Imageamento GPR e medidas petrofísicas de rochas carbonáticas carstificadas da formação salitre, norte da Bahia

Abstract

Three-dimensional distribution of karstic zones and estimated porosity were obtained in fractured carbonate units of the Salitre Formation (Northern of the Bahia State) from GPR data and petrophysical. Low-amplitude 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 with the set of passages controlled by ~N-S and E-W oriented fracture system, observed on surface in unmanned aerial vehicle images. Measurements in plug samples indicate that the local karstification modified the density, dielectric permittivity and porosity of the studied carbonate rocks. While the average density and relative dielectric permittivity decreased from 2714 to 2713 kg/m3 and from 7.36 to 6.81, respectively, the average porosity slightly increased from 0.95% to 1.08%. These changes in the rock properties affect the EM impedance contrasts between carbonate bedding, causing attenuation of the reflected GPR signal. The correlation between dielectric permittivity and porosity could be analyzed using the complex refractive index model (CRIM), which uses EM velocity estimated from GPR data. The CRIM porosities are somewhat lower than porosities derived from plug sample due to different measurement scales. However, the fact that porosity is slightly higher in the karstified carbonates was maintained. In addition, GPR instantaneous amplitude and frequency attributes were used to estimate spatial variability in porosity throughout the whole GPR data volume. The crossplot of these two GPR attributes show that karstified zones generally cluster in narrow ranges of GPR instantaneous amplitude and in broad range of GPR instantaneous frequency. Assuming empirical correlation between physical properties and GPR attributes two predicted porosity volumes were established for the karstified carbonates in the studied area.