# Reconstruction of the Subsurface of Al-Hassa Oasis Using Gravity Geophysical Data
## 1 Introduction  
The Al-Hassa region in Eastern Saudi Arabia is well-known for being home to the largest oasis in the world and the most extensive naturally irrigated lands on a global scale [1]. Al-Hassa is located next to Ghawar, which is the largest conventional oil field in the world. Due to its historical dependence on more than 280 natural springs, some of which provide warm water for agricultural irrigation [2], the area's diverse water quality indicates the existence of intricate underground geological formations that require careful investigation. Prior research on the hydrochemistry and hydrogeology of the Al-Hassa region provides a thorough comprehension of the intricate subsurface conditions in the study area. [PERSON] et al. [3] studied the hydrogeology of the Umm Er Radhuma aquifer. Subsequently, the authors [PERSON] and [PERSON] [4] assessed hydrochemical facies in the same aquifer. [PERSON] [5] emphasized the hydrogeological and hydrochemical aspects of groundwater, specifically highlighting the risks of seawater intrusion. In their study, [PERSON] and [PERSON] [1] examined the hydrogeology of the Al-Hassa springs and predicted thefuture availability and quality of water. They concluded that if the current water extraction rates continue, the situation will become increasingly dire. In his 2009 study, [PERSON] [6] introduced a spatial modeling approach to analyze changes in groundwater salinity over time. [PERSON] [7] and [PERSON] [2] expanded upon this by assessing groundwater suitability for irrigation purposes and analyzing the elemental compositions. [PERSON] et al. [8] analyzed the quality of the groundwater, while [PERSON] et al. [9] provided further information on the geological aspects of the Umm Er Radhuma aquifer. In their recent studies, [PERSON], [PERSON], [PERSON], et al. [10] and [PERSON], [PERSON], [PERSON], et al. [11] utilized advanced methodologies, such as machine learning and spatial mapping, to examine trace-element pollution and nitrate concentrations, respectively. These studies provided state-of-the-art findings on groundwater susceptibility and pollution in Al-Hassa. The Ghawar oil field was first discovered in 1948 and began production in 1951. This extensive oil field is projected to maintain its current maximum production capacity of 3.8 million barrels daily until at least 2050. Ghawar is the primary supplier of associated gas for Aramco, and the greater Ghawar area also contains several significant non-associated gas fields. The genesis of the prolific Ghawar oil field can be traced back to the Amar Collision, occurring approximately between 640 and 620 million years ago (Ma). This collision initiated a series of structural events across the Arabian Plate, giving rise to distinct north-, northeast-, and northwest-oriented structural patterns, such as the En Nala Anticline, Qatar Arch, and Khurais-Burgan Anticline [12; 13]. After this, an extensional regime between about 570 and 530 Ma catalyzed the development of the Najd fault system and engendered the formation of northeast-trending rift basins, laying the foundation for the creation of major subsidence zones during Precambrian and Early Cambrian times [12; 14; 15; 16; 17]. During this period, the proximity of the Arabian plate to the Equator [15] facilitated the deposition of extensive salt layers within these basins, including the Hormuz, Ghaba, Fahud, and South Oman salt basins [12; 18]. Notably, the Hercynian Orogeny during the Devonian period was pivotal in reactivating pre-existing basement structures, thereby generating prominent north-south oriented host blocks across Central and Eastern Saudi Arabia. These geological events and processes cumulatively contributed to the formation of structural traps favorable for hydrocarbon accumulation, amongst which Ghawar stands as a preeminent example [13]. Geophysical investigations are a set of techniques that are used to analyze and describe the properties of materials in the subsurface.