Investigation of cavities and karstic features at Al-Amal city, Qattamiya, Egypt by using 3-D inversion of VES data

摘要

In the past few years, construction extended extraordinarily to the southeast of Cairo, Egypt, where limestone caves occur. The existence of caves and sinkholes represents a hazard for such new urban areas. The state is planning to construct a new housing community there. Therefore, it is important to map and delineate these natural voids and cavities before construction.Recently, cavity imaging by means of geophysical surveys became more common. In this paper, DC resistivity survey had been applied at Al-Amal city, east of Qattamiya, Cairo to image the shallow subsurface and possibly karstic features and cavities. Thirty-two vertical electrical soundings were measured using the common Schlumberger array of electrode configuration with a maximum AB/2 spacing of 200 m. A 3-D inversion scheme was applied to the measured data. The results of VES data inversion subdivided the shallow subsurface section in the study area into three main geoelectrical units. The uppermost unit composed of sand, gravel and fossiliferous limestone with high resistivity values and small thickness. The second unit is composed of sandy clay and clay with low resistivity values. The third unit is considered to be the bed rock, where it consists of consolidated sand, limestone and marl with relatively high resistivity values.The results elucidated many geoelectrical variations which could be interpreted as litho-facies variations. It was also revealed that, the area is dissected by some normal faults affecting the second and third geoelectrical units. In addition,numerous cavernous structures could be elucidated at shallow to intermediate depths. These features are too important to be taken into account during the housing development plans.