The environmental radioactivity monitoring programs started in late 1950’s of the 20^th century following the global fallout from testing of nuclear weapons in the atmosphere, becoming a cause of concern regarding health effects. Later, the necessity of world industrialization for new energy sources led to develop national plans on electricity production from nuclear technology, initializing in this context worldwide exploration for fuel minerals: uranium exploration gained a particular attention in late 1940’s in USA, Canada and former USSR and in 1951 in Australia with respective national plans [1] .

The principal sources of environmental radioactivity of monitoring interest are due to the presence of ²³⁸U, ²³²Th and ⁴⁰K in the Earth’s crust. Generally, other major and trace elements like ²³⁵U and ⁸⁷Rb are negligible for radioactivity monitoring purposes. The world average abundances of the continental upper crust for ²³⁸U, ²³²Th and ⁴⁰K are respectively 2.7 ppm, 10.5 ppm and 2.3% [2] . Many countries have already monitored the distribution of natural radioactivity, finalized with the construction of the radiometric maps of their territories (USA, Canada, Australia, Switzerland, Slovakia, Slovenia, Czech Republic, UK, etc.). Gamma-rays are the most penetrating radiation from natural and man-made sources. It is a powerful tool for the monitoring and assessment of the radiation environment. It is widely used in geological mapping, soil surveying, mineral exploration and regolith studies. The use of the method as a mapping tool requires an understanding of the geochemistry of the radio elements in rocks and soils and the processes that effect their distribution and mobility [1] .

Airborne and spaceborne remote sensing techniques are of great interest especially in poor accessible remote regions such as playa (Sabkha) environs, subdued topography and sand dunes [3] - [7] .

Multispectral data like Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) were successfully used for mapping landcover types in playa regions [3] [8] [9] .

The modern Landsat 8 (L8) sensor provides multispectral imagery data of higher spectral (11 bands) and radiometric (16-bit) resolutions than the commonly used Landsat 5 and 7. The main objective of the present work is using remote sensing technique (Landsat 8 data processing) integrated with detailed field geology and ground gamma-ray spectrometric survey for geologic mapping and detecting radioactive anomalies as well as dose-rate assessment of alluvial fan Quaternary sediments in a poor-accessible desert area (Wadi Diit) along the Red Sea coast in the southeastern part of Egypt.

Wadi Diit alluvial fan sediments is located at the southern part of Shlatin town by about 70 km along the Red Sea coastal plain. It drains a vast area from the central part of the Sudanese Eastern Desert. It is located in the southeastern part of Egypt, between latitudes 22˚23'00"N - 22˚39'00"N and longitudes 36˚4'00"E - 36˚15'00"E (Figure 1). The studied area is generally characterized by an arid climate. It has a gentle topography, thick cultivation and is divided by a dendritic seasonal drainage network. It has a triangular shape with its apex at about 40 km from the Red Sea shoreline forming an alluvial fan. This fan is considered as one of the most obvious geomorphic landform along the foot-slopes of the Red Sea mountains.

The integrated data between Landsat 8 and detailed field investigation were used to identify the exposed rock types at Wadi Diit area (Figure 9). They show that the exposed rock units in the alluvial fan of the study area are mostly composed of Quaternary sediments, which comprise unconsolidated wadi sediments, costal-eolian sabkhas, sandy conglomerates, sandy dunes and sheets, mud cracks and black-sand lens.

Wadi sediments are mostly comprised sands and gravels, which are broadly categorized as undifferentiated wadi alluvium on the map sheet. They include outwash sediments, channel fill, terraced piedmont sands and gravels of angular, subangular, subrounded to rounded fragments of serpentinite, metagabbros, metavolcanics, Hamammat sediments, older and younger granites, felsite and quartz.

