Vol.:(0123456789) 1 3 Environmental Earth Sciences (2020) 79:115 https://doi.org/10.1007/s12665-020-8861-4 ORIGINAL ARTICLE Environmental hazard analysis of a gypsum karst depression area with geophysical methods: a case study in Sivas (Turkey) Sevda Özel 1  · Naşit Darıcı 1 Received: 26 June 2018 / Accepted: 14 February 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract In the study area, the geophysical properties of gypsum and flling materials, karst structures that developed in gypsum, and groundwater level were investigated. According to the results of geophysical studies, it was thought that the depression and dissolution/weathering areas in the karst could be completely or partially flled with water, air, gypsum particles, and one, several, or all of the alluvial units. It was determined that resistivity and S-wave velocity (average < 26.4 Ω m, ~ 220 m/s) in these areas were low and corresponded to low seismic velocity zones (LSVZ). In fll and cover unit areas where water did not afect or was less afected, resistivity and S-wave velocity (< 67.3–131 Ω m, average < 245 m/s) were determined low. It was found out that resistivity and S-wave velocity of the gypsum bedrock was very high (> 10,263–14,412 Ω m, aver- age > 655 m/s), and resistivity and S-wave velocity of LSVZ in the gypsum surfaced far from the source were lower (< 91.3 Ω m, ~ 585 m/s) compared to the gypsum bedrock. Therefore, it was observed that the resistivities and seismic velocities of the gypsum in the surfaced area were higher compared to the depression area. The results were associated with karst as the main reason for dissolutions and depressions, because the water draining/leaking/spreading from the water source progressed by creating low-resistivity areas in the permeable flling material and the groundwater was near the surface. Therefore, karst structures will probably continue to develop in this area, and natural hazards may occur in the future. Keywords Geophysics · Gypsum · Karst · Depression area · Natural hazard Introduction Karst structures can be analyzed as 1D (one dimension), 2D (two dimension), and 3D (three dimension) by determin- ing diferent physical/geometric parameters (such as elec- trical resistivity, seismic velocity, cavity limits, geological layer geometry (thickness, depth, length, and slope), and groundwater level) using geophysical methods. Especially nowadays, geophysical tomography methods are largely used for this purpose. These are direct current electrical resistiv- ity tomography (ERT), seismic multi-channel analysis of surface wave (MASW), and electromagnetic ground pen- etrating radar (GPR) methods applied in the tomography technique. Along with the development of multi-channel or multi-electrode electrical resistivity measurement systems among them, cavities in karst and other possible karst struc- tures are successfully detected by the ERT method (Hamdan et al. 2012). Possible structures in gypsum can also be inves- tigated by applying the vertical electrical sounding (VES) technique instead of ERT to study deeper vertical changes (discontinuities) (Duvarcı 1994; Tanıdır and Karlı 1996; Manoutsoglou et al. 2010). For this purpose, the Schlum- berger electrode array is preferred in taking measurements with the VES technique. In the ERT method, the Wenner, Wenner–Schlumberger, and dipole–dipole (DD) types of arrays are mostly preferred in shallow investigations. The MASW method, one of the seismic tomography methods, is also very successful in gypsum environments (Xavier and Abraham 2000; Grandjean and Leparoux 2004; Xu and Butt 2006). The GPR method, which operates at high frequen- cies, is commonly used to search for karst structures near the surface (Ulugergerli and Akça 2006; Darıcı 2015; Darıcı and Özel 2018). With the ERT, MASW, and GPR meth- ods, cavities, fractures, cracks, and dissolution/weathering areas developing in gypsum have become the subjects of study of many researchers (Toshioka et al. 1995; Cardimona * Sevda Özel sozel@cumhuriyet.edu.tr 1 Department of Geophysical Engineering, Engineering Faculty, Cumhuriyet University, Campus, 58140 Sivas, Turkey