Remove Radon Effect

Use the RPS > Remove Radon Effect option (geogxnet.dll(Geosoft.GX.Radiometrics.RemoveRadonEffect;Run)*) to correct airborne radiometric data for atmospheric radon background radiation. In addition to the upward-looking crystal method, you may also select either the table-based or over-water survey method. For more details, refer to the Application Notes below.

To rerun the process with previous settings, select the header cell of any channel created by the process, then right-click to open the context menu. The last item in the menu represents the most recently executed process (GX). Select it to reopen the associated dialog. From there, you can rerun the process with the existing settings, adjust parameters before execution, or simply close the dialog. Learn more about Dynamic Process Links (Makers).

Remove Radon Effect dialog options

Input channel suffix	If Remove Aircraft and Cosmic Effects 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.REMOVE_RADON_EFFECT_INPUT_SUFFIX
Method	Select the desired method: Upward facing crystal (Default) Over water Background table Refer to the Application Notes below for further details. Script Parameter: SPECTRO.RADON_REMOVAL_METHOD [ 0: Upward facing crystal; 1: Over water; 2: Background table]
Upward Facing Crystal Method
Specify the cutoff values for low-pass filtering to smooth noisy data. See the Application Notes* for more details.*
Upward U cutoff	Enter a cutoff value for the Upward Uranium channel. Typical value: 200 Script Parameter: SPECTRO.UPUSWAV_RADON
U cutoff	Enter a cutoff value for the Uranium channel. Typical value: 200 Script Parameter: SPECTRO.UTSWAV_RADON
Th cutoff	Enter a cutoff value for the Thorium channel. Typical value: 200 Script Parameter: SPECTRO.THTSWAV_RADON
Skyshine Coefficients Specify the skyshine coefficients – enter new values or accept the displayed defaults. See the Application Notes* for more details.*
A1	Enter a value for the A1 coefficient. Typical value: 0.036 Script Parameter: SPECTRO.SPECTRO.UPA1
A2	Enter a value for the A2 coefficient. Typical value: 0.022 Script Parameter: SPECTRO.SPECTRO.UPA2
Calibration Factors Specify the atmospheric radon calibration constants – enter new values or accept the displayed defaults. See the Application Notes* for more details.*
Potassium (K)	Enter the a and b calibration constants for the Potassium data. Typical values: 0.8 and 0 Script Parameters: SPECTRO.UPAK SPECTRO.UPBK
Uranium (U)	Enter the a and b calibration constants for the Uranium channel. Typical values: 0.25 and 0 Script Parameters: SPECTRO.UPAU SPECTRO.UPBU
Thorium (Th)	Enter the a and b calibration constants for the Thorium channel. Typical values: 0.1 and 0 Script Parameters: SPECTRO.UPAT SPECTRO.SPECTRO.UPBT
Total count (TC)	Enter the a and b calibration constants for the Total count data. Typical values: 12 and 0 Script Parameters: SPECTRO.UPAI SPECTRO.UPBI
Over Water Method
Reference channel	Select the overwater reference channel. Script Parameter: SPECTRO.WBREF
Filter cutoff	Enter the filter cutoff value. Choose an appropriate filter width for the average length of the overwater segments but not so long as to contaminate the shorter segments with adjacent ground-source data. Default: 75 Script Parameter: SPECTRO.WBSWAV_RADON
See the Application Notes* for more details*.
Background Table Method
Reference channel	Select the channel for background removal. Script Parameter: SPECTRO.REFERENCE
Table file	Select the background table or atmospheric radon correction file. Script Parameter: SPECTRO.TABLE
See the Application Notes* for more details*.

Output channel suffix	Specify the suffix to append to output channels. Default: `_rad`. As you type, the information string below the field updates to reflect the new output channel names. Script Parameter: SPECTRO.REMOVE_RADON_EFFECT_OUTPUT_SUFFIX

Application Notes

Radon Background Radiation

Radon gas decay in the atmosphere is one of the most challenging sources of background radiation to remove in airborne radiometric surveys. Atmospheric radon ²²²Rn and its daughter products are the main source of background radiation and produce a spectrum virtually identical to that of the uranium decay being measured. Radon diffusion in air is influenced by several environmental factors, including:

Air pressure
Soil moisture
Ground cover
Wind speed and patterns
Temperature

These variables fluctuate throughout a survey, affecting background radiation levels.

Radon background calibration

To measure the radon spectrum, a calibration flight is conducted over water in the presence of radon. Aircraft and cosmic radiation contributions are subtracted to isolate the radon signal.

Removing Radon Background Using Upward-Looking Data

The most common method for atmospheric radon removal involves an additional, upward-looking detector mounted above the downward-facing crystals. The downward crystals partially shield the upward detector. The upward-looking crystal permits distinguishing between atmospheric radon and terrestrial sources of radon.

If your survey system includes upward-looking uranium detectors, these can be used to correct for radon background. Corrections are based on measured count rates—adjusted for cosmic and aircraft backgrounds—and calibration factors outlined in Section 4 of the IAEA technical report ^[1].

Required Inputs

Filtering Parameters:

The filtering settings are designed to remove residual high-frequency noise, ensuring the data is smoothed as effectively as possible. To select appropriate filter cutoff values, examine the profile view to identify visible noise and choose a cutoff that reduces this noise without distorting the underlying signal.

Skyshine Coefficients:

When using the upward facing crystal method, the system prompts for the skyshine coefficients, A1 and A2.

