Remove Aircraft and Cosmic Effects

Use the RPS > Remove Aircraft and Cosmic Effects option (geogxnet.dll(Geosoft.GX.Radiometrics.RemoveAircraftAndCosmicEffects;Run)*) to remove aircraft and cosmic background contributions and produce background-corrected airborne radiometric data. For more details, refer to the Application Notes below.

Remove Aircraft and Cosmic Effects dialog options

Input channel suffix	Suffixes from channels generated by earlier processes (Along-Line Filtering, Account for Dead Time, or Generate Radioelement Counts) are automatically detected and listed. By default, the suffix from the most recent process is preselected. Once you select a suffix, the associated channels are listed below the field. Script Parameter: SPECTRO.REMOVE_AIRCRAFT_AND_COSMIC_EFFECTS_INPUT_SUFFIX
Correction Coefficients - Aircraft and Cosmic Specify the aircraft background and cosmic stripping values for your instrument configuration or survey characteristics. These values are retained after you run the tool and will be available the next time the dialog is opened. See the Application Notes below for details.
Potassium (K)	Enter the aircraft and cosmic correction coefficients for the potassium (K) data. Script Parameters: SPECTRO.KAIRBACK SPECTRO.KCOSSTRIP
Uranium (eU)	Enter the aircraft and cosmic correction coefficients for the equivalent uranium (eU)* data. Script Parameters: SPECTRO.UAIRBACK SPECTRO.UCOSSTRIP
Thorium (eTh)	Enter the aircraft and cosmic correction coefficients for the equivalent thorium (eTh)* data. Script Parameters: SPECTRO.THAIRBACK SPECTRO.THCOSSTRIP
Total count (TC)	Enter the aircraft and cosmic correction coefficients for the total count (TC) data. Script Parameters: SPECTRO.TCAIRBACK SPECTRO.TCCOSSTRIP
Upward Uranium (UpU)	Enter the aircraft and cosmic correction coefficients for the upward uranium (UpU) data. If the channel does not exist, this field will be disabled (greyed out). Script Parameters: SPECTRO.UPAIRBACK SPECTRO.UPCOSSTRIP
* Refer to the Radioactive Decay and Gamma Ray Emission section under Application Notes for more details.
Output channel suffix	Enter a suffix to append to output channels. Default: bg 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.REMOVE_AIRCRAFT_AND_COSMIC_EFFECTS_OUTPUT_SUFFIX

Application Notes

This tool operates on pre-processed channels generated by Along-Line Filtering, Account for Dead Time, or Generate Radioelement Counts. It applies aircraft and cosmic background corrections to these inputs and produces background‑corrected output channels, labeled *_bg by default.

Radioactive Decay and Gamma Ray Emission

Potassium concentrations in rocks and soils are commonly estimated using gamma‑ray spectrometry, which detects the 1461 keV gamma rays emitted by the radioactive isotope potassium-40 (⁴⁰K). Unlike ⁴⁰K, which decays directly to a stable daughter isotope, uranium‑238 (²³⁸U) and thorium-232 (²³²Th) decay through long chains of intermediate, unstable daughter products.

For gamma-ray spectrometry, the energies associated with these decay series are identified through their most prominent daughter isotopes:

• ²³⁸U → ²¹⁴Bi (bismuth)

• ²³²Th → ²⁰⁸Tl (thallium)

Characteristic gamma‑ray energy peaks — most notably the 1765 keV line from ²¹⁴Bi and the 2615 keV line from ²⁰⁸Tl — serve as markers for the uranium and thorium decay chains. The intensities of these emissions are then scaled to estimate concentrations of uranium and thorium, reported as equivalent uranium (eU) and equivalent thorium (eTh).

Cosmic and Aircraft Background Corrections

Cosmic background radiation results from high-energy cosmic ray interactions with the atmosphere. The intensity of cosmic rays increases with altitude—approximately doubling every 2,000 meters—and exhibits minor variations with latitude.

In addition to cosmic radiation, background radiation is also generated by the aircraft  and its onboard equipment. This is due to trace amounts of naturally occurring radioactive elements such as potassium, uranium, and thorium present in the materials used in the aircraft and its systems.

High-Altitude Cosmic Background Flights

The aircraft’s spectral contribution is assumed to be constant. The cosmic spectrum at each observation point is estimated by scaling a normalized cosmic spectrum by the cosmic window count rate. Each measured spectrum represents the sum of the aircraft component (constant) and the cosmic component, both of which are then subtracted from the total gamma-ray counts.

Over open water, contributions from ground sources and atmospheric radon are minimal. To estimate the aircraft spectrum and the normalized cosmic spectrum, high-altitude flights are performed offshore, ideally in regions with low radon concentrations. Measurements are taken at multiple altitudes (e.g., 1.0, 1.5, 2.0, 2.5, and 3.0 km above sea level), with background count rates recorded across all energy channels. At each altitude, data is collected for a minimum of 2 minutes and up to 15 minutes, and then averaged.

Aircraft and Cosmic Spectra

The measured spectra are each the sum of the aircraft component (constant) and the cosmic component. Since in the energy range of interest (700 to 3072 keV), the count rate in the cosmic window is linearly related to the count rate in the i‑th energy channel, a linear regression of the cosmic window count rate against a given channel yields the cosmic sensitivity (slope of regression) and the vessel background (zero intercept) for that channel as follows:

• Slope (b): Cosmic sensitivity

• Intercept (a): Aircraft (vessel) background

For each of the 4 count windows, a least-squares (LSQ) linear fit is applied to determine the slope (b) and intercept (a) ^[1]:

where:

• N_i — Aircraft + cosmic background count rate in the i'th channel

• a_i — Aircraft background contribution in the i‑th channel

• b_i — Cosmic background contribution in the i‑th channel, normalized to unit counts in the cosmic window

• n_cos — Cosmic window count rate

The illustration below of aircraft and cosmic spectra is taken from the referenced IAEA International Atomic Energy Agency document ^[1] (p. 61).

Reviewing Results

Once processing is complete, review the raw, filtered, levelled, and corrected channels in the profile window to assess the impact of each correction step. If the results are unsatisfactory, adjust the parameter values and rerun the GX.

References