Focused Electric (Guardlog)
The focussed resistivity (LL3) measurement provides excellent vertical resolution and a reasonable depth of investigation. The guard log replaces the classic Elog in conditions of low mud resistivity and high formation resistivity.
Principle of Measurement:
The probe includes a central current-source electrode between two guard electrodes, maintained at the same potential by internal electronics. Current from the centre electrode is constrained to a thin disk by the presence of the guards and returns to the cable armour above a 10m insulated section. The potential of the central electrode with respect to a surface voltage-reference stake and the measured current are combined by a down-hole microprocessor to calculate apparent formation resistivity.