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 applied to the current database, the suffix for the generated channels is automatically detected and preselected. If multiple channel sets exist, all detected suffixes are listed, with the most recent one selected by default. After selecting a suffix, the associated channels are listed below. Script Parameter: SPECTRO.COMPTON_STRIPPING_INPUT_SUFFIX
STP altitude channel	Select the STP-corrected altitude channel. Defaults to  RALTSTP if it exists in the database. Script Parameter: SPECTRO.RALTSTP
Stripping Ratios Specify the spectral correction factors for your instrument configuration. These values are retained after running the tool and are available the next time the dialog is opened. See the Application Notes for more details.
alpha	Specify the α spectral ratio. Script Parameter: SPECTRO.ALPHA
beta	Specify the β spectral ratio. Script Parameter: SPECTRO.BETA
gamma	Specify the γ spectral ratio. Script Parameter: SPECTRO.GAMMA
a	Specify the a spectral ratio. Script Parameter: SPECTRO.ASTRIP
b	Specify the b spectral ratio. Script Parameter: SPECTRO.BSTRIP
g	Specify the g spectral ratio. Script Parameter: SPECTRO.GSTRIP
Output channel suffix	Enter the suffix to append to output channels. Default: strip As you type, the information string below the field updates to show the resulting channel names. Each name is formed by combining the radiometric element name with the suffix (letters and numbers only). Output channels follow the pattern element_suffix. Script Parameter: SPECTRO.COMPTON_STRIPPING_OUTPUT_SUFFIX

Application Notes

This tool applies Compton stripping corrections to channels that have already been corrected for atmospheric radon background radiation. It operates on preprocessed channels labeled *_rad when Remove Radon Effect has been applied to the current database and the default channel suffix has been retained. The tool then generates Compton‑stripped output channels, which are labeled *_strip by default.

Correcting Spectral Overlap: Stripping

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 records gamma rays originating from the other two elements.

• Gamma rays from the thorium decay series contribute to both the uranium and potassium windows.

• Gamma rays from the uranium decay series contribute to the potassium and thorium windows.

Correcting for this spectral overlap is known as stripping. This process applies a set of corrections to remove these unwanted contributions, allowing the counts recorded in the K, U, and Th windows to represent only their respective elemental signals, 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 follows:

where:

• α (Th into U) — Ratio of counts in the U window to those in the Th window for a pure Th source

• a (U into Th) — Reverse ratio; counts in the Th window to those in the U window for a pure U source

• β (Th into K) — Ratio of counts in the K window to those in the Th window for a pure Th source

• b (K into Th) — Reverse ratio; counts in the Th window to those in the K window for a pure K source

• γ (U into K) — Ratio of counts in the K window to those in the U window for a pure U source

• g (K into U) — Reverse ratio; counts in the U window to those in the K window for a pure K source

How Are Stripping Ratios Determined?

Stripping ratios are empirically derived using specially constructed calibration pads containing known concentrations of K, U, and Th. These measurements are typically performed and provided by the instrument manufacturer as part of system calibration. 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 correction factors per metre of altitude are shown below:

Stripping Ratio	Increase per Metre
α	0.00049
β	0.00065
γ	0.00069

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

Reverse stripping ratios (a, b, and g) are typically 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