Geophysical Mapping: Method Details
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Method Name: Ground spectrometry
Method Type:   Radioactivity
Assigned Problems:
+ Monitoring Hazardous Waste
+ Regional Mapping Regional Mapping
0 Fractures Groundwater
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Ground spectrometry is a technique that provides information about the distribution of the natural isotopes (e.g., elements Potassium (K), Uranium (U) and Thorium (Th)) as well as of artificial isotopes (e.g., 137Cs).
Keywords:   Gamma spectrometry; radiometric surveys; natural and artifical isotopes; radiometric element concentration; gamma rediation
  • Target must be characterised by sufficient radioelement contrast
  • For uranium: secular equilibrium in the decay series must prevail
  • Survey data corrections needed (e.g., background scattering, cosmic radiation background)
  • Spectrometry should be postponed for about 12 to 24 hours following a 'ground soaking' rain, to allow soil moisture levels to return to 'pre-rain' levels
  • For larger area, most radiometric surverys are done from the air
Resolution:   A single measurement provides an average surface concentration for an area of several square metres, composed of variable proportions of bedrock, overburden, soil moisture, water and vegetation.
Spectrometry measures effects generated within the first 0.3 m of the underground, thus, has almost no penetration depth.
Expected Results:  
  • Measured parameter: counting rate in counts / min or counts / s
  • Data analysis: count rates can be converted to ground concentrations (in [ppm] for Uranium and Thorium; in Percent for Potassium [%]) or activities (Becquerel per volume ( m3) [Ci / l] or Becquerel per mass [Bq / kg]). Data is plotted in contour maps
  • Interpretation: the interpretation requires an understanding of the nature of the surficial materials and their relationship to bedrock geology
Combination with other Methods:  
  • Required additional information: state of weathering of surface outcrops (relevant for secular equilibrium)
  • Related add-on information: rock, soil, hydro or bio-geochemical sampling
  • Independent additional information: magnetic, electromagnetic or electric data; seismic data
Operating Expense:  
  • Crew size: 1 key person
  • Acquisition speed: 1 measurement using a gamma-spectrometer: around 3 min; additional: time for moving between measurement points
  • Processing: 0.5 - 1 day per acquisition day
  • Equipment rental costs: intermediate
Parameters to specify:  
  • Variables to be measured, instruments required (i.e., Integral, range of interest (ROI) or spectral measurements; number of channels)
  • Spacing between measurements (for engineering-scale surveys usually less than 2 m; depend on degree of lateral homogeneity)
  • Measurement period (usually around 3 min)
QC Documents:  
  • Calibration information
  • Document applied corrections (e.g., subtraction of background counts)
  • Field notes (e.g., all activities, effective time schedule, present personnel)
  • Maps of total gamma radiation, radiation within a range of interest, or spectral fractions of the radiation spectrum
  • Maps of radiometric element-concentrations
  • Interpretation
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