Tool
Lab

Natural gamma: the rock's own radiation

What it measures. Gamma rays emitted by potassium-40, uranium and thorium in the rock, counted by a scintillation crystal. A spectral tool sorts the counts by energy into the three elements; a total-count tool reports the sum in API units.

How it works. The crystal flashes when a gamma photon deposits energy; a photomultiplier turns each flash into a pulse whose height is the photon's energy. Counts are averaged over the sensor's length, so thin beds read as smoothed edges. No source is needed and the log works through casing with a correction.

What it tells you. Shale and clay content, potassium-rich alteration, uranium associated with organic-rich or reef horizons, and bed boundaries for depth-matching every other log. In the lab, watch the count rate jump from pyroxenite to shale, and drop when steel casing absorbs part of the flux.

Density: gamma rays scattered off electrons

What it measures. Bulk density in grams per cubic centimetre, from which porosity and, with sonic velocities, dynamic elastic properties are derived.

How it works. A shielded caesium-137 source on a pad pressed to the wall sends gamma rays into the rock. They Compton-scatter off electrons and lose energy with each collision; two detectors at different spacings count those that return. The denser the rock, the more electrons, the more scattering and the fewer counts. Comparing the two detectors compensates for mudcake and small standoffs, reported as ΔRHO.

Limits. The pad must touch the wall: washouts and casing defeat it. In the lab, add washout in the shale and watch ΔRHO grow while the compensated density drifts.

Resistivity: how easily current flows through the rock

What it measures. The electrical resistivity of the formation in ohm-metres, or its inverse, conductivity in millisiemens per metre.

How it works. A galvanic tool injects current from an electrode on the tool into the formation through the borehole fluid, and measures the voltage between electrodes further up. The current path's geometry sets the depth of investigation; spacing the electrodes wider looks deeper. Rock matrix is an insulator, so what conducts is the pore water and its salinity, plus clay and metallic minerals. Chromitite and massive sulphides read low; quartzite and dolerite read high.

Limits. Galvanic tools need conductive fluid and open hole. Steel casing short-circuits them and a dry hole gives no current path, which is why induction tools exist. In the lab, switch to air or steel and the reading disappears.

Magnetic susceptibility: how much the rock magnetises

What it measures. Volume magnetic susceptibility in SI units, a proxy for magnetite and other ferrimagnetic minerals.

How it works. A coil on the tool generates a low-frequency alternating magnetic field. Magnetic grains in the rock magnetise in response and change the coil's inductance; the change is calibrated to susceptibility. The sensitive volume is small, a few centimetres into the wall, so the reading is sharp at contacts.

What it tells you. Lithology in layered intrusions, dolerite dykes, alteration and magnetite content, and ground truth for airborne magnetic surveys. It is also the warning flag for magnetic survey tools: where susceptibility is high, the electronic multishot's compass will be deflected.

Caliper: the shape of the hole

What it measures. Borehole diameter, continuously, from spring-loaded arms pressed against the wall. Three arms give an average diameter; more arms or an acoustic caliper reveal ovality.

How it works. Each arm's pivot angle is read by a potentiometer and converted to radius. The arms are opened at the bottom of the hole and the tool is logged upward.

Why it is the first log we look at. Every contact and nuclear measurement is corrected for hole size; washouts show where the rock is weak; breakouts, elongation perpendicular to the maximum horizontal stress, reveal the stress direction; and in casing the caliper becomes a casing inspection. In the lab, raise the washout slider and watch the arms follow the wall.

Full-waveform sonic: the speed of sound in the rock

What it measures. Compressional (P) and shear (S) wave velocities, from which dynamic Young's modulus, shear modulus, bulk modulus and Poisson's ratio are calculated when combined with density.

How it works. A transmitter emits an acoustic pulse into the borehole fluid. It refracts along the wall as P and S waves, faster than in the fluid, and arrives at an array of receivers spaced up the tool. The time difference between receivers divided by their spacing is slowness, the inverse of velocity. The full recorded waveform lets the interpreter pick P, S and Stoneley arrivals separately.

