# Reconstruction of the Subsurface of Al-Hassa Oasis Using Gravity Geophysical Data
## 5 Discussion  
A detailed and extensive gravity survey was applied over the Al-Hassa Oasis to characterize the subsurface and reveal any tectonic or geological information. The data possessed significant value, and their analysis aided in my comprehension of the study area's intricacy and distinctiveness. The residual gravity field (Figure 3) indicates the influence of shallow structures on the recorded signal. It reveals low gravity anomalies in the southwestern part of the study area and some other areas, namely in the north, northeast, and central parts of the area under investigation. Low-density anomalies (blue and dark-blue colors) are usually represented by fractured/eroded rocks, semi-cohesive sediments, sedimentary rocks, and, sometimes, loose sediments. On the other hand, areas of high gravity anomalies, depicted by the colors red and pink, are found in the northwestern and southeastern parts of the study area. Hard sediments and rock formations usually produce such high anomaly densities. The rest of the study area is characterized by mid-to-low gravity (density) anomalies, typically ranging from \(-0.4\) to \(0\) mGal, represented by green and light-yellow colors. Based on the above, the low-density anomalies can be considered as deep basins where only the area they cover can be calculated. Other information, such as the thickness of the sedimentary basin, can be extracted from other results. The high-density anomalies appear next to the basins, with very sharp NNW-SSE and W-E contacts. Assuming that the lateral density change can be considered as a fault and considering the proximity of the study area to the Ghawar anticline, the high-density anomalies are probably uplifted rock formations. The previous assumption regarding the N-S or NNW-SSE basins and the E-W discontinuities was verified by utilizing the enhancement filters and the produced map showing the TDR, seen in Figure 4a. The lines depicted on the TDR map could be interpreted as either tectonic lineaments or geologic contacts. The dip direction is also specified (Figure 4a). The lineament analysis identified the borders of the geological structures and emphasized the regions where basins (characterized by low density and appearing blue) of the predicted north-south and west-east general orientation are formed. A rose diagram is created to interpret the lineaments (Figure 4b). All linear features (black lines in Figure 4a) were segmented into \(100\) m length lines, and the strike for each one was recorded. From the total number of linear segments, the rose diagram was constructed where the dominant lineaments in NNW-SSE, NNE-SSW, and E-W orientations were determined. Although those orientations are depicted from the subsurface, they show a good agreement with surface features documented by [PERSON] et al. [21]. The tilt-depth approach was utilized, and the terracing map was estimated to enhance comprehension of the formation and progression of the basins (Figure 5). This investigation presented data on the depths at which geologic contacts and/or tectonic structures are anticipated and provided useful insights for subsurface modeling. The structural basin in the southwestern area of Al-Hassa Oasis is situated at significant depths, with the basin rim being approximately \(200\) m below the ground surface and extending to a depth of almost \(1.8\) km. The rest of the basins are shallower. The obtained data were subjected to 3D inversion to incorporate and portray the complexity of the research area, including changes in density, both in-depth and laterally. Figure 6a displays regions with low density (shown by blue colors) and high density (indicated by pink colors). The low-density structures are observed in a north-south pattern. This is much clearer in Figure 6b, where only the low-density anomalies are shown, represented by the sedimentary basins covering the study area. The figure shows that almost all three basins are formed in the main N-S orientation.