An electrical conductivity/resistivity well log based on electromagnetic induction. See Figure I-4. A high-frequency alternating current of constant intensity induces current flow in a formation (Foucault current).
This current (also called ground loop) produces an alternating magnetic field that causes a current in a receiving coil; the current is nearly proportional to the conductivity of the formation. Induction sondes may have several transmitting and receiving coils to produce highly focused logs. While most effective with fresh mud, an induction log can be recorded in air-filled boreholes or moderately conductive muds. A dual induction log measures different depths of penetration; IL indicates deep and IL medium penetration. Generally superseded by focused array induction tools; see array induction log.