Geophysical surveys provide non-visual information to assist in core/chip logging
Lithologies that may be difficult to distinguish visually may be readily distinguishable by geophysical properties
Rock properties, including metallurgical parameters, can be measured continuously throughout the borehole, with high resolution and rapid results
Proxies can be developed for properties that are not measured directly, such as geochemistry
Can be used to constrain and refine geophysical 3D models
Identify off-hole anomalies
Above: Certain geophysical properties correlate with certain lithological units. Notice geophysical properties show some variation that is not captured in the lithology logs, potentially providing valuable insights to the geologist.
Above: Geophysical inversions for lithological interpretation can be useful to highlight variation that may not be captured visually by the geologist.
Above: Constraining surface geophysical surveys with downhole surveys increase depth resolution, improve 3D model accuracy, and (as shown here) potentially identify new drill targets.
Induced Polarization, Resistivity (Elog), and Inductive Conductivity
Characterize lithologies, alteration and ore zones based on resistivity, conductivity, and chargeability
Resistivity and Induced Polarization require water in hole
Conductivity can run in dry or wet hole, and behind non-conductive casing
Resistivity & conductivity anomalies are associated with a variety of metal deposits, and chargeability anomalies (from Induced Polarization) are commonly associated with disseminated suphides
Above: Lithology log with resistivities adjacent. Notice some variation in the Resistivity log is not captured in the Lithology log.
Cross-hole (Induced Polarization/Resistivity only)
Induced Polarization/Resistivity cross-sections can be generated between holes
Identify mineralization not intercepted by borehole
Cross-hole surveys provide superior depth resolution relative to surface surveys
Above: Cross-hole Induced Polarization survey, showing anomaly pattern.
Measures natural electrical fields in formation and drill mud/water interface
Correlates with clay content and salinity contrasts
Also used for stratigraphic characterization and correlation
Measures the amount of magnetic material in the formation
Map lithologies and identify ore zones by magnetic highs or lows
Track iron content for metallurgy
Above: Magnetic Susceptibility log, shown with iron assay results delivered months later.
3D Magnetic Field Mapping
Measures magnetic field strength and direction
Detect and vector-in on off-hole magnetic anomalies
Above: Schematic representation of magnetic field mapping, showing the variation in direction of the magnetic field downhole
Traditional measurement of total radioactivity in the near-hole environment
Commonly used for lithology and stratigraphic characterization
Used to detect K-alteration
Above: A Natural Gamma log, showing correlation with lithology. Note some variation in the Natural Gamma log is not reflected in the Litho log.
Subdivides radiometric signature into concentrations of K, U and Th
Used for mineral/lithology identification, especially clays
Delineate U ore from K alteration.
Above: A Spectral Gamma log, showing Total Gamma as well as K/U/Th concentrations. The upper gamma spike is from K, whereas the lower spikes are primarily from U.
Above: A common clay classification scheme based on Th/K ratio.
Measures P & S wave velocities
Combine with Density to calculate mechanical and elastic properties such as Young/s/Bulk/Shear Moduli, Poisson’s Ratio, Compressibility
Test efficacy of seismic methods & calibrate seismic model
Calculate primary vs secondary porosity
Above: Fullwave Sonic and Density logs, with calculated mechanical properties, showing contrast at a geological contact.
Measures electron density, from which bulk density is calculated
Survey to test efficacy of seismic methods
Combine with Neutron or Magnetic Resonance for dry weight density, used for ore tonnage and reserves calculations
Distinguish lithologies (e.g. coal seams) by density contrast
Above: Fullwave Sonic and Density logs, with calculated mechanical properties, showing a contrast with geology.