Characterizing Jointed Systems by Azimuthal Resistivity Surveys

ABSTRACT Anisotropy, directional connectivity, and porosity of fracture systems can be estimated from surface azimuthal electrical resistivity surveys. Azimuthal resistivity surveys utilize conventional resistivity equipment and are performed by rotating a Wenner array about a fixed center point and measuring apparent resistivity as a function of azimuth. The depth of investigation is determined by the electrode spacing which is generally limited by practical considerations to approximately 70 m. The azimuthal survey generates an apparent resistivity ellipse from which the properties of a subsurface joint system can be determined. Theoretically the major axis of the resistivity ellipse will coincide with strike of the primary joint set. The ratio of major axis to minor axis of the ellipse defines the coefficient of anisotropy associated with the joints, and from this coefficient, secondary porosity can be calculated. Application of azimuthal surveys at 60 sites in bedrock and clayey till throughout Wisconsin generally yielded joint strikes within a few degrees of direct observations. Porosities calculated for jointed bedrock are within a few percent of values determined by other methods. The absolute values of porosities calculated for the tills appear unreliable due to conduction effects along the surface of clay minerals. Although the theoretical foundation for this technique is based on idealized jointing, the field results indicate that the method is effective in media with multiple joint sets. When mean joint length exceeds the electrode spacing, azimuthal resistivity peaks closely coincide with the mean strikes of observed joint sets. When joint lengths are less than the electrode spacing, the major ellipse axis indicates the most conductive path through the joint system. In both cases, ellipse shape indicates the direction (s) having the greatest joint connectivity, which are also the directions of greatest aquifer permeability.