Deep cavity systems detection in Al-Ain City, UAE, based on gravity surveys inversion

Sinkholes and subsurface cavities can pose a real challenge to the stability of engineering constructions due to the risk of ground collapse and subsidence. The city of Al-Ain (UAE) is known to have several geo-hazards related to shallow sinkholes and karst cavities that have been reported in the locally outcropping formations. In this study, a micro-gravimetric survey was carried out around Al-Ain city in order to explore the subsurface density distribution of the karstified formations. A total number of 452 gravity measurements, covering an area of approximatively 1600 km² were used to invert the 3D density distribution to a depth of 1500 m. From this inversion, we have distinguished seven abnormally low density bodies ranging in size from 75 m to 750 m within the cavernous limestone Asmari Formation, consistent with a complex deep cavity system. The discovered cavities follow the local structural trend along the NW-SE direction, and lie within the high-risk areas of the geohazard map of Al-Ain city, in zones considered unstable in former geotechnical studies. The likely origin of the cavity systems is the dissolution of bedrock in water circulating in the limestone formations, along a connected shallow and deep fracture network. Further geophysical studies will be required to adequately map and investigate each deep cavity system, and to distinguish between the deep cavities and the shallow cavities reported in the area.