# Reconstruction of the Subsurface of Al-Hassa Oasis Using Gravity Geophysical Data
## 3 Gravity Survey  
The gravity survey was carried out by the Geophysics lab of the King Fahd University of Petroleum and Minerals in three phases, taking place in the summer of 2021, the summer of 2022, and the spring of 2023. A total of 571 gravity stations were measured, at urban and semi-urban areas north and east of Al Hofufty, as well as within the Al Hassa National Park area. The data were collected in grids and profiles and cover an area of approximately 350 km\({}^{2}\), with varying station spacing (150-900 m) according to local conditions (gridded/profile data, remote/urbanized areas, accessibility, etc.). The characteristics of urban areas (e.g., roads, buildings, telecommunication cables, large metallic objects, buried pipes and cables, and sewers) do not allow for the usage of magnetic or electromagnetic methods, so the gravity method can be used to gain preliminary insights into the subsurface regime of such areas in a quick and cost-effective manner [34]. However, due to the complexity of these areas, there are limitations in the application of the gravity method as well. The measurements were acquired using two Scintrex CG-5 AUTOGRAVTM gravitometers, which have a high resolution of 1 uGal and use the optimum acquisition parameters depending on the natural and/or cultural noise of the area under investigation. We set the gravitometers to automatically detect and discard spikes caused by high external noise, as well as to consider the Earth's tide effect and the instrument's tilt during transportation and make the necessary corrections to the measurements. To avoid the extra minor noise caused by the high air temperature that is typical for the area during the summer months, the data were acquired during the morning hours and stopped before noon. The gravity measurements were taken in sets of three or five, with a duration of 30-60 s, according to the environmental or cultural noise in the nearby area. To ensure high-quality data, any set with a difference of more than 10 uGal was repeated with additional measurements. Stations with a tilt greater than 15 arcsecs were not used, and the gravity readings were repeated. The average standard deviation (SD) for all the acquired data was 0.037 mGal. Three points were selected to be used as base stations (one for each measurement period), which were located in a central location near the rest of the measurements, in a relatively quiet area (with no external noise sources). Each day, an opening and a closing measurement were taken on the base station. To estimate the gravitometer's height correction, the distance between the center of the instrument's mass and the elevation of the instrument's location were measured so that every measurement refers to the actual elevation of the topographic relief. The geographic coordinates and elevation of each station were measured by Trimble's R10 Differential GPS (Global Positioning System), with an accuracy error of less than 1 cm, using either a base station or the KSA-CORS (Kingdom of Saudi Arabia Continuously Operating Reference Station) network (for the measurements that cover wide areas within the city). The latter consists of a service provided for the entire country that increases the geographic and elevation accuracy of measured positioning. For urban areas, there is a need to minimize the influence of buildings and other relevant constructions on gravity measurements. The mass, height, and vibrations of a tall construction cause the so-called \"Building Effect\", which needs to be eliminated from the data by applying the \"Building Correction\", using information from detailed ground elevation datasets such as LIDAR [35]. A minimum distance of 75 m is proposed to exist between a large construction and the installed gravity station [35], while in many areas where a distance like this cannot be achieved, smaller distances might be used, up to 30 m [36].