Sedimentary basins -- Florida -- Biscayne Aquifer

In southeastern Florida, the majority of drinking water comes from the Biscayne aquifer. This aquifer is comprised of heterogeneous limestones, sandstones, sand, shell and clayey sand with zones of very high permeability. Visualizing the spatial variations in lithology, porosity and permeability of heterogeneous aquifers, like the Biscayne, can be difficult using traditional methods of investigation. Using the Roxar IRAP RMS software multi-layered 3D conceptual geomodels of the lithology, cyclostratigraphy and porosity were created in a portion of the Biscayne aquifer. The models were built using published data from borehole geophysical
measurements, core samples, and thin sections. Spatial relations between lithology,
cyclostratigraphy, porosity, and preferential flow zones were compared and contrasted to
better understand how these geologic features were inter-related. The models show local areas of differing porosity within and cross-cutting different cycles and lithologies. Porosity in the Biscayne aquifer study area follows a hierarchy attributed to lithofacies with a pattern of increasing porosity for the high frequency cycles. This modeling improves understanding of the distribution and interconnectedness of preferential flow zones, and is thus an invaluable tool for future studies of groundwater flow and groundwater contamination in the Biscayne aquifer.

The karst Biscayne aquifer is characterized by a heterogeneous spatial
arrangement of porosity, making hydrogeological characterization difficult. In this
dissertation, I investigate the use of ground penetrating radar (GPR), for understanding
the spatial distribution of porosity variability in the Miami Limestone presented as a
compilation of studies where scale of measurement is progressively increased to account
for varying dimensions of dissolution features.
In Chapter 2, GPR in zero offset acquisition mode is used to investigate the 2-D
distribution of porosity and dielectric permittivity in a block of Miami Limestone at the
laboratory scale (< 1.0 m). Petrophysical models based on fully saturated and unsaturated.
water conditions are used to estimate porosity and solid dielectric permittivity of the
limestone. Results show a good correspondence between analytical and GPR-based
porosity estimates and show variability between 22.0-66.0 %.
In Chapter 3, GPR in common offset and common midpoint acquisition mode are
used to estimate bulk porosity of the unsaturated Miami Limestone at the field scale
(10.0-100.0 m). Estimates of porosity are based on the assumption that the directly
measured water table reflector is flat and that any deviation is attributed to changes in
velocity due to porosity variability. Results show sharp changes in porosity ranging
between 33.2-60.9 % attributed to dissolution areas.
In Chapter 4, GPR in common offset mode is used to characterize porosity
variability in the saturated Biscayne aquifer at 100-1000 m field scales. The presence of
numerous diffraction hyperbolae are used to estimate electromagnetic wave velocity and
asses both horizontal and vertical changes in porosity after application of a petrophysical
model. Results show porosity variability between 23.0-41.0 % and confirm the presence
of isolated areas that could serve as enhanced infiltration or recharge.
This research allows for the identification and delineation areas of macroporosity
areas at 0.01 m lateral resolution and shows variability of porosity at different scales,
reaching 37.0 % within 1.3 m, associated with areas of enhanced dissolution. Such
improved resolution of porosity estimates can benefit water management efforts and
transport modelling and help to better understand small scale relationships between
ground water and surface water interactions.