## 3 Results and Discussion  
### Two-Dimensional Resistivity Tomography Data  
To tie the inverted 2-D resistivity tomograms with the available lithological and hydrogeological information, previous work conducted by different authors, for example [16; 20; 22; 24; 25], have been correlated with 2-D inverted resistivity tomograms. Using an iterative smoothness-constrained least-squares inversion method [37; 38], apparent resistivity data collected by the 2D-DC resistivity system were inverted using AGI Earthmager 2D Software (after Advanced Geosciences, Inc.) to create a subsurface resistivity model that approximates the actual subsurface resistivity distribution [38]. Figure 7 shows the inversion of profile P-6 (see Figure 4 for location), where the resistivity field-data is presented on the upper part of the section, resistivity models are presented on the middle part of the section, and synthetic-data inversions are shown on the lower part of the section. Figure 6: Diagram of an EM38 m showing the principles of operation. Investigations of the constructed 2-D resistivity tomograms along with the data from the previous work conducted by different authors at Al-Ahsa Oasis, as well some soil pits dug by local farmers, helped in the interpretation of the resistivity tomograms and in the mapping of the near-surface condition of the soil along the study area which has its impacts on soil characterization. Figure 8 shows the combined tomograms with their locations across the Al-Ahsa Oasis; vertical and lateral variations of different lithological units are recognized. Referring to Figure 8 and investigations of these 2-D Tomograms of the location of both of Al Asfar and Al Uyoun lakes, it is found that the main three identified geoelectric-lithologic units along these tomograms are:  
Figure 7: Two-dimensional resistivity section for the location P-6. The upper section is the measured resistivity (**A**), the middle section is the calculated resistivity (**B**), and the lower section is the inverted resistivity (**C**) (see Figure 4 for location). Layer-1 is a topsoil zone of loose to weakly consolidated sand. This zone corresponds to the upper part of the very resistive layer. The resistive nature of this layer is indicative of dry conditions in the upper part of the measured 2-D transects. However, and referring to Figure 8, it is noticed that resistivity values of this layer have relatively high values (above 10 Ohm-m) along 2-D transects of profiles 1, 10, and 7 (P-1 to P-10, see Figure 4 for locations); these are attributed to both dry conditions and the coarse grains sand at these locations. On the other hand, this layer at the 2-D transects of profiles 1, 10, and 2 (P-1, P-10, and P-2, Figure 8) have relatively low values due to the effect of Sabkha and clayey soil and the effects of the existence of both the Al Yuoun evaporation lake at the north of Al-Ahsa Oasis and of the Al Asfar Lake at the east of the Oasis. These match well with the results of [PERSON] et al. and [PERSON] et al. [27; 39]. Referring to Figure 8, it is worth mentioning the alternating of high and low resistive parts at this layer (see, for example, P-9 at a distance of 30-50 m, Profile 10 at a distance of 60-75 m, and Profile 3 at a distance of 90-105 m. This indicates the inhomogeneity of this layer and variations of irrigation practices at the same farm (part of the farm is cultivated by vegetables and the other is date palm). Moreover, some farmers transport soils from other places to some locations at their farms. Layer-2 has a moderate resistivity range of (2 to 10 Ohm-m). This layer exists at different depth intervals (for example, 2-D transects of profiles P-4, P-5, P-6, P-8, and P-9, Figure 8). This layer is interpreted as a partially saturated layer, which makes its resistivity values less than of the topmost soil layer.