# Reconstruction of the Subsurface of Al-Hassa Oasis Using Gravity Geophysical Data
## 2 Study Area--Geological and Tectonic Settings  
The study area is located about 60 km west of the Arabian Gulf, in the eastern province of Saudi Arabia, known for its prominent geological characteristics. It is situated between latitudes 25.351462 and 25.547711, and longitudes 49.557074 and 49.754603, covering an area of approximately 350 km2 (Figure 1a-c). The elevations in the area vary between 120 and 220 m above sea level. A sand layer covers the study area and lacks any extended outcrops that can support any geological/tectonic study. Therefore, no geological maps have been published that include information about the types of rocks in the studied area (Figure 1a). Within the study area, there is a spot where rock outcrops, particularly the Hoftu formation, were found. These rock formations would probably extend to other areas in the eastern province. The region is mainly characterized by sand dunes and Quaternary deposits, with occasional exposed sections that reveal the Upper Miocene to Lower Pliocene Hoftu Formation (Figure 1b). In general, there is a scarcity of published geological research in the area because it is located close to the Ghawar oilfield. Additionally, the groundwater wells in the area lack any information about the types of rocks present. The Hoftu Formation is distinguished by the presence of conglomerates and calcareous sandstones alternating with reddish mudstones and limestones [25]. The Hoftu Formation is located above the Dam Formation and consists of limestones and reddish marks. The Hadrukh Formation follows, consisting of sandstones, limestones, and marks [26]. Following that, the sequence comprises the dolomitic and limestone formations of the Eocene Dammam Formation. The underlying formations include the early Eocene Rus Formation, which is made up of dolomitic limestone and gypsum, and the Paleocene Umm Er Radhuma Formation, which contains limestone and dolomite with quartz geodes and chert nodules [26; 27]. The Neogene sedimentary sequence in the Arabian Gulf area is distinguished by an elongated NW-SE asymmetrical basin called the Zagros Syncline, located within the East Arabian Block [26; 28]. The basin area is thought to have diminished during the Neogene period, likely due to a global increase in sea levels during the Mid-Oligocene period, followed by the upward movement of the Earth's crust from the Oligocene to the Miocene epochs [29]. The Hoftu Formation is formed due to the Neogene development of the East Arabian Block. This is supported by features such as grapes, strike-slip faults, and transpressive folds, as documented by [PERSON] and [PERSON] in 1978 [30], [PERSON] et al. in 1991 [31], and [PERSON] in 1999 [29]. Dome formation in the Arabian-African Shield occurred from the Late Cretaceous to the Eocene, while rift formation and the separation of Africa from Arabia occurred during the Late Eocene to the Oligocene. Tectonic activity occurring from the Late Miocene to the Pliocene caused the Red Sea floor to spread, which resulted in the drainage of the uplifted Arabian Shield and the deposition of substantial clastic sediments. This process led to the formation of the Miocene Hudrukh and Mio-Piocene Hofu' formations [25]. The topography of the region, which slopes towards the east, allowed for the movement of sediment from west to east because of erosion of the underlying bedrock. Hydrogeologically, the area is recharged by the mountainous western part of Saudi Arabia, where fresh groundwater flowing to the east and northeast is observed in many  
Figure 1: **(a)** The coverage of Saudi Arabia by geological maps released by the Saudi Geological Survey is presented by the colored rectangles. A thick black square depicts the location of the study area. (**b**) Geological map of the wider area (modified from [PERSON] [32]). A red rectangle shows the location of the caves map in Figure 1d. (**c**) The study area with all the acquired gravity geophysical measurements is presented. The camera icon shows the location of the photo shown in Figure 1f.