The upward detector ground component is related to the downward detector ground components by the linear equation (IAEA,1991²):

Where:

u_g, U_g, and Th_g are ground-originated contributions in the respective window and must be calculated independently.
a₁ (or A1) and a₂ (or A2) are the calibration factors and are determined using a least squares method.

The calibration requirements for the upward-facing crystal method are comprehensively described in IAEA, 1991 ^[2].

Atmospheric Radon Calibration Constants:

These constants are typically provided by the survey contractor and calculated according to IAEA guidelines:

a_k, a_u, a_t, a_tc: Ratios between various downward and upward window counts
b_k, b_u, b_t, b_tc: Intercepts when uranium count = 0. Radon intercepts (b-values) are usually small and often set to zero.

Expand for more details:

a_K: K_Down/U_Down

a_u: U_Up/U_Down

a_t: Th_Down/U_Down

a_tc: TC_Down/U_Down

b_k: K intercept for U = zero (0)

b_u: Upward U intercept for U = zero (0)

b_t: Th intercept for U = zero (0)

b_tc: TC intercept for U = zero (0)

Radon Component (Background) Calculation

The radon contribution to the uranium window of the main detector package (i.e., the “downward” U window) is calculated using the following formula (IAEA, 2003 ^[1]):

Where:

U_r = Radon background in the "downward" U window

u = Count rate in the "upward" U window

U = Count rate in the "downward" U window

T = Count rate in the "downward" Th window

a₁, a₂, a_u, a_t, b_u, and b_t = Calibration constants (see sections above for details on skyshine coefficients and calibration factors)

Applying Radon Corrections to Radiometric Windows

The contributions to the other four windows, attributed to atmospheric radon, are estimated via linear regression and the background in these channels can be derived from the background in the uranium channel by suitable calibration:

Where:

U_pU_r = Radon component in the upward-looking uranium window

U_r, K_r, Th_r, and TC_r = Radon components in the various downward detector windows

The “a” and “b” coefficients are the required calibration factors (see list above).

Radon Removal

The calculated radon background to remove is applied using the following formula (example shown for the output uranium channels):

Where:

U_out = Output uranium channel with radon background removed

U_pUr_out = Output upward uranium channel with radon background removed

Interim Channels

This method also generates interim channels in the database used for levelling: UTEMP, THTEMP, UPUTEMP, and UPURADREF. If deleted, these channels are automatically regenerated the next time the GX is run.

Removing Radon Background Using the Over-Water and Background Table Methods

Survey flights over water bodies—such as lakes and rivers—yield no gamma-ray response from the ground. This means the uncorrected radiometric data collected in these areas reflects only the background contributions from:

Aircraft
Cosmic radiation
Atmospheric radon

The tool leverages this principle to offer two methods for radon background removal:

Background Table Method

This method uses a predefined table of radiometric values collected over water. These values represent background levels and are used to correct survey data.

It also generates interim channels in the database to store the radon backgrounds: URADREF, KRADREF, THRADREF, and TCRADREF. If deleted, they are regenerated the next time the GX is run.

Over-Water Method

This approach creates a reference channel that flags overwater locations with a value of 1. The system then automatically selects the corresponding radiometric values and applies radon corrections (interpolates background values between all fiducials marked by a value of 1 and removes the radon background from the potassium, thorium, total count and uranium channels).

Select a filter width that matches the average length of over-water segments, but avoid widths so large to contaminate the shorter segments with adjacent ground-source data.

The radon backgrounds are stored in channels URADREF, KRADREF, THRADREF and TCRADREF. Display these data as profiles to verify that the calculated backgrounds are reasonable and within expected ranges.

This method does generate a RpsWtBk.tbl file in the working directory. It is populated with values for the following parameters: _RpsFlight, _ RpsFid, URADREF, KRADREF, THRADREF, TCRADREF.

The Over water method is preferred because it eliminates the need to manually create a background table. However, the Background table method remains useful when you need to simulate or adjust background values manually.

Assumptions and Setup for the Background Table Method

To use the Background Table method effectively, the following conditions should be met:

Your survey includes flight lines over lakes or rivers.
Data was collected at consistent survey altitude (important because radon concentration decreases with altitude).

Table Configuration Considerations

The structure and content of your background table depend on several factors:

Survey procedures and flight path
Equipment specifications
Availability of over-water measurements
Recorded fiducial or time channels
Data quality and consistency

Sample Background Table

A sample table named rpsbackg.tbl is included with the Oasis montaj Sample Data.

This table provides a reference for how to format and populate your own background data for use in radon correction.

*GX.NET tools are embedded in the geogxnet.dll file located in the \Geosoft\Desktop Applications\bin folder. To run this GX interactively (outside the menu), first navigate to the bin directory and provide the GX.NET tool in the required format. See the Run GX topic for more guidance.

References

[1] G. Erdi-Krausz et al.(2003), Guidelines for Radioelement Mapping Using Gamma Ray Spectrometry Data, IAEA-TECDOC-1363, International Atomic Energy Agency.
https://www-pub.iaea.org/MTCD/Publications/PDF/te_1363_web.pdf
[2] IAEA (1991), Airborne Gamma Ray Spectrometer Surveying, Technical Reports Series - 323, International Atomic Energy Agency.
[3] R.L. Grasty and B.R.S. Minty (1995), A Guide to the Technical Specifications for Airborne Gamma-Ray Surveys, Australian Geological Survey Organisation, 1995/60.
https://www.ga.gov.au/bigobj/GA7667.pdf