Compton Stripping

Use the RPS > Compton Stripping option (geogxnet.dll(Geosoft.GX.Radiometrics.ComptonStripping;Run))*) to correct airborne radiometric data for spectral overlap effects ("stripping"). For additional information, refer to the Application Notes below.

Compton Stripping dialog options

Input channel suffix	If Remove Radon Effect has been run on the current database, the suffix for the generated channels is automatically detected and preselected. If multiple sets exist, all suffixes are listed, with the most recent one selected by default. Once a suffix is chosen, the associated channels are listed below the field. Script Parameter: SPECTRO.COMPTON_STRIPPING_INPUT_SUFFIX
STP altitude channel	Select the STP-corrected altitude. Default name: RALTSTP Script Parameter: SPECTRO.RALTSTP
Stripping Ratios Specify the spectral stripping ratios by entering new values or accepting the displayed defaults. See the Application Notes for more details.
alpha	Specify the α spectral ratio. Default: 0.24 Script Parameter: SPECTRO.ALPHA
beta	Specify the β spectral ratio. Default: 0.37 Script Parameter: SPECTRO.BETA
gamma	Specify the γ spectral ratio. Default: 0.7 Script Parameter: SPECTRO.GAMMA
a	Specify the a spectral ratio. Default: 0.05 Script Parameter: SPECTRO.ASTRIP
b	Specify the b spectral ratio. Default: 0 Script Parameter: SPECTRO.BSTRIP
g	Specify the g spectral ratio. Default: 0 Script Parameter: SPECTRO.GSTRIP
Output channel suffix	Specify the suffix to append to output channels. Default: _strip. As you type, the information string below the field updates to reflect the new output channel names. Script Parameter: SPECTRO.COMPTON_STRIPPING_OUTPUT_SUFFIX

Application Notes

In airborne gamma-ray spectrometry, the energy spectra of potassium (K), uranium (U), and thorium (Th) naturally overlap. As a result, each spectral window—intended to isolate a specific radioelement—also detects gamma rays from the other two.

• Gamma rays from the thorium decay series appear in both the uranium and potassium windows.

• Gamma rays from the uranium decay series appear in the potassium and thorium windows.

Correcting Spectral Overlap: Stripping

In airborne gamma-ray spectrometry, correcting for spectral overlap is known as stripping. To isolate the true signal from each radioelement, a correction process called spectral stripping is applied. This ensures that the recorded counts in the K, U, and Th windows reflect only their respective elemental contributions—free from contamination by overlapping spectra.

What Are Stripping Ratios?

Stripping ratios are spectral correction factors used to adjust the count rates in each window for cross-element interference. They quantify how much signal from one element appears in another element’s window when measured from pure sources of K, U, and Th.

These ratios are denoted as:

How Are Stripping Ratios Determined?

Stripping ratios are empirically derived using specially constructed calibration pads containing known concentrations of K, U, and Th. These calibration measurements are typically performed and provided by the instrument manufacturer. Additionally, spectrometers are periodically returned to the manufacturer for recalibration and measurement.

Altitude Corrections

Due to changes in gamma-ray attenuation and scattering with altitude, the three primary stripping ratios —α, β, and γ—exhibit a systematic increase with elevation above ground level. To account for this, corrections are applied based on the STP Standard Temperature and Pressure equivalent altitude.

In airborne surveys, it is standard practice to measure these stripping ratios at ground level and then calculate their increase at the actual flight altitude. The corrections factors per metre of altitude are shown below.

Increase in stripping ratios with altitude (after IAEA, 1991 ^[2]):

Stripping Ratio	Increase per Metre
α	0.00049
β	0.00065
γ	0.00069

Reverse stripping ratios (a, b, and g) are generally small and do not require altitude correction.

Applying Stripping Ratios

The stripping process involves two key steps:

• Ratio Calculation: The α, β, and γ stripping ratios are corrected for each record based on the STP equivalent altitude.

  

  

  

  Where: 

  h_e = equivalent height AGL (above ground level) at STP 

• Spectral Stripping: The adjusted ratios are then used to perform spectral stripping—that is, to separate the true elemental contributions in each window.

  Expand to see the calculated stripped count rates in the Potassium, Uranium and Thorium channels.

  

  

  

  Where:

  KSTRIP = Output Compton stripped potassium

  USTRIP = Output Compton stripped uranium

  THSTRIP = Output Compton stripped thorium

  And where:

  KLEVL = Input potassium levelled count channel

  ULEVL = Input uranium levelled count channel

  THLEVL = Input thorium levelled count channel

   

  BK = ALPHA2*GAMMA2-BETA2;

  CK = ASTRIP*BETA2-GAMMA2;

  DK = 1-ASTRIP*ALPHA2;

   

  BU = GSTRIP*BETA-ALPHA

  CU = 1-BSTRIP*BETA

  DU = BSTRIP*ALPHA-GSTRIP

   

  BTH = 1-GSTRIP*GAMMA2;

  CTH = BSTRIP*GAMMA2-ASTRIP;

  DTH = ASTRIP*GSTRIP-BSTRIP;

  Where:

  ALPHA2 = ALPHA + 0.0004895*RALTSTP

  BETA2 = BETA + 0.0006469*RALTSTP

  GAMMA2 = GAMMA + 0.0006874*RALTSTP

  And:

  RALTSTP = Input STP corrected radar altimeter channel value (m)

  ALPHA = Input Compton stripping alpha factor (default: 0.24)

  BETA = Input Compton stripping beta factor (default: 0.37)

  GAMMA = Input Compton stripping gamma factor (default: 0.70)

  ASTRIP = Input Compton stripping a factor (default: 0.05)

  BSTRIP = Input Compton stripping b factor (default: 0.0)

  GSTRIP = Input Compton stripping g factor (default: 0.0)

  And where:

  

References