An Application Of Electrical Anisotropy In Hydrogeological Modeling

The electrical anisotropy of near surface aquifers can be determined through azimuthal<br>resistivity surveys, which measure the apparent resistivity as a function of compass bearings.<br>Systematic azimuthal variations in the apparent resistivity may result from the presence of<br>preferred current flow through joints or fractures, which can also be paths for preferred<br>groundwater flow. Hence, electrical anisotropy should provide a geophysical means to improve<br>groundwater models. To establish an empirical relationship between measured electrical<br>anisotropy and the horizontal hydraulic anisotropy required for groundwater models, azimuthal<br>resistivity surveys were conducted at an established test site near Sturgeon Bay, Wisconsin. This<br>site is well documented by the Wisconsin Geological and Natural History Survey (WGNHS)<br>through extensive studies of groundwater flow in the fractured media of the area. Fracture<br>azimuths were found to be 75”, 115”, and 160”. The average electrical anisotropy was 1.2.<br>Groundwater modeling of a pump test conducted at the site indicated a hydraulic anisotropy of<br>2.6. The independent measurements of electrical and hydraulic anisotropy were used to establish<br>empirical relationships between the two quantities.