Imaging gypsum–anhydrite interface by a structure‐based conceptual resistivity model: A case study from Bala (Turkey) gypsum deposits

摘要

Abstract Principal calcium sulphate rocks, such as gypsum and anhydrite, originate from evaporitic processes. Gypsum is considered an industrial raw material with high economic value, but anhydrite has no economic interest. Therefore, delineating the interface between sedimentary cover and gypsum, and differentiating gypsum from anhydrite has significant importance in the production planning of open pits. An electrical resistivity tomography study was carried out in the Bala gypsum formation (Central Anatolia, Turkey) to map the gypsum–anhydrite interface. Because open‐pit mining is economical up to a certain depth due to the cost of excavation, delineation of the upper boundary of gypsum is also necessary. As the survey's primary purpose is to delineate the interfaces of the sedimentary cover–gypsum and gypsum–anhydrite layers in shallow depths (50 m), a structure‐based model search scheme that includes a sequential use of global and local search methods has been applied for the derivation of the subsurface resistivity model. The conventional cell‐based conceptual model using the damped least‐squares inversion method is also used for comparison purposes. The strong resistivity contrast leads to an easy identification of cover and gypsum for both conceptual models. However, only the structure‐based conceptual model delineates the interface between gypsum and anhydrite. The estimated interfaces derived from the structure‐based model are consistent with the borehole data. This case history for gypsum investigation indicates the value of selecting an appropriate conceptual model for a specific exploration problem.