Africa Surveyors May-June issue 2023 digital

FEATURE
The important aspect of
Radiometrics Survey
By Dorcas Kang'ereha
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 gammarays
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).
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,
20 May-June issue l 2023 www.africasurveyorsonline.com