Limits. The hole must contain fluid to couple the sound in. Shear waves are hard to pick in slow, soft rock. In steel casing the first arrival is often the casing itself. In the lab, the red band is the P wave and the blue the slower S wave; compare coal with pyroxenite.

Acoustic televiewer: an ultrasound image of the wall

What it measures. A 360-degree image of the borehole wall in two channels: the amplitude of the reflected pulse, which depends on acoustic impedance, and its travel time, which is the hole radius at that point.

How it works. A transducer fires an ultrasonic pulse at a rotating mirror, sweeping the beam around the hole several hundred times per metre. A magnetometer and accelerometers orient every sweep to magnetic north and the high side of the hole. Planar features crossing the hole appear as sinusoids on the unwrapped image; the sinusoid's amplitude gives the dip and its low point the dip direction.

Strengths and limits. Works in mud and opaque fluid, resolves millimetre fractures, maps breakout from the travel-time channel, and in casing inspects the casing. It needs fluid: in a dry hole use the optical televiewer. In the lab, open fractures show as dark sinusoids in the strip on the right; switch to steel casing to see orientation fall back to gyro.

A real log: an NQ investigation hole in the Western Cape

Choose "Real log" under Data source to replace the simulation with a televiewer log recorded in an NQ investigation borehole in the Western Cape in May 2024. Everything you see is derived from the raw instrument record: each of the 2,622 scans carries 180 travel-time and amplitude samples around the hole, the tool's three-axis magnetometer and accelerometer readings, the signal gain, the acquisition window and the transducer and electronics temperatures.

What the raw record shows. The accelerometers say the hole is inclined about 33° from vertical, and the travel-time channel confirms the tool lies against the low side of the hole, which is how the images are oriented to high side. The acoustic caliper, travel time converted to radius with the sound speed in water, reads 76 mm in gauge rock, exactly NQ bit size, and opens to 90 mm just below the casing shoe at 4.2 m. Above the 2 m fluid level there is no coupling and the image is blank. Between 10 and 11.5 m the amplitude and travel-time images both show open fractures as sinusoids; the automatic fits give apparent dips of 55° to 68°. The polar scan at the bottom of the track is one raw revolution at the tool depth: radius from travel time, brightness from amplitude.

What it takes to turn this into a deliverable. A gyro survey to rotate the picks from high side to true north and correct apparent dips for the hole's inclination, a geologist's classification of each feature, and the caliper, RQD and fracture-frequency tracks that appear on the composite plot. That is the Tech-Log™ product; this page shows only the physics behind it.

Optical televiewer: a true-colour photograph of the wall

What it measures. A continuous, oriented, true-colour image of the borehole wall at sub-millimetre resolution.

How it works. A ring of lights illuminates the wall and a camera views it through a conical mirror, capturing a full circle per scan line. Orientation comes from the same magnetometer and accelerometer package as the acoustic tool. Because it records colour and texture, it shows lithology, veins, healed structures and mineralisation that the acoustic image cannot.

Limits. It needs clear water or air; drilling mud or turbid water blinds it. Image quality drops if the tool is off-centre. In the lab, set the fluid to mud and the image disappears; set it to air and it is the only imager still working.

Borehole radar: seeing tens of metres beyond the hole

What it measures. Reflections of a radio-frequency pulse from contrasts in the rock's electrical properties away from the hole: contacts, reefs, dykes, faults, fractures and voids.

How it works. A transmitter antenna radiates a short electromagnetic pulse, typically around 250 megahertz, into the formation. Where the dielectric permittivity changes, part of the energy reflects back to a receiver antenna further up the tool. Plotting echo time against depth gives a radargram; the range is the echo time times the radar velocity in rock, roughly a tenth of the speed of light. The system is omnidirectional, so reflector orientation is inferred from geology or from televiewer images.

Limits. Conductive rock absorbs the pulse: range is tens of metres in resistive pyroxenite or quartzite and collapses in shale or brine. Steel casing screens it completely. Press "Off-hole view" in the lab to watch the pulse reach a reflector and return.

