The ELOG or electric log is the classic water-well combination probe combining shallow, medium and deep penetrating resistivity measurements with self-potential (SP) and natural gamma (optional).
Principles of Measurement
Resistivity
The down-hole ELOG probe is equipped with electrodes, measurement electronics and an insulated bridle. A low-frequency bi-directional electric current from a source electrode on the probe returns through the formation to the cable armour above the bridle. Potentials due to this current flow are measured on various sense electrodes on the probe with respect to a voltage-reference ‘fish’ normally located at the surface. The spacing between the source and individual sense electrode determines the depth of investigation of the measurement. These measurements are converted to apparent formation resistivities within the probe and digitally transmitted to the surface.
SP
The SP is a voltage measurement between a non-polarising down-hole electrode and a reference ‘fish’ at the surface.
Features
- 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 manual range switching
- Automatic real-time correction for borehole diameter and well-fluid resistance
- Optional natural-gamma measurement and temperature
Measurements
- 16" Normal resistivity
- 64" Normal resistivity
- Single-point resistance
- Self potential (SP)
- Natural-gamma
- Temperature (optional)
- 8" and 32" Normal resistivity (optional)
Applications
Water
Determination of water quality Indication of permeable zones and porosity
Engineering/Minerals
Bed-boundary positions Strata correlation between boreholes
Operating Conditions
Borehole type
Open-hole, water-filled
The bridle top must be immersed in well fluid, restricting top logging depth to 10m below the fluid level. A short bridle may be used to allow resistivity logging to within 3m of the fluid surface. However, 64" normal results in this case will be qualitative. Best results are obtained in fresh mud with low to medium formation resistivities.
