The radiometric measurement of soil gamma radiation, or gamma ray spectroscopy, is an important geological mapping tool which has been applied to many environmental and mineral exploration projects. The use of gamma-ray spectrometry for the detection and mapping of mineral deposits has developed rapidly over the past 40 years. Spectrometers (gamma-ray spectrometers) are used to detect gamma-rays emanating from natural and man-made sources. They use a number of spectral windows, covering specific gamma-ray energies, to detect and identify the source. The data obtained is usually centered on three energy bands which indicate Uranium, Thorium and Potassium concentrations in the earth’s crust. These bands are typically centered around the Bi214, Tl208 and K40 energy peaks at 2.62 MeV, 1.76 MeV and 1.46 MeV respectively.
With reference to Howard Barrie, President at Terraquest, Radiometric Surveys detect and map gamma rays, and can be performed on the ground, in the air, or in a borehole. Airborne gamma-ray spectrometry or radiometric surveying for mineral exploration measures the gamma-rays that are emitted from naturally occurring radioactive isotopes that are found in rocks and soils near the surface. The most abundant of these radioactive isotopes are potassium (K40), uranium (U238), and thorium (Th232).
Africa Surveyors May-June issue 2023 digital
Howard further elaborates on the usefulness of airborne radiometric surveys for mineral exploration and geological mapping depends on the following:
1. The measured radioactive isotope distributions relate to different lithologies in the bedrock (normal litholigic signatures)
2. Normal lithologic signatures are distinctly modified by mineralizing processes (alteration zones)
3. Bedrock signatures migrate into surficial materials (exposed bedrock, sediments, affected by soil and vegetation types, and surface water)
4. Surficial materials can be spatially related to bedrock sources.
Ramesh Acharya Gundi, Chief Geophysicist at Mcphar international, says that, “Radiometric survey more specifically Gamma ray Spectrometric survey measures the natural gamma radiation, in the range of (0 – 3000 KeV), emanated by the radio nuclides of Potassium (K), Uranium (U) and Thorium (Th) present in geological formations. The relative presence of the radio nuclides varies from acidic to basic geological formations. Mapping the distribution of K, U, Th and the ratios (U/K, U/Th and Th/K) provide great detail on surface geology and the alteration zones (radiometric halos). Integrated analysis of spectrometric data along with other geophysical data sets such as: magnetic, electromagnetic in a given geological setup aids in mineral exploration programs.”
“High resolution magnetic and spectrometric surveys on an airborne platform are routinely being carried out over huge areas in understanding the subsurface structure and mineral potential and in narrowing down the areas of interest for subsequent followup. Airborne spectrometric surveys are also being conducted to track the radioactive plumes in the event of nuclear disasters in atomic power plants for safety from environmental perspective,” adds Ramesh.
Gamma-ray spectrometry can also be used to map the concentration of radioactive elements in the earth’s crust and to calculate elemental ratios. These are important in geochemical exploration because they allow the presence of hydrocarbons or other buried reservoirs to be detected by assessing their abundance at various depths.
The spectrometer has a series of crystals that have different properties and a photomultiplier tube that converts the gamma-ray energy into a voltage output. This output is used to determine the count rate, which in turn reflects the gamma-ray intensity.
There are several types of spectrometers, all of which are designed to measure the gamma-ray intensity from a variety of sources. The sensitivity of the detector is controlled by a number of instrumental parameters including the ionization resistance, which can be influenced by the temperature of the crystal and the photomultiplier tubes. It is therefore essential that the sensitivity of the detector is closely monitored during operations. Daily calibration checks using standard isotope sources are also recommended.
High sensitivity systems, which can sense gamma radiation with a count rate of over one million counts per second (cps), are essential for airborne geochemical and geological mapping surveys. However, the sensitivity of these systems is limited by the total volume of crystals which can be carried on aircraft and by the speed and altitude of the aircraft. Larger crystal volumes require larger aircraft, which are heavier and more expensive to operate.
Medium sensitivity reconnaissance surveys, which may not be intended to detect a wide range of spectral windows or uranium or thorium concentrations in particular, can be flown with smaller crystal volumes and less sensitive spectrometers. This can reduce costs and make the survey more economical.
Sensitivity constants for the spectral windows of interest are experimentally determined from measurements on calibration pads made of concrete containing known amounts of U, Th and K. These constants are usually referred to as the attenuation coefficients, mTh, mU and mK, for each of the three energy windows; and their values are dependent on the size of the crystals and detector altitude.
