Ground penetrating radar

As sea level rises, saltwater migration can threaten coastal ecosystems and beach-dune environments, which negatively impacts coastal flora. This study uses ground penetrating radar (GPR) to evaluate the spatiotemporal variability of saltwater migration in the near shore at high lateral resolution (i.e. cm) by using daily micro tidal cycles as analogs to infer saltwater migration. Time-lapse GPR profiles were collected at low and high tide capturing phase lags of the tidal flux through different substrates. GPR measurements were collected at two sites in Miami with contrasting lithologies: a) Crandon Park, composed of unconsolidated sand; and b) the Barnacle Historic State Park, composed of the Miami Limestone Formation. Laboratory-scale GPR measurements were collected over samples mimicking field conditions. The results may be helpful to identify regions vulnerable to saltwater migration in the near shore based on lithological variability, and to mitigate negative impacts for flora in beach-dune habitats during sea level rise.

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.

Peat soils are known to be a significant source of atmospheric greenhouse gasses. However, the releases of methane and carbon dioxide gasses from peat soils are currently not well understood, particularly since the timing of the releases are poorly constrained. Furthermore, most research work performed on peatlands has been focused on temperate to sub-arctic peatlands, while recent works have suggested that gas production rates from low-latitude peat soils are higher than those from colder climates. The purpose of the work proposed here is to introduce an autonomous Ground Penetrating Radar (GPR) method for investigating the timing of gas releases from peat soils at the lab scale utilizing samples originating from Maine and the Florida Everglades, and at the field scale in a Maine peatland. Geophysical data are supported by direct gas flux measurements using the flux chamber method enhanced by timelapse photography, and terrestrial LiDAR (TLS) monitoring surface deformation.

Peatlands cover a total area of approximately 3 million square kilometers and are one of the largest natural sources of atmospheric methane (CH4) and carbon dioxide (CO2). Most traditional methods used to estimate biogenic gas dynamics are invasive and provide little or no information about lateral distribution of gas. In contrast, Ground Penetrating Radar (GPR) is an emerging technique for non-invasive investigation of gas dynamics in peat soils. This thesis establishes a direct comparison between gas dynamics (i.e. build-up and release) of four different types of peat soil using GPR. Peat soil blocks were collected at peatlands with contrasting latitudes, including the Everglades, Maine and Minnesota. A unique two-antenna GPR setup was used to monitor biogenic gas buildup and ebullition events over a period of 4.5 months, constraining GPR data with surface deformation measurements and direct CH4 and CO2 concentration measurements. The effect of atmospheric pressure was also investigated. This study has implications for better understanding global gas dynamics and carbon cycling in peat soils and its role in climate change.