Environmental hazard analysis of a gypsum karst depression area with geophysical methods: a case study in Sivas (Turkey)

In the study area, the geophysical properties of gypsum and filling 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 filled 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 fill and cover unit areas where water did not affect or was less affected, 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, average > 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 filling 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.