Detecting cavernous features in Al Kharj, Saudi Arabia: advancements in TEM and ERT geophysical measurements

The cavernous features are considered one of the geological hazards that are dominant in the carbonate rocks as a results of dissolution effect of groundwater. The dissolution process extends for many years at the subsurface layers and causes catastrophic collapse if migrated to the surface layers. The problem become crucial in the urbanization areas where there are wide varieties of construction activities. Beside the natural weathering process, the human interventions, representing in the wastewater, can accelerate the weathering process of the prevailing cracks. Al Kharj governorate is one of the promising areas close to Riyadh, the capital of Saudi Arabia, that attracts wide aspects of urbanization, agriculture and industrial activities. The study area is located inside an urban neighborhood at the western part of Al Kharj city. The site is surrounded by buildings extensions from three sides and the fourth side was excluded from urbanization activities due to existence of surface cracks along the area especially at the northern direction. Excavations and boring tests reveal that the exposed cracks are extended downward and may be connected to subsurface karstic features in the limestone rocks. Two surface geophysical measurements have been planned to provide clear information about the locations, horizontal and vertical extensions of the cavernous features along the study site. The Time Domain Electromagnetic (TEM) and Electrical Resistivity Tomography (ERT) methods were applied for investigating the subsurface characteristics up to 40m depth with different resolutions. The data have been acquired along profiles path across most of the surface fractured features. In spite of the inverted TEM data representing the subsurface resistivity variations in 1D mode, the dense TEM stations and the direct comparing the results with the available borehole lithology, enabled to trace the subsurface cavernous features laterally and vertically with reasonable resolution. Moreover, the 2D ERT processed data have been compared with the TEM results along some localities for guiding the data interpretation to locate the subsurface cavernous features. The two methods are analyzing the subsurface in two different resolutions, the TEM can explore the deeper parts and the layer successions, whereas the ERT methods can provide the lateral extension tracing of cavernous features. Based on the results 3D fence diagrams, most of the cavernous features that are located along the