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.
Features
- 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
Measurements
- Focussed resistivity
- Natural gamma
Applications
- 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.
Specifications
- Diameter: 38mm
- Weight: 8kg
- Temperature: 0-70⁰C (extended ranges available)
- Max. pressure: 20MPa
- Measurement range:1 to 2,000 ohm-m
