Electromagnetic methods for rapidly characterizing porosity distributions in the upper part of the Biscayne aquifer, southern Florida

Gregory J. Mount1, Xavier Comas1, and Kevin J. Cunningham2 1Department of Geosciences, Florida Atlantic University, Boca Raton, Florida 33431 2U.S. Geological Survey, 3110 SW 9th Avenue, Fort Lauderdale, Florida 33315 Although conventional hydrological techniques of aquifer characterization, which rely on data obtained from boreholes and wells can provide very valuable direct information about porosity, storativity and transmissivity, they are invasive and can often become time consuming and relatively expensive. Near-surface electromagnetic techniques, such as ground penetrating radar (GPR), provide indirect measurements of aquifer properties that complement traditional point measurements and provide a laterally continuous subsurface image in an efficient and cost effective manner with a minimal impact on the environment. We investigated the carbonate rocks of the uppermost part (3-5 meters) of the Biscayne aquifer in Everglades National Park to better understand the distribution of karst features that can create concentrated flow of groundwater, nutrients, and contaminants. As the Biscayne aquifer is the primary source of drinking water for millions of people in southern Palm Beach, Broward, and Miami-Dade counties, knowledge about these features could create a more complete understanding of a critical natural resource. These macroporous elements contribute to the overall storage, permeability, and transmissivity of the aquifer and for that reason, delineation of their distribution and areal extent should aid in the development of more accurate groundwater flow models. The macroporous elements create numerous hyperbolic diffractions in GPR common offset profiles, and these diffractions are used directly used to estimate two-dimensional (2D) models of electromagnetic (EM) wave velocity in the subsurface. Such models are further contrasted with one-dimensional (1D) velocity models using GPR common mid-point surveys at selected locations. In order to estimate measurements of bulk porosity from the data available, a complex refractive index model (CRIM) was used. This allows for the delineation of the distribution of estimated porosity values from both 1D and 2D velocity models. All data are further constrained with terrain conductivity measurements and porosity values measured from whole-core samples. Porosity estimates calculated using the CRIM, range from about 10 to 45% and strongly correspond with the porosity values from the whole-core samples This study illustrates the potential of electromagnetic methods for quickly mapping areas of enhanced porosity. Furthermore, this information may improve understanding of surface water-groundwater exchange in the Everglades, which is a key component of ongoing and future restoration efforts.