Porosity Estimation and Geometric Characterization of Fractured and Karstified Carbonate Rocks Using GPR Data in the Salitre Formation, Brazil

Abstract

Three-dimensional distribution of karstic zones and estimated porosity were obtained in fractured carbonate units of the Salitre Formation (Brazil) from GPR data. Low-amplitude (shadow) GPR zones could be associated with mineral dissolution halos that developed around fractures and bedding planes. GPR volumes show a 3D mazelike geometry of the vertical karstified carbonate zones, similar to the set of passages controlled by ~ N–S and E–W oriented fracture systems, observed on the surface in unmanned aerial vehicle images. Measurements in plug samples indicate that the mineral dissolution caused bulk density and dielectric permittivity decline and incipient increase in porosity from 0.95 to 1.01%. These changes in the rock properties affect the EM impedance contrasts between carbonate bedding, causing loss of the reflected GPR signal. The correlation between dielectric permittivity and porosity could be analyzed using the complex refractive index model (CRIM), which is used to convert the relative dielectric permittivity into porosity. The CRIM porosities are somewhat lower than plug sample porosities due to different estimation methodologies and measurement scales. However, the fact that porosity is slightly higher in the dissolved carbonates was maintained. In addition, GPR amplitude and frequency instantaneous attributes were used to estimate spatial variability in porosity throughout the whole GPR data volume. The crossplot of these two GPR attributes show that altered zones generally cluster in narrow ranges of instantaneous GPR amplitude and in broad range of instantaneous GPR frequency. Assuming empirical correlation between physical properties and GPR attributes two predicted porosity volumes were established for the karstified carbonates in the studied area.