Geophysical logging—also known as borehole logging or well logging—is the process of measuring the physical properties of geological formations intersected by a drillhole using specialized downhole probe instruments, with measurements recorded as a continuous or depth-sampled log against depth. In bauxite, gold, iron ore, and diamond exploration and mining, geophysical logging provides continuous, objective, and high-resolution characterization of the subsurface that complements and extends the information obtainable from drill core or chip sampling alone.
A wide suite of geophysical logging tools is available, each measuring different physical properties of the rock formation or contained fluids. Density logging uses a gamma-gamma nuclear source to measure formation bulk density, critical for resource tonnage calculation and the calibration of ore tonnage models in gold, iron ore, and bauxite resource estimation. Natural gamma logging measures the natural radioactivity of formations (primarily from uranium, thorium, and potassium), providing excellent lithological discrimination between potassic alteration zones and unaltered host rocks, clay-rich versus clean sandstone or BIF units, and laterite profile horizons.
Magnetic susceptibility logging measures the in situ magnetic susceptibility of the formation directly in the drillhole, providing high-resolution discrimination of magnetite-bearing versus non-magnetite BIF horizons in iron ore exploration, pyrrhotite-bearing versus pyrite-bearing sulfide assemblages in gold exploration, and fresh versus weathered kimberlite in diamond mining.
Resistivity logging measures the electrical resistance of the formation, reflecting variations in porosity, fluid content, clay content, and metallic mineral concentration. In base metal and gold sulfide exploration, resistivity logs identify zones of conductive sulfide mineralization. In bauxite exploration, resistivity contrasts between conductive clay-rich and resistive ferricrete horizons assist in correlating auger hole and drillhole intersections across the laterite profile.
Acoustic televiewer (ATV) and optical televiewer (OTV) logs produce high-resolution oriented images of the borehole wall, enabling direct measurement of structural features (joints, faults, foliation, veins) encountered in the drillhole without requiring oriented core recovery. These data are particularly valuable in structurally complex gold and diamond mining environments where the orientation and frequency of discontinuities are critical inputs to geomechanical design.
In iron ore exploration, full-waveform sonic logging measures compressional and shear wave velocities, providing inputs to seismic modeling and geomechanical property estimation. Neutron porosity and spectral gamma logs further assist in lithological characterization and clay mineral identification.
Geophysical log data are managed within geological databases and integrated with geological core logs and geochemical assay data to produce comprehensive composite logs that support geological interpretation, resource modeling, and mine planning across all mining commodity types.