Question 7 of 10: Three Downhole Geophysical Tools — Principles and Interpretive Use

Question 7: Three Downhole Geophysical Tools — Principles and Interpretive Use (16.66% of paper)

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1. Natural gamma-ray log. A passive scintillation or Geiger-Mueller detector lowered down the hole records total (or spectral K-U-Th) natural gamma radiation continuously with depth, with no source required. Geological use: gamma activity is diagnostic of lithology — shale and clay-rich units carry naturally elevated K (from clay minerals) and are readily distinguished from cleaner sand, sandstone or carbonate on the log, allowing bed boundaries and formation tops to be picked precisely and correlated hole-to-hole even through cased sections (gamma penetrates casing, unlike many other logging tools). Helps interpret other geophysical data: the downhole gamma log directly calibrates a surface or airborne gamma-ray spectrometric survey — it ties the surface K/U/Th anomaly pattern to the actual lithology and depth interval producing it, resolving ambiguity about whether a surface radiometric high reflects near-surface alteration or is simply reading through to a shallow radioactive unit.

2. Resistivity/induction log. A short-normal or induction-coil array is lowered down the hole and measures formation electrical resistivity as a function of depth (induction tools work in air- or oil-filled as well as fluid-filled holes, since they do not require direct galvanic contact with the borehole wall). Geological use: resistivity strongly depends on porosity and pore-fluid salinity (Archie's law), so the log discriminates porous, fluid-bearing zones (aquifers, permeable sand) from tight, low-porosity rock, and can flag zones of clay alteration or sulphide mineralization as sharp low-resistivity spikes. Helps interpret other geophysical data: the downhole resistivity-versus-depth profile is the direct ground-truth needed to convert a surface DC-resistivity sounding's apparent-resistivity curve into a correctly layered true-resistivity depth model, resolving the equivalence ambiguity inherent to surface soundings alone.

3. Sonic (acoustic) velocity log. A transmitter/receiver pair (or array) lowered down the hole emits an acoustic pulse and measures the travel time of the compressional (and often shear) wave along a fixed source–receiver spacing, giving interval velocity as a function of depth. Geological use: velocity correlates with rock competency, porosity and degree of fracturing, so the log identifies weak, weathered or highly fractured zones (low velocity) versus competent, intact rock (high velocity) — directly useful for rock-mass quality assessment in a geotechnical hole. Helps interpret other geophysical data: the sonic log (calibrated against a check-shot velocity survey in the same hole) is the key input to a synthetic seismogram, converting the depth-domain lithology at the hole into the time domain so that surface seismic reflection events can be tied confidently to specific geological horizons — without this tie, a surface seismic section can be read structurally but its reflectors cannot be assigned a definite geological identity.