Borehole magnetic resonance: counting the water in the pores

What it measures. Total porosity split into free and bound fluid, from which permeability and hydraulic conductivity are estimated, independent of lithology.

How it works. Permanent magnets on the tool create a static field that aligns the spins of hydrogen nuclei in the pore water within a thin shell around the hole. A radio-frequency pulse tips them out of alignment; as they relax back they emit a signal the antenna records. The initial signal strength is proportional to how much water is present, so it is porosity directly. The decay time, T2, depends on pore size: water in large pores relaxes slowly and is free to flow, water in small pores or bound to clay relaxes fast.

Limits. The sensitive shell sits a fixed distance from the tool, so a large washout fills it with borehole fluid and the reading goes high. Steel casing blocks the radio pulse; PVC does not. In the lab, watch the spins tip on the pulse and relax faster in shale than in sandstone.

Impeller flowmeter: where the water enters the hole

What it measures. Vertical flow of water in the borehole, and from changes in flow between depths, where water enters or leaves the hole and how much each zone contributes.

How it works. A small impeller spins at a rate proportional to the fluid velocity past it. Logged under ambient conditions it shows only the zones flowing naturally at that moment. Holding the well under a steady pumping or injection pressure while logging, induced flow, makes every permeable zone contribute, so thin fracture inflows that would otherwise be missed appear as steps in the profile. A modified form of Darcy's law converts the flow contribution of each interval into hydraulic conductivity.

Why it matters. It can site packer tests, fill the gaps between them or replace them, giving a hydraulic conductivity value for every metre of hole. In the lab, toggle induced flow and watch the impeller and the flow curve respond to the open fractures.

Fluid temperature and conductivity: the water column tells its story

What it measures. The temperature and electrical conductivity of the fluid in the hole, logged continuously downward before any other tool disturbs the column.

How it works. A fast thermistor and a conductivity cell in the nose of the tool. Undisturbed, the temperature follows the geothermal gradient smoothly and conductivity is uniform. Water entering from a fracture arrives at a different temperature and salinity, producing a kink in the temperature log and a step in conductivity at the inflow depth.

What it tells you. Inflow and outflow zones, mixing between aquifers, brine versus fresh water, and the fluid resistivity that resistivity logs need for interpretation. In the lab, choose the lithium brine profile and watch conductivity jump by two orders of magnitude.

North seeking gyro: azimuth from the Earth's rotation

What it measures. The borehole's azimuth relative to true north and its inclination, at each survey station, from which the 3-D path of the hole is computed.

How it works. The Earth turns once a day, about fifteen degrees per hour. A gyroscope held still in the hole senses that rotation, and the component it measures depends on which way the sensitive axis points relative to the Earth's spin axis. From that, the tool finds the direction of true north with no reference to the magnetic field. Accelerometers give inclination from gravity. Because nothing magnetic is involved, it works inside steel casing, drill rods and magnetic ore bodies.

Trade-offs. Stations must be still while the gyro seeks, so surveys are slower than a magnetic multishot, and the tool is more delicate. In the lab, put the tool in steel casing or in the chromitite reef and note that the gyro's azimuth does not change while the multishot's does.

Electronic multishot: azimuth from the Earth's magnetic field

What it measures. Borehole azimuth relative to magnetic north, inclination, and tool-face angle at each station, converted to true north with the local magnetic declination.

How it works. Three orthogonal magnetometers measure the Earth's magnetic field vector and three accelerometers measure gravity. Together they fix the tool's orientation in space. The total field strength and magnetic dip are computed as quality checks: if they differ from the known local values, something magnetic is disturbing the reading.

Limits. Steel casing, drill rods, nearby steel and magnetic rock such as chromitite or dolerite deflect the field and corrupt the azimuth. Surveys must be run in open hole below the casing with the rods pulled clear. In the lab, switch to steel casing and watch the field vector swing and the quality flag reject the station.