Martadi, S. Pambudi, and O. Verdiansyah, “Optimizing of Drill Hole Spacing For Coal Resources Classification in Kelay Blok 3 Area, Labanan Formation, Berau Regency, East Kalimantan”, IJBAS, vol. 14, no. 8, pp. 667–675, Jan. 2026, doi: 10.14419/nbt0tq29.