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Method Name: |
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Nuclear 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 natural Gamma radiation (primarily from K-40, U-238, and Th-232) or measurement of the interactions of artificial radiation with the surrounding rock.
Measurements of natural radiation:
- Gamma log (integral or spectral)
Measurements with artificial sources:
- Gamma-Gamma log (bulk density, lithology)
- Neutron-Neutron log (porosity)
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Keywords: |
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Gamma log; natural-gamma log; neutron-activation log; radioactive probes; density log; litho-Density log; porosity log |
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Prerequisites: |
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- Measurements in open and cased, wet or dry boreholes possible
- Probes containing radioactive sources need to be the last to be run in an uncased well. They never are run if other probes encounter problems
- Laboratory- and on-site-calibration of the instruments is crucial
- Consider regulations for storage, transportation and use of radioactive materials (SUVA)
- Absorption and borehole corrections (e.g., diameter; casing) for quantitative uses
- Mud cake or borehole fluid have to contain radioactive materials
- Large holes inappropriate
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Resolution: |
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In general, the vertical resolution strongly depends on the logging speed and device length:
Gamma-ray logging:
- Normal measurements: velocity between 3 and 5 m / min: resolution between 15 and 30 cm
- High-resolution measurements: velocity between 1 and 2 m / min: resolution about 10 cm.
- Very small layers may not be accurately resolved; parameters to consider: contrast in radioactivity, velocity of the logging instrument and sampling rate. Depth of investigation: 10 - 20 cm (depending on rock density)
Gamma-Gamma density logging: Between 20 and 60 cm (i.e. source-detector distance). Depth of Investigation 5 to 12 cm (depending on rock density)
Neutron-Neutron log: Between 25 and 70 cm (i.e., source-detector distance). Depth of investigation for Neutron-Neutron and Neutron-Gamma logs: from 16 to 60 cm (depending on the water content) |
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Expected Results: |
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- Measured parameter: Gamma log: counting rate in counts / min or counts / s, often converted to API, the standard unit of the American Petroleum Institute (Unit for Gamma Ray); other logs
- Data analysis: The statistical nature of radioactive decay must be considered when running or interpreting nuclear logs. Knowledge of the local geology may be needed for reliable identification of probed rocks
- Interpretation: Gamma log: determination of clay content and predominant clay minerals is possible. Gamma-Gamma Density log: determination of bulk density in kg / m3 that can be converted to porosity log.
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Combination with other Methods: |
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- Required additional information: Caliper log
- Related add-on information: NA
- Independent additional information: Other logs; laboratory analyses
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Operating Expense: |
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- Crew size: 1 - 2 persons
- Acquisition speed:see ""Parameters to specify""
- Processing: low
- Equipment rental costs: high
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Parameters to specify: |
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- Instrument type
- Sampling rate (digital devices) or time constant (analogue devices)
- Logging speed: Normally between 3 and 5 m / min, for high-resolution investigations: 1 to 2 m / min
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QC Documents: |
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- Calibration data: last shop-calibration and or on-site calibration
- About 20 m of repeated measurements
- Borehole information (i.e. casing (type and 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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