## 4 Conclusions  
Implementing electrical resistivity tomography (ERT) and modern electromagnetic induction techniques contributed to soil characterization salinity and mapping hardpan layer problems at Al-Ahsa Oasis, Eastern Province, Saudi Arabia. The tomograms of ten acquired 2-D resistivity profiles and data of the excavated soil pits judged by local farmers indicated the different geoelectrical layers. Information about the depth thickness of these geoelectric layers, including the topmost soil, was given. In some of the 2-D tomograms and at shallow depths, the hardpan layer was depicted, and this layer was composed of marl, clay, mudstone, or shale. This layer directly affects the level of soil water. Thus, the accumulation of salt in some locations results in some of the sites affected by the salt composed sabkha, especially if it is very shallow, which increases the soil conductivity. The 2-D tomographs measured at the southwest displayed resistivity values different to those those measured at the north and east of the study area, which were characterized by low resistivity values at greater depths due to effect of the sabkha and clayey soil and effects of both the Al Yuoun evaporation lake at the north of the Al-Ahsa Oasis and of the Al Asfar Lake at the east of the Oasis. The results of the twenty measured electrical conductivity profiles using the EM38-MK2 in the vertical mode were conducted at two different coil separations (1 m and 0.5 m) at the same locations of the measured 2-D profiles. The electrical conductivity results showed that the salinity variation along the measured profiles changed laterally and vertically due to changes in soil type/texture, soil moisture, and the existence of sabkha. The measured conductivity at most lines indicated high values of conductivities at coil separation of 1 m (greater depth); these profiles were located at the north and east of the study area, while only a few lines showed high values of conductivities measured at a coil separation of 0.5 m (shallow depth) compared to those at 1 m coil separation, these lines were located at the southwest of the study area. Moreover, it is found that the farms located in the southwest of the study area were characterized by low conductivities, while relatively high conductivities characterized the farms located to the north and east in the Al-Ahsa oasis. Thus, modern geophysical techniques proved to be efficient and effective tools for the nonintrusive gathering of continuous agricultural field data on many soil types, soil conditions, soil salinity, and near-surface conditions. Monitoring data about both soil and irrigation water was very important to prevent the degrading of agricultural soils in Al-Ahsa oasis. Degrading of agricultural soils will lead to poor agricultural productivity. The productivity limiting factors of these soils were mainly integrated into their chemical properties represented by the increased salinity. Such contribution provides technical support for planners, decision makers, and researchers in the field of agricultural sciences for ensuring agriculture sustainability in this Oasis.  
Conceptualization, A.E.M., M.M. and Y.A.; methodology, A.E.M.; software, A.E.M.; validation, M.M., A.E.M. and Y.A.; formal analysis, A.E.M.; investigation, A.E.M.; resources, M.M., A.E.M. and Y.A.; data curation, A.E.M.; writing--original draft preparation, M.M., A.E.M. and Y.A.; writing--review and editing, M.M., A.E.M. and Y.A.; visualization, M.M. and A.E.M. All authors have read and agreed to the published version of the manuscript.  
The authors appreciate the Deanship of Scientific Research, King Faisal University, and extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia, for funding this study through project grant no. 150220.