# Reconstruction of the Subsurface of Al-Hassa Oasis Using Gravity Geophysical Data
## 4 Results  
Figure 2 presents the mapping of the resulting complete Bouguer anomaly (CBA). The CBA anomalies range from 20.893 to 24.276 mGal. It is shown that the southwestern part of the study area is dominated by lower CBA values than the rest of the area, whilst the highest values are measured in the southeastern and northern parts of the area under investigation. Figure 2: The complete Bouguer anomaly map of the Al-Hassa study area. The white points correspond to the locations of the gravity stations. Figure 3 shows the residual gravity field. In the residual field map, which shows the influence of shallow structures on the gravity field, the anomalies range from \(-1.643\) to \(1.498\) mGal. This indicates a measured signal with a cumulative amplitude of \(3.141\) mGals. Here, it is important to note that the gravitometer has a precision of \(0.001\) mGal. The residual gravity field indicates regions of low gravity anomalies (lower densities) in the southwestern area and locally, in some spots supported by several stations, in the north, northeast, and central part of the study area, represented by the dark-blue color. Conversely, areas of high gravity anomalies, depicted by the colors red and pink, are found in the northwestern and southeastern parts of the study area. The rest of the study area is characterized by mid-to-low gravity anomalies, typically ranging from \(-0.4\) to \(0\) mGal, represented by green and light-yellow colors. Figure 4a shows the TDR map generated using the residual gravity field. The TDR's contours at zero-value were employed to analyze the subsurface lineaments depicted as black lines. The dip direction of the lineaments is also indicated in Figure 4a, represented by the direction of the semi-circles. The subsurface lineaments can be considered as lateral geophysical (density) discontinuities and with low uncertainty can be assumed as fracture zones. Based on this assumption, all lineaments were segmented in equal lengths, and their orientation was recorded and used to prepare the rose diagram in Figure 4b using the GeoRose v0.4.3 software ([[http://www.yongtechnology.com](http://www.yongtechnology.com)]([http://www.yongtechnology.com](http://www.yongtechnology.com)), accessed on 17 January 2024). Figure 3: The residual gravity-field map with the locations of the gravity stations (white points). The black rectangle indicates the boundaries of the 3D modeling area. Figure 5, displays the terracing transformation map, resulting in an improvement of the subsurface characteristics, suggesting the presence of similar areas with regards to gravity response. The depth solutions obtained from the tilt-depth method are displayed as colored circles according to their values over the residual gravity field (Figure 5). A 3D inversion was applied to the gravity data, resulting in the construction of a voxel-based 3D representation of the subsurface density anomalies. This representation depicts the density contrast, which can be either positive or negative, as illustrated in Figure 6a. The area was divided into a mesh consisting of 41 \(\times\) 43 \(\times\) 12 voxels in the X (easting), Y (northing), and Z (depth) directions, with a resolution of 500 m. The inversion reached a maximum depth of approximately 4.2 km. To emphasize the presence of bodies with a low-density contrast, we employed upper and lower boundary values of 0.5 g/cm\({}^{3}\) and \(-\)0.5 g/cm\({}^{3}\), respectively, in the inversion procedure for the visualization of the 3D gravity model. The negative (green and blueish color) density anomalies are between \(-\)0.002 and \(-\)0.48 g/cm\({}^{3}\) (Figure 6b). Figure 4: (**a**) The tilt derivative and the interpreted lineaments (black lines), which correspond to structural edges (tectonic lineaments and/or rock contacts), are presented. (**b**) Rose diagram of the interpreted lineaments (insert into the Figure with the tilt derivative and the lineaments). Figure 5: The terracing map overlaid by the results of the tilt-depth method (shown as colorful dots).