Sabkha is the Arabic term adopted by geologists for low-lying salt flats subject to periodic inundation. An

irregular, buffy crust of loosely cemented salts, fine sediment is common on dry sabkha and macroscopic marine organism remnants (Figure 10(a) and Figure 10(b)). Expansion due to the crystallization of salts may create raised polygonal patterns, whereas shrinkage due to desiccation may create polygonal “mud cracks”. Sabkha plain and salt encrustations are formed due to the invasion of sea water during high tides. In the studied area the sabkha may extend more than 40 km along the Red Sea shoreline and 4 to 5 km width inland. The coastal sabkha is extremely flat above the level of normal high tides.

・ Kappa Coefficient: Cohen’s kappa (k) is commonly used for accuracy assessment. For example, these are useful when building models that predict discrete classes or classifying imagery. They help provide a sense of how accurate or useful the model is.

Three general types were recognized, on the basis of dominant physical processes of their environmental formation (Figure 11) [18] [19] .

1) Coastal marine: Coastal sabkha, as the name implies, forms at or near the marine shoreline.

2) Lacustrine/playa: It is formed in association with river or lake drainage systems in arid areas.

3) Eolian/interdunal: Inland or interdunal sabkha is found in low-lying basins within the sand desert.

Wadi Diit sabkha plain is costal-eolian dominated which distinguished by a mixture of sediments comprising salts, carbonate-sulfate and mud (Figure 10(a) and Figure 10(b)).

Sandy conglomerates are composed mainly of basement fragments of different size range between 30 and 60 cm, of angular, subangular, subrounded to rounded shapes embedded in yellowish friable sands.

The major sand dunes and sheets belt are found in the eastern part of the studied area. Their formation is controlled by a combination of wind strength and direction, as well as sediment supply. They were transported to Red Sea via Wadi Diit (Figure 9) and formed its delta fan promontory, followed by parallel waves of this promontory that pushed sands onto the beach. Prevailing onshore wind blowed sands inland, where they were accumulated as sand dunes, sheets and barchans (Figure 10(c) and Figure 10(d)). Sand dunes comprise small linear dunes, also called longitudinal or seif dunes, that are oriented parallel to the prevailing wind direction. Their mode of formation is not well understood and various explanations were proposed, including, inter alia, consistent high wind velocities, bi-directional wind regime and helical air flow along the troughs between dunes. They have NNE-SSW trends. The elevation of these sand dunes ranges between 0.5 and 12 m. The prevailing wind direction in the studied area is mainly from the NNE to SSW. However, field observations indicate that sand movement in the study area is from NNE-SSW direction i.e., from the coastal plain of Wadi Diit fan (40 km from Red Sea shoreline) to basement rocks inland, where sand overrides hill slopes. Sand dunes in the southern part of the study area overlie basement rocks forming barchan.

Mud cracks formation is a natural process in clayey soils upon drying (Figure 10(e)). As drying declines downwards through the sediments, mud cracks have generally been theorized to nucleate near the surface, propagate downward and terminate at depth [20] . Clay content, mineralogy and physical boundary conditions govern the characteristics of a crack network that forms and evolves with decreasing water content [21] . The cracks create weakness zones in a soil mass causing reduction in the overall mechanical strength and increase in the compressibility [22] . Formation of cracks is also one of the important factors effecting hydraulic conductivity of soils. An increase in hydraulic conductivity results in an increase in the rate of transportation of pollutants in the soil.

Crack formation is a natural process observed in clayey soils as a result of decrease in water content. Among the factors effecting crack formation, grain size, temperature, initial water content, thickness and surface characteristics of base material on Ankara Clay [23] as the following results:

1) As the thickness or grain size of the samples increased, the surface area of cracks decreased.

2) As the water content and temperature decreased, the surface area of cracks decreased.

3) As the friction between clay and base material decreased, the surface area of cracks increased.

The black-sand beach deposits are known as a source of strategic and economic heavy minerals that are considered as raw materials for nuclear industry. Besides, they are considered of thorough importance in many of the metallurgical and engineering industries. The black-sand lens in the present study along the shoreline extends for about 1.5 km along the Red Sea coast, with a width varies that from 3 to 20 m and a thickness which changes from 0.4 to 1.0 m (Figure 10(f)).

