Electrical resistivity tomography and induced polarisation techniques applied to the identifiaction of gypsum rocks

Abstract

Gypsum deposits are the currently exploited sulphate rocks with industrial purposes. In addition to the expensive drilling projects, geophysical techniques can be considered to estimate the economical potential of these deposits. An electrical resistivity tomography survey has been carried out in the Pira gypsum formation (SE of the Catalan margin of the Tertiary Ebro Basin, Spain). Additionally, a continuous coring drill was performed in order to support the study. Electrical imaging has been successfully applied to identify the gypsum deposits interlayered in lutite units. Nevertheless, the gypsum-lutite boundaries are diffuse and uncertain in the tomographic lines. Comparison of the cores of the borehole and the electrical response shows a meaningful correlation between electrical resistivity and purity of gypsum. The electrical resistivity tomography profiles display a rather wide range of electrical resistivity value (from 20-1000 Ωm) for the investigated gypsum facies. The highest values are attributed to sulphate layers with >90% of gypsum mineral. Lutite units display higher values than expected due to the presence of evaporite minerals within them (>10 Ωm). Additionally, induced polarization measures have been performed in order to study the chargeability of gypsum deposits. It has been evidenced that impure gypsum rocks (with important presence of lutites within) are slightly chargeable. This property has allowed distinguishing between lutite levels and clay-rich gypsum rocks. Electrical resistivity tomography lines are useful in the prospection of gypsum deposits. However, electrical imaging prospection should be supported by an accurate petrological study of the deposits, in order to properly interpret the resistivity profiles.