A [[log]] recorded in uncased boreholes which involves the use of electromagnetic induction principles for the measurement of [[formation]] [[conductivity]] or [[resistivity]]. The induction logging [[tool]] has advantages for use in nonconductive borehole fluids (air. oil gas) where other electrical resistivity logging tools cannot be easily used or should not be used. The induction log is widely used in electrically conductive drilling [[mud|muds]] where it works well provided the formations are not too resistive and borehole effects are known and not too great (i.e., mud not too saline and hole diameter not too large).

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Practical induction [[sonde|sondes]] include an array of several [[transmitter]] and [[receiver]] coils designed to provide focusing and deep investigation and to minimize borehole and adjacent-formation effects. A high-frequency alternating current, constant in magnitude, is passed through the transmitter coils. The resulting alternating magnetic field induces currents in the formation which flow in circular ground-loop paths coaxial with the sonde. Those ground-loop currents generate their own magnetic fields which induce in the receiver coils [[signal|signals]] which at low conductivities are essentially proportional to formation conductivity. At high conductivities, the reading may be affected by [[skin effect]]. Receiver-coil signals produced by direct coupling with the transmitter coil are balanced out by the measuring circuits.

Induction tools can be run separately or can be combined with other devices to run combination services. Integrated tools, combining in one tool the devices necessary to perform different resistivity-measuring operations, are commonly used in the well-logging industry. Examples of such tools are the induction device with a deep [[depth of investigation]] in combination with: another induction device having a shallower depth of investigation, [[invaded zone]] investigative devices (e.g., short [[normal device]], short [[laterolog]] or [[guard log]], or [[Spherically Focused Log|Spherically Focused Logging]] device), long [[lateral device\]], and [[SP]].