During a low altitude survey, the presence of atmospheric radon and cosmic rays can also affect the count rates in each of the three spectral bands. The effect of these non-geologic sources on the count rates is primarily due to Compton scattering and the fact that they have a lower energy than gamma rays originating from the earth’s surface.
To reduce the impact of background noise on the spectral windows and count rates, a special spectrometer called a “differential spectrometer” is used. The difference between this spectrometer and the multichannel spectrometer is that it does not use “windows” of fixed energy width; instead, a narrower range of energy windows are measured. The detectors are of a larger size, and the light flash counting rates are much lower than with the multichannel spectrometer.
The optimum line spacing depends on the survey type and the amount of detail that is required. For fixed wing surveys, a mean terrain clearance of 130 metres is recommended, and the line spacing should be about three times this.
For unmanned aerial geophysical systems (UAS) this spacing is not as critical, and the distance between the surveyed strip and the closest point on the ground should not exceed half its diameter. For this reason a circle of investigation is often used for the strip, which moves with the aircraft. The main value of low altitude radiometrics is the ability to map uranium dispersion halos, and to indicate local anomalies in the U/Th and U/K ratios as “red-balls” or “black balls”. In suitable areas these maps can be useful as aids in geological interpretation.
“High resolution magnetic and spectrometric surveys on an airborne platform are routinely being carried out over huge areas in understanding the subsurface structure and mineral potential and in narrowing down the areas of interest for subsequent follow-up. Airborne spectrometric surveys are also being conducted to track the radioactive plumes in the event of nuclear disasters in atomic power plants for safety from environmental perspective,” says Ramesh.
Important aspect of Radiometrics survey
The important aspect of radiometric surveys is that they can detect subtle variations in the natural background radiation. These changes are generally not detectable from the ground, but can be detected by using “upward looking” crystals that are shielded from atmospheric radiation and can therefore monitor these variations in real time during the survey.
“One important aspect of radiometric surveys is their ability to provide reliable, consistent, and calibrated data. Variations in lighting conditions or camera settings can be accounted for and do not compromise the quality and comparability of the imagery captured over time,” affirms Susanne Scholz, Application Engineer at Vexcel Imaging. “Precise calibration of the imaging system is a fundamental prerequisite to achieving such a high level of reliability. In our calibration lab, our UltraCam aerial camera systems undergo both geometric and radiometric calibration processes, transforming them into precise measurement instruments,” she adds.
Changes in the natural background can lead to errors in the radiometric survey and it is therefore important that a careful control of the background radiation is maintained during the survey operation. For example, the survey area can be surveyed in stages to ensure that there is no deterioration in the natural background before the next stage.
“During the radiometric calibration, a series of images is captured, including color targets, white fields, and dark frames. The use of calibrated shutters ensures a consistent amount of light enters the camera system. The illumination of the calibration environment is crucial and requires a stable light source that closely approximates the spectrum of the sun. By employing a calibrated spectrometer, the light source can be carefully monitored throughout the calibration process,” enlightens Susanne.
“The most important aspect of radiometric surveys in my candid opinion is, ACCURACY,” asserts David Armah, Director of Business Development and Admin at A-M Surveys Ltd.
“The measuring and analysing the levels of radiation in a given area to identify and characterize radioactive materials or sources. These Accurate measurements are crucial for various reasons:
Safety: In the survey conducted to assess potential radiation, hazards to; and the safety of individuals and the environment must be ensured. Accurate measurements help identify areas with elevated radiation levels, allowing for appropriate safety measures and precautions to be implemented.
For Environmental Monitoring: Radiometric surveys play a vital role in monitoring environmental radioactivity, such as in nuclear power plants, waste management facilities, or areas affected by nuclear accidents. Accurate measurements are necessary to track any changes in radiation levels over time and detect any potential environmental contamination.
Resource Exploration: Particularly in the mining industry, to identify and assess deposits of radioactive minerals such as uranium or thorium. Accurate measurements are crucial for determining the extent, grade, and economic viability of these deposits.
Radiometric surveys contribute to Scientific Research; in various fields, including geology, archaeology, and environmental studies. Accurate measurements provide reliable data for studying geological formations, dating artefacts using radioactive isotopes, and understanding natural radiation background levels,” highlights David.
David emphasizes that,to ensure the accuracy in radiometric surveys as stated above, several factors should be considered, including
1. calibration of instruments
2. Appropriate sampling techniques,
3. proper shielding from external radiation sources,
4. Quality assurance and Quality control procedures,
5. Expertise of trained professionals who can accurately interpret and analyse the collected data.
