applied sciences  
Applications of Electromagnetic Induction and Electrical Resistivity Tomography for Digital Monitoring and Assessment of the Soil: A Case Study of Al-Ahsa Oasis, Saudi Arabia  
[PERSON] 1,2,*2,*3, [PERSON] 3 and [PERSON] 4  
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techniques are becoming an important practical tool for many agricultural applications. Agricultural geophysical analyses are focused on delineating features over large areas within the soil profile (0 to 2 m depth) [5]. The most common geophysical methods predominantly used for agricultural purposes are direct current (DC) resistivity and electromagnetic induction. With the recent advent of precision agriculture, the ability to generate a large amount of information rapidly is becoming more critical than ever [5; 6]. Site infrastructure assessment, hydrologic monitoring, and environmental investigations are just a few current and potential uses of near-surface geophysical methods in agriculture applications. In addition, modern geophysical techniques are being incorporated within site-specific agriculture to detect soil horizons, soil compaction, soil salinity, mapping of hardpan layer, and soil texture [6]. Today, with the advent of precision agriculture which employs data from multiple sources to enhance crop yields and assess soil quality to improve the cost-effectiveness of crop management strategies, it is necessary to advance the agricultural geophysics methods for clay-pan depth determination, soil hydrologic characterization, soil nutrient monitoring, crop/tree root biomass surveying, identification of subsurface flow pathways, subsurface irrigation, drainage systems infrastructure detection, and soil compaction evaluation [7; 8; 9]. Electromagnetic (EM) instruments have been used extensively to determine the spatial distribution of soil attributes, including clay content [10; 11], moisture [12], and soil salinity [13; 14; 15; 16]. In addition, different studies have used EM instruments to determine the depth of a clay layer [17], soil types, and organic carbon fraction [18]. Al-Ahsa Oasis is located 70 km to the west of the Arabian Gulf, and it is also one of the main agricultural areas in Saudi Arabia (Figure 1). The cultivated area in the Al-Ahsa oasis is about 12,000 hectares. Groundwater was considered the main water source, if not the only source, for the different purposes of Al-Ahsa Oasis. As a consequence of the overexploitation of groundwater resources, a decline in groundwater levels is observed. The famous springs in the Al-Ahsa oasis are  
Figure 1: Location map of Al-Ahsa Oasis, Eastern Province, Saudi Arabia. running dry. Furthermore, deterioration of groundwater quality is occurring, caused by upcoming of deeper saline groundwater [19]. Due to groundwater depletion, treated wastewater is currently used to meet the shortage of irrigation water at Al-Ahsa Oasis. As a result, most groundwater is discharging from the Neogene aquifer. Today, due to excess overpumping, water depletion, and water deterioration, the signs of soil salinity at Al-Ahsa have become dominant [20; 21]. Water and soil are the most critical factors that determine the success of agricultural activity in any location based on their characteristics. Some excavations at Al-Ahsa oasis indicated hardpan layers at different soil depths. Generally, hardpan is a term for a dense layer of soil below the uppermost topsoil layer. The negative impacts of the hardpan layer are that it is mainly impervious to water, restricts root growth, and makes it difficult for air, water, other gases, and organisms to move through the soil. Therefore, determining the existence of the hardpan layer nature and depth of its presence is very important in crop selection [22].