GEOPHYSICAL FLOW ANALYSIS OF ANISOTROPY IN A KARST SYSTEM: A CASE STUDY OF SNAPPER CREEK MUNICIPAL WELL FIELD, MIAMI, FL

This study presents hydrogeophysical measurements of aquifer anisotropy at an active hydrological site consisting of a municipal wellfield transversed by a surface drainage canal. 1-D azimuthal electrical soundings, 2-D and 3-D resistivity imaging, and azimuthal self potential gradient (ASPG) techniques were employed together with well data to map the subsurface behavior at Snapper Creek Municipal Well Field, in Miami, Florida. Azimuthal electrical measurements at the site indicate that the direction of minimum resistivity (maximum hydraulic conductivity) changes from NNW-SSE to NE-SW with depth. The 1-D resistivity-depth profile shows a decrease in resistivity between 2 and 4 m depth, followed by a more resistive layer below 4 m depth. The 2-D and 3-D imaging illustrates higher resistivity at the surface in the unsaturated zone followed by a decrease in the saturated zone with significant horizontal and vertical changes due to low resistivity zones. The variation is attributed to solution cavities similar to those evidenced in monitoring well lithology at the site. The measured self potential during pumping and non-pumping of groundwater changes on the average by 3 mV. The estimated potential polarities indicate that flow direction trends SSE which is similar to the trend of azimuthal resistivity survey from the surface to 4 m and the direction of the surface water flow in the canal at the site. The results demonstrate that the change in hydraulic anisotropy might be related to solution cavities, the surface canal and the groundwater extraction wells. This study shows the potential for hydrogeophysical measurement as a useful tool in providing information about the anisotropy in areas of complex surface and groundwater interaction.