Impact of Geological–Geophysical Data Integration on Coal Tonnage and Stripping Ratio Estimation in an Open Pit Coal Mine

Open pit coal plans rely on geological models built from widely spaced drill holes, so errors in seam continuity pass directly into tonnage and stripping ratio forecasts. This study quantifies how integrating infill drilling and electrical resistivity tomography (ERT) with existing drill hole data changes the seam model, and the coal tonnage, overburden volume and stripping ratio derived from it, within an unchanged pit boundary. At a reopened pit in Ogan Komering Ulu, South Sumatra, the initial model predicted 34,518.86 t of coal for the first 2.84 ha excavated, whereas truck counts recorded 2,615.6 t. Two infill holes and four Wenner–Schlumberger ERT lines (600 m in total) were interpreted with the existing drill holes, Seams X and Y were remodelled, and volumes were recalculated within the same 22.41 ha boundary. The decisive new information was that the southern infill hole did not intersect Seam Y and that the borehole-calibrated coal response (above 5,000 Ωm) was continuous in the west but faded towards the south and southeast. Seam roofs shifted by −11 to +6 m, and the seams were terminated where both data sets indicated their absence. Estimated coal therefore fell by 34.50% (490,346.96 t, from 1,421,125.56 t to 930,778.60 t), overburden rose by 10.11%, and the stripping ratio increased from 2.48 to 4.17 BCM/t. These changes quantify the effect of the revised seam geometry; support for the revised model comes separately, from its agreement with the infill drilling and with what excavation has exposed. Because the ERT inversions have high RMS errors and only two infill holes were available, the revised estimates are provisional, and the early-mining shortfall cannot be attributed to the initial model alone. For open pit planning, the case shows that a low-cost geophysical check tied to drill holes can change the economics of a short-term pit design before most of it is excavated.