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Method Name: |
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Electrical and induction logging |
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Method Type: |
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Borehole Methods
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Assigned Problems: |
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Principle: |
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Measurement of the electrical resistivity of the borehole environment and surrounding rock. The resistivity is related to various geological parameters such as the mineral and fluid content, fractures and porosity.
Electrical log: Single-Point resistance logging; Normal-resistivity logging (16" and 64"); Focused-resistivity logging (Laterolog; Guard log, FEL); Micrologging; Diplog or Dipmeter log (DIP); Spontaneous potential logging (see for more information "Self potential"); IP logging
Electromagnetic log: Induction log; Dielectric log, or Electromagnetic-propagation log.
Refer to DC Sounding, DC Profiling, DC Tomography, and Geoelectrical Crosshole Tomography for more information. |
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Keywords: |
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Electrolog, Resistivity log, Induction log, Dielectric log; Electric field, Electromagnetic field, |
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Prerequisites: |
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- Limited to water-, or mud-filled (conductive borehole fluid) open holes. FEL can be measured in PVC-filter tube. Exception: Induction log: PVC-cased and/ or air-filled boreholes possible
- Proper calibration for quantitative interpretation
- Active measurements: resistivity of formation must be higher than resistivity of borehole fluid
- Stray currents may influence the measurements significantly
- Measurements are very sensitive to equipment interferences
- Rocks with high clay content
- Corrections required (e.g., for borehole diameter, fluid resistivity, temperature; Induction logging may require a Skin-depth correction)
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Resolution: |
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The resolution varies from several cm to few m: e.g., Microresistivity: 2 to 5 cm; Induction logging: 0.5 - 1.2 m (proportional to resistivity); Dip log: 1 - 1.5 cm thick bedding
The depth of investigation varies from several cm to few m: e.g. Microresistivity: 3 to 5 cm; Dual-Induction log: 0.5 to 3 m (proportional to resistivity) |
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Expected Results: |
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- Measured parameter: Resistivity in Ωm or conductivity in mS / m (Induction logging); Self potential log: Voltage in mV
- Data analysis: Resistivity function with depth (log); Pitfall: SP-logs may be difficult to interpret in aquifers
- Interpretation: Qualitative interpretation: classification, correlation; Quantitative interpretation: estimation of porosity and permeability in some cases
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Combination with other Methods: |
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- Required additional information: Often needed: temperature log for corrections
- Related add-on information: Mud or fluid resistivity; caliper log
- Independent additional information: Other loggings; petrophysical laboratory analyses
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Operating Expense: |
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- Crew size: 1 - 2 persons
- Acquisition speed: logging speed 2 - 8 m / min
- Processing: low
- Equipment rental costs: intermediate to high
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Parameters to specify: |
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- Instrument type
- Electrode configuration and separation
- Velocity of logging tool (2 - 8 m / min)
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QC Documents: |
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- Calibration data: last laboratory-calibration or on-site calibration
- At least 20 m of repeated measurements
- Borehole information (i.e., casing type, length, diameter, deviation, fluid properties)
- Complete headers
- Field notes (e.g., all activities, effective time schedule, present personnel)
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Products: |
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- Field logs (displayed using specified scales)
- Composite logs
- Crossplots
- Listings; histograms
- Interpretation
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