Detection of Cover Collapse Doline and Other Epikarst Features by Multiple Geophysical Techniques, Case Study of Tarimba Cave, Brazil

Abstract

Reliable characterization of the karst system is essential for risk assessment where many associated hazards (e.g. cover-collapse dolines and groundwater pollution) can affect the natural and built environments threatening public safety. The use of multiple geophysical approaches may offer an improved way to investigate such cover-collapse sinkhole and aid the geohazard risk assessment. In this paper, covered karst, which has two types of shallow caves (vadose and fluvial) located in Tarimba (Goias, Brazil), was investigated using various geophysical methods to evaluate their efficiency in the delineation of the geometry of sediments filled sinkhole. Three methods were used including Electrical Resistivity Tomography (ERT), Seismic Refraction Survey (SRS), Seismic Refraction Tomography (SRT) and Very Low Frequency Electromagnetic (VLF-EM) method. The study developed several resultant (2D) sections of the measured physical properties, including P-wave velocity and electrical resistivity, as well as the induced current (because of local bodies). For the analysis and processing of the data obtained from these methods, the following approaches were adopted: ERT inversion using a least-square scheme, Karous-Hjelt filter for VLF-EM data, and time-distance curves and Vp cross-sections for the SRS. The refraction data analysis showed three-layered stratigraphy topsoil, claystone, and carbonate bedrock, respectively. The findings obtained from ERT (three-layered stratigraphy and sediment-filled doline) as well as VLF-EM (fractured or filled caves as a positive anomaly) were found to be consistent with the actual field conditions. However, the SRS and SRT methods did not reach the depth of the cave because of shorter profile lengths. The study provides a reasonable basis for the development of an integrated geophysical approach for site characterization of the karst system, particularly the perched tank and collapse doline.