The electrical properties of calcium sulphate rocks from decametric to micrometric scale

Abstract

Sulphate rocks have a sedimentary evaporitic origin and are present in many deposits worldwide. Among them, gypsum (dihydrated calcium sulphate) is the most common and is exploited for industrial purposes. Anhydrite (calcium sulphate) is frequently found in gypsum quarries and in non-outcropping sulphates. The greater hardness of anhydrite compared to gypsum causes a problem for gypsum extraction; quarry fronts have to be halted as soon as anhydrite is found. In this work the electrical properties of calcium sulphates have been studied by means of geoelectrical methods. A direct relationship between the electrical conductivity values of the calcium sulphate rocks and their lithological composition has been established with the lutitic matrix being the main controlling factor when it is well connected. When the matrix is under the percolation threshold the sulphate phases are dominant, and the electrical response of the rocks depends on the percentage of each phase. When the rock is matrix dominant, the electrical resistivity trend fits with the Hashin- Shtrikman lower bound for multiphase systems (considering gypsum, anhydrite and matrix as the components). On the other hand, when the rock is calcium sulphate dominant the trend shows the one of the Hashin-Shtrikman upper bound. The reference electrical resistivity value of pure anhydrite rocks has been defined as 104 29 Ω.m and geoelectrical classification for calcium sulphate rocks has been elaborated. With this classification it is possible to differentiate between calcium sulphate rocks with different composition from their electrical resistivity value. This classification has been checked with field examples and calculating the theoretical resistivity value of thin section photographs with the program ELECFEM2D. The electrical behavior of calcium sulphate rocks is a good reference for other type of rocks with electrically differentiated components, and similar methods can be used to define their geoelectrical responses. 36 37 Keywords: Gypsum, Anhydrite, Sulphate, Electrical Conductivity