RESUMEN

Resistivity and self-potential tomography can be used to investigate anomalous seepage inside heterogeneous earthen dams. The self-potential (SP) signals provide a unique signature to groundwater flow because the source current density responsible for the SP signals is proportional to the Darcy velocity. The distribution of the SP signals is also influenced by the distribution of the resistivity; therefore, resistivity and SP need to be used in concert to elucidate groundwater flow pathways. In this study, a survey is conducted at a small earthen dam in Colorado where anomalous seepage is observed on the downstream face at the dam toe. The data reveal SP and direct current resistivity anomalies that are used to delineate three anomalous seepage zones within the dam and to estimate the source of the localized seepage discharge. The SP data are inverted in two dimensions using the resistivity distribution to determine the distribution of the Darcy velocity responsible for the observed seepage. The inverted Darcy velocity agrees with an estimation of the Darcy velocity from the hydraulic conductivity obtained from a slug test and the observed head gradient.

Asunto(s)

RESUMEN

Low-frequency polarization of sands and sandstones seems to be dominated by the polarization of the Stern layer, the inner part of the electrical double layer coating the surface of the silica grains and clay particles. We investigate a simple model of Stern layer polarization combined with a simple complexation model of the surface of the grains immersed in a 1:1 electrolyte like NaCl. In isothermal conditions, the resulting model can be used to predict the complex conductivity of clayey materials as a function of the porosity, the cation exchange capacity of the clay fraction (alternatively the specific surface area of the material), and the salinity of the pore water. A new set of experimental data is presented. This dataset comprises low-frequency (1 mHz-45 kHz) complex conductivity measurements of saprolites and sandstones that are well characterized in terms of their petrophysical properties (porosity, permeability, specific surface area or CEC, and pore size). This dataset, together with incorporating additional data from the literature, is used to test the Stern layer polarization model. We find an excellent agreement between the predictions of this model and this experimental dataset indicating that the new model can be used to predict the complex conductivity of natural clayey materials and clay-free silica sands.