The observed patterns of gamma-ray spectrometry are reflections of their radioactive property of normal constituents in rocks. The measured radio spectrometric data were treated statistically to determine the distribution characteristics of the three radioelements; K in %, eU in ppm, eTh in ppm and their activities in different Quaternary sediments in the study area (Table 5 and Figure 12). Contour maps also enable to define the mean absorbed dose rate (D), annual dose effective equivalent (AEDE) and external hazard index (H_ex) (Figures 13-15), respectively.

The comparison of K, U and Th specific activities registered by the present study and worldwide mean values Table 5 and Figure 12(a) show that wadi sediments, coastal-eolian sabkhas, sandy conglomerates, sand dunes and sheets and mud cracks have lower contents than permissible Worldwide. Meanwhile, the black-sand lens has higher eU and eTh specific activities than permissible Worldwide related to monazite and zircon [33] but it has low K contents.

The results of the statistical treatment, as illustrated in Table 5, reveal that, the values of mean absorbed dose rate, annual dose effective equivalent and external hazard index for the wadi sediments, costal-eolian sabkhas, sandy conglomerates, sand dunes and sheets and mud cracks remain in safe side and within maximum permissible safe radiation dose rate, without harm to the individual (Figure 12(b)). Meanwhile, the absorbed dose rate of the black-sand lens attains 100.77 nGy/h, which exceeds the world average value of 54 nGy/h [27] related to the enriched radioactive mineralization monazite and zircon [33] and remains under the world average value for the annual dose effective equivalent and the external hazard index (Figure 12(b)). So, the inhabitants will receive a relatively high radioactive dose related to monazite and zircon.

A detailed geologic map (scale 1:100,000) of the different landcover units of Wadi Diit area was obtained by the analysis of Landsat 8 OLI satellite data. Maximum likelihood supervised classification followed by Majority- Minority analysis was applied successfully (with 89.51% overall accuracy) based on selected ROIs representing the different lithologic units. This shows the potential of remote sensing technique for geologic mapping in hard accessible environs like the study area. The integrated work between image processing of Landsat 8 data and detailed field investigation show that the exposed rock units at Wdi Diit fan are represented by unconsolidated wadi sediments, costal-eolian sabkhas, sandy conglomerates, dunes and sheets, mud cracks and black-sand lens. sand

The absorbed dose rate (D), annual effective dose equivalent (AEDE) and external hazard index (H_ex) from the terrestrial gamma-radiation of Quaternary sediments were found to be within the recommended international limits, with no significant health threat to human lives and, therefore, the environment is said to be radiological hazard safe.

From the results of the present work, the authors recommend that the inhabitants of Adilldat and Shakret El-Delam villages at Wadi Diit fan can spread horizontally through the fan, except the black-sand lens, because it is considered as one of the most safety environments along the Red Sea coast. On the other hand, black-sand lens along the Red Sea coast of the studied area is not recommended to be used in building materials, because the absorbed dose rate of the black-sand lens attains 100.77 nGy/h that exceed the world permissible value.

The authors would like to express their gratitude to Prof. Dr. Ahmed A. Ammar, Emeritus Professor of applied geophysics, Exploration Sector, Nuclear Materials Authority of Egypt, for constructive critical revising of the manuscript for his revisions and fruitful discussions. They also, cordially thank the reviewers of this work for their comments and recommendations.

Ahmed E. Abdel Gawad,Atef M. Abu Donia,Mahmoud Elsaid, (2016) Processing of Landsat 8 Imagery and Ground Gamma-Ray Spectrometry for Geologic Mapping and Dose-Rate Assessment, Wadi Diit along the Red Sea Coast, Egypt. Open Journal of Geology,06,911-930. doi: 10.4236/ojg.2016.68069