Geophysical Mapping: Method Details
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Method Name: Nuclear logging
Method Type:   Borehole Methods
Assigned Problems:
+ Density Civil Engineering
+ Gravel, clay, limestone, salt exploration Natural Resources
+ Ice thickness Natural Hazards
+ Landslides Natural Hazards
+ Porosity / Permeability Groundwater
+ Quality / Thickness of aquifer/aquitard Groundwater
+ Quality and thickness (Natural resources) Natural Resources
0 Building stability Buildings and Structures
0 Depth of Overburden-bedrock interface Civil Engineering
0 Groundwater table Groundwater
0 Heat mining Natural Resources
0 Host sediments, hydogeological settings Hazardous Waste
0 Permafrost and ice detection Natural Hazards
0 Quality / Thickness of concrete Buildings and Structures
0 Soil / rock quality Civil Engineering
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   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)
Keywords:   Gamma log; natural-gamma log; neutron-activation log; radioactive probes; density log; litho-Density log; porosity log
  • 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
Resolution:   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)
Expected Results:  
  • 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.
Combination with other Methods:  
  • Required additional information: Caliper log
  • Related add-on information: NA
  • Independent additional information: Other logs; laboratory analyses
Operating Expense:  
  • Crew size: 1 - 2 persons
  • Acquisition speed:see ""Parameters to specify""
  • Processing: low
  • Equipment rental costs: high
Parameters to specify:  
  • 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
QC Documents:  
  • 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)
  • Field logs (displayed using specified scales)
  • Composite logs
  • Crossplots
  • Listings; histograms
  • Interpretation
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