# Reconstruction of the Subsurface of Al-Hassa Oasis Using Gravity Geophysical Data
## 3 Gravity Survey
#### Gravity Data Processing  
After their acquisition, the data were imported to Seequent's Onsis montaj v2023.2 software for processing and enhancement. For the corrections to the raw data, the Gravity and Terrain Correction extension was used; a useful tool integrated within Oasis montaj that handles gravity data obtained from ground surveys. The common gravity corrections of instrumental height and drift were applied to eliminate the factors affecting the measurements' quality. Since no absolute gravity station was available to use its coordinates and elevation for the latitude correction, the base station of the 2021 survey was used as the reference point, and all gravity measurements were tied based on that point. Earth's normal gravity was calculated using the 1980 Geodetic Reference System (GR80) [37] so that the theoretical gravity value of the 2021 base station location was considered equal to zero. Topographic highs and lows strongly affect the local gravity field, and this dependence should always be considered when processing gravity measurements [36]. The effect of the local topography was reduced after applying the topographic correction using the Oasis montaj standard algorithm. To run the terrain correction algorithm, it is necessary to import a detailed and coarser Digital Elevation Model (DEM) or gridded elevation data for the area of the survey and the wider area, respectively. The software will produce a new grid that represents the terrain corrections; thus, the correction at each gravity station location can be accurately calculated using the methods described by [PERSON] [38] and [PERSON] [39]. In our study, Shuttle Radar Topography Mapping (SRTM) at a 30 m resolution was used as the local elevation model, and SRTM at 90 m was used as the regional elevation model. The elevations of each gravity station were measured with high accuracy using the Trimble R10 Differential GPS system. The Free Air and Bouguer corrections were applied to the corrected data to determine the complete Bouguer anomaly (CBA). This anomaly considers the influence of extensive and deep-seated regional structures, as well as shallower local bodies. Adjustments to this overarching workflow may be necessary based on the specific goals of the study and the nature of the survey [34]. In order to separate the influence of the shallow structures, we obtained the residual gravity-field map by calculating and subtracting a first-order polynomial that simulates deeper sources (regional field) from the CBA field. To continue the process of identifying subsurface geotectonic structures, such as rock contacts and faults, that have a significant difference in density and to compare the results of the gravity analysis with the geology of the study area, we utilized the terracing transformation and the tilt derivative (_TDR_) filters on the residual field data. The technique of terracing transformation, as described by [PERSON] and [PERSON] [40], can identify distinct regions of uniform gravity-field intensity with well-defined boundaries [41]. To produce the terracing transformation map, we run 30 iterations of the terracing algorithm using Oasis montaj software. The zero contours of the _TDR_ can be utilized to enhance the structural edges [42; 43]. This technique can also improve the signals of gravity anomaly sources that are deeper or weaker. The calculation of _TDR_ can be determined using Equation (1):  
\[TDR=\arctan\left(\frac{\frac{\partial G}{\partial z}}{\sqrt{\left(\frac{ \partial G}{\partial x}\right)^{2}+\left(\frac{\partial G}{\partial y}\right) ^{2}}}\right) \tag{1}\]where \(\partial G/\partial x\) and \(\partial G/\partial y\) are the first-order derivatives of the gravity field in the \(x\) and \(y\) directions, and \(\partial G/\partial z\) is the first-order vertical derivative. It was applied to the residual gravity-field data to define the locations of structures' edges corresponding to faults and/or vertical/sloping rock contacts that act as the causative anomaly sources.