Dictionary:Induction log

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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).

FIG. I-4. Induction log. (a) Schematic two-coil induction logging sonde. (b) Dual-induction log. ILd is a deep induction log, ILm a medium induction log. The separation of the resistivity curves shows a variation of resistivity with distance from the borehole, probably because of invasion. Laterolog-8 (LL8) and SP logs were run at the same time. (Courtesy Schlumberger.)

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 Generally superseded by focused array induction tools; see array induction log.


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