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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
high-sensitivity multi-channel radiometric measurements were
made at 92 meters interval at a nominal sensor altitude of 120
meters terrain clearance, using twin-engine Cessna-404, Titan type
aircraft. A high sensitivity 256-Channel airborne gamma-ray
spectrometer-having approximately 50 liters Nal (TI) detector was
used as the primary sensor elements in the Aero-Service
CODAS/AGRS 3000 F computer based digital data acquisition
system (Aero Service 1985). The obtained data were displayed in
stacked profiles or contour maps, and interpreted visually.
In the present work, airborne radiometric digital data were
converted such to images format and different image processing
techniques were applied. These images were designed to show
intensities distribution of U, Th and K*. The preprocessing steps
are as follows:
a- Converting the data of each element (U, Th, K*) to an image
file with a set of floating (real) density numbers (DN).
b- Mapping each element image file to the range 0-255 (integer
one byte) because the one-byte image file is much easier in
processing and manipulation.
c- Merging the three elements image files to one multi-bands
image that could be manipulated and processed as an
ordinary image file. The composite color image representing
U, Th and K? distribution and intensity, i.e. blue represents
high U and poor Th + K*, green represents high Th and poor
U--K" and red represents high K^? and poor U + Th.
d- Resampling has been made to project the produced image from
the Egyptian Transverse Mercator (ETM) system to
Universal Transverse Mercator (UTM) system. This process
was necessary to achieve compatibility between the two data
types, and to ensure the coincidence between the different
layers that could be extracted from both types of data. The
various layers in this sense could be easily used in GIS
technique.
e- The areas that have uranium to thorium ratio greater than 30
and uranium content greater than 50 ppm have been
allocated. This was done by developing a spatial model to
discriminate the desired percentage, producing output raster
layers that delineate the areas having values greater than 30
and 50 ppm. The output raster layers in this form were used
in GIS modeling with combination of the other layers.
2.1.3 Geographic Information System (GIS):
a- System input
The input layers used in this study are mixture of vector and
raster layers including:
- Geological and structural vector maps, which have been
extracted from the enhanced Landsat TM image.
- Ground troth information as vector layer.
- U/Th raster layer.
b- GIS model
GIS model has been designed and implemented by ARC/INFO
(version 7.02) software package and it is based on the intersection
of the buffering zones of each input layer.
The controlled parameters governing the buffering zones
have been selected to define the most promising radioactive areas
as follows:
Geological unit All units
Structure 100 m far from faults
U more than 50 ppm
U/Th more than 30
451
c- Output decision
The decision of defining the most promising areas of uranium has
resulted as an output image. Figure ( | ) shows the concentrated
areas of uranium (U>50 ppm and U/Th >10 ) superimposed on
Landsat TM image using the above selected controlled
parameters. These sites lie between Wadi Sharm El Bahari in the
north and Wadi Um Greifat in the south.
2.2. Field and laboratory work
Two field trips for scintilometer measurements and sampling of
the recorded sites of high radioactivity. Six sites showing
anomalous radioactivity > 50 ppm U on the processed airborne
radiometric images were prospected in the field by scintillometer
and sampled. The scintillometer used is a “SAPHYMO — STEL”,
type S.P.P.2. NF.
Thin sections and polished surfaces for 30 samples were
studied microscopically to reveal the mineralogical composition of
the rock units and the hosted radioactive anomalies.
Eighteen rock samples were collected from the 6 sites
representing the highly anomalous radioactivity sites. Then,
laboratory gamma ray measurements for the these samples were
carried out using the Hyper Pure Germanium Gamma-
Spectrometer (HpGe). The collected rock samples were crushed,
sieved and packed in 100 ml volume containers. The containers
were then sealed for four weeks to get secular equilibrium
between "Th and its daughters. The gamma ray analyses of the
samples have been carried out by HpGe detector with 8K
multichannel analyzer (MCA). The detector (Canberra type) has
40% efficiency with 1.95 keV resolution at 1332 keV of “Co. The
system was energy calibrated by using multi gamma ray sources;
137Cs, Co and ’Co. The efficiency of calibration and factors to
be used for quantitative calculations of thorium and uranium were
carried out. All the measurements have been carried out at the
Central Laboratory for Environmental Measurements, National
Center for Nuclear Safety and Radiation Control.
Uranium was identified based on its daughter transition, 63
keV of **Th. Thorium was determined based on its daughters
gamma transitions; 583keV of 208TI and 911keV of 228Ac. The
activity (A) of ?*U and ?"Th was calculated by the following
equation;
A = R.Fn/p Bg/Ks......-e- I
where R - the count rate in the specific photo peaks
Fn = the factor of normalization
J = the sample density (g/em”)
The activities (Bq/g) of uranium and thorium were then
converted to concentration in ppm (ug/g) by using the following
equation:
A = (W/A.wt). Nf. (0.693/T,») ......... 2
where w -the weight of ?*U and ?"?Th (ug/g)
A.wt = the atomic weight of U and Th,
Nf = the Avogadro’s number (6.02 x 1023)
T1/2 = the half life time of U and Th.
The IAEA's Reference materials were used for quality
control of the calculations and results.
