3D Geological Modelling of the Subsurface Adjacent to Cementa’s Quarry in Skövde, Sweden

Abstract: Limestone is one of the main components of cement production. Limestone has been quarried in Skövde, Sweden, since the end of 19th century and Cementa AB has been operating the quarry since 1973. Aside from limestone, there are also Alum shale of Cambrian age, mudstones as well as bentonite layers of Ordovician age present in the quarry.  The production of cement evidently is important for Sweden’s infrastructure, and the quarry in Skövde is one of few known locations in the country with limestone with the right composition. Therefore, it is important to increase the knowledge regarding the character of the limestone to make accurate predictions for the future regarding the cement production. The geological knowledge of the area is already extensive; however, the aim of this thesis is to expand this knowledge further by constructing a 3D geological model. The data which has been used to construct the model are field observations, drill core data, chemical data, high-resolution pictures (photogrammetry) and resistivity measurements (field and samples). The resistivity measurements were done using the multiple gradient array, and apparent resistivity was inverted using Res2Dinv. The geological modelling was done using Leapfrog geo (© Seequent Systems, Incorporated). Two models have been proposed as a result of this project; one where high-grade limestone of lesser quality and whitestone has been regrouped with two other units (A) and one including all units (B). In the most recent drilling campaign, the nomenclature used to distinguish the units has been modified and does not differentiate high-grade limestone of lesser quality and whitestone from the rest. This affects coherence of the model and for this reason two models have been built. Both models show roughly flat lying units in the area of interest. Both low-grade limestone and lower waste stone units have consistent thickness in both models. The major difference between the models is how the high-grade limestone unit is modelled as a consequence of the additional units in model B. The result from the resistivity measurements shows unexpectedly low values, when compared to values from the literature as well as the measurements on hand samples from the quarry. The reason for these low values is still unclear, and therefore resistivity data has been used with caution. Considering this, it appears that resistivity measurements is not a suitable technique to characterize the subsurface in this particular area.  The models produced in this project provides information regarding thickness and extent of the units and overlying soil. As such, the new knowledge can be used to plan future prospecting campaigns, make projections, and estimates within current mining permits and evaluate how future mining can be conducted.