using geophysical tools. On the density log, coal appears as a low-density feature with a low natural-gamma count. Televiewer logs show distinctive dark bands indicating high absorbance for both optical and acoustic signals and also reveal fractures. A sonic log, used together with the density log, indicates rock strength, a useful pointer for future mine planning. Coal has a high electrical resistivity, depending on moisture content and the electric log can indicate local variations in calorific value.
Thin coal layers or thin clay/shale partings within coal beds can be detected using log measurements with appropriate resolution. The optional Bed-Resolution Density (BRD) detector on RG’s sidewall density probe and the focussed electric log (Guardlog) are examples of tools specifically designed to differentiate strata on this small scale.
Coal Identification and Correlation