Focussed Electric - Probe

Focussed Electric Probe (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

Operating Conditions

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


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

Focussed Electric Probe

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