Two- and three-dimensional heterogeneity in carbonate sediments using resistivity imaging

Abstract

Volume heterogeneity was investigated in carbonate sediments using fine-scale electrical resistivity data, which are sensitive to porosity and to sediment macrostructure and microstructure. Variability in sediment density and porosity was assessed by X-radiographic and two-dimensional (2D) electrical resistivity measurements in 3-cm-thick slabs collected from the Dry Tortugas in the lower Florida Keys. A comparison of the 2D correlation lengths calculated from each assessment indicates that, using methods that differ in resolution, these two methods measure quite different horizontal and vertical fluctuations in sediment density and porosity. Other electrical resistivity experiments were conducted using a 2D network of resistivity electrodes on the surface of freshly collected box cores. The resulting three-dimensional (3D) resistivity data provide a unique insight into the spatial variability of sediment porosity and structure at the cm scale. Complementary X-radiograph cores provide 2D datasets of resistivity and porosity at higher resolution. These data are used to establish the porosity–resistivity–microstructure relationships for these carbonate sediments, with microstructure being described in terms of tortuosity. These relationships are used to extend our interpretation in terms of porosity and tortuosity to the corresponding 3D box core resistivity datasets. Sediment tortuosity is investigated by numerically modeling the flow of electrical currents through a range of pore morphologies in three dimensions. The results show particular sensitivity to the intra-particle porosity, particle shape and the relative sizes of pores and throats. The 3D methodology shows promise as a non-invasive measurement of buried inhomogeneities that could lead to improving models for predicting acoustic backscattering from the sediment volume.