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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.


  • Good depth of penetration with excellent bed-boundary resolution
  • Down-hole calibration check using internal resistor
  • Digital down-hole measurement avoids errors due to cable effects in deeper boreholes
  • Constant-power down-hole current source give 4 decades of measurement without range switching


  • Focussed resistivity
  • Natural gamma



  • Determination of water quality
  • Indication of permeable zones and porosity
  • Minerals/Engineering
  • Strata correlation between boreholes
  • Indication of fractures and permeable zones
  • Bed-boundary and thickness measurements
  • Moisture determination in coal


Diameter: 38mm
Weight: 8kg
Temperature: 0-70⁰C (extended ranges available)
Max. pressure: 20MPa
Measurement range: 1 to 2,000 ohm-m

Operating Conditions

Borehole type: open-hole, water-filled
Centralisation: standoff recommended. The logging cable armour should be insulated for 10m above probe head.

Sales Information

I002078 Focussed electric probe includes natural gamma


Download Focused Electric (Guardlog) sheet


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Robertson Geologging Ltd., 2017

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