NS23A-01
Imaging Karst Aquifers with Multichannel Seismic Data in Biscayne Bay: Conventional Wisdom Defied
Conventional wisdom reasons that acquisition of useful seismic data in shallow-marine carbonate environments is not possible because: (1) water-bottom multiples will dominate; (2) receiver offsets will be too short to image deep reflectors; (3) normal move out is too small to effectively calculate velocities; (4) air-gun source arrays are not appropriate or frequency band-limited; and (5) it is folly to over-sample the seismic data and process very large digital data sets. In 2007, about 108 km (17 individual profiles) of marine, multichannel, high-resolution, seismic data were acquired almost entirely inside Biscayne National Park in water depths ranging from 0.9 to 100 m. The data were collected using a 48-trace, towed-streamer array; an interdependent air-gun as the seismic source; and a proprietary 52-channel, 24-bit recording system. The seismic vessel was a fast, shallow-draft catamaran capable of continuously acquiring data in water as shallow as 0.7 m. The set of seismic images from 17 profiles show well-defined reflections from near surface to the Eocene Oldsmar Formation (including the karstic Boulder Zone in the Lower Floridan aquifer). The profiles also display distinctive geologic features that include karst, clinoformal prograding strata, unconformities, fractures, stratal truncation, and evidence for breaching of confining units.
NS23A-02
Marine High-Resolution Seismic-Reflection Data in Southeastern Florida: Indications of a Regional Seal Bypass System
In southeastern Florida during 2007, about 108 km of marine, multichannel, high-resolution, seismic-reflection data were acquired almost entirely inside Biscayne National Park at water depths ranging from about 0.9 to 100 m. Fourteen profiles were acquired between the shoreline of the Florida peninsula and a series of small keys that separate Biscayne Bay from the Atlantic Ocean. Additionally, three profiles were collected eastward of the islands with two extending seaward of the present-day shelf margin and its discontinuous reefs. The set of seismic images from the 17 profiles is providing recognition of intriguing geologic features beneath and beyond Biscayne Bay. For example, the seismic sections provide clues as to the sealing capacity of confining units above a highly permeable zone (Boulder Zone) in the lower part of the Floridan aquifer system used on the southeastern peninsula for deep well injection of treated wastewater. Many of the seismic profiles exhibit continuous vertical disturbances in parallel seismic reflections that correspond to the rocks of the karst Floridan aquifer system and overlying intermediate confining unit. These features indicate fractures that disrupt seismic reflections representative of confining units and may allow ground water to flow across confinement. Combined, the fractures could act as a regional seal bypass system. If this bypass system allows cross-stratal fluid migration, it could provide many pathways for upward directed ground- water flow with leakage to higher hydrostratigraphic levels or to the surface as submarine ground-water discharge. Future research will include acquisition of additional marine seismic profiles and the use of streaming marine resistivity profiling and radon water column mapping. These data will be used to investigate the source waters of submarine ground-water discharge to Biscayne Bay, which could be associated with the fractures imaged on the seismic sections.
NS23A-03
Lattice Boltzmann Methods Applied to Three-Dimensional Virtual Cores Constructed from Digital Optical Borehole Images of a Karst Carbonate Aquifer
Recovery of whole-core samples from macroporous karst carbonate is nearly impossible with conventional drilling technology. Thus, the most porous part of coreholes drilled in karst systems rarely yield whole-core samples. The consequent lack of samples for measurement of fluid-flow properties in karst carbonate aquifers impedes characterization of ground-water flow within these systems. This study uses advanced modeling techniques together with geophysical corehole data acquired from the karst carbonate Biscayne aquifer of southeastern Florida, USA, to explore a combination of innovative technologies designed to compensate for the lack of macroporous whole-core sample data. Specifically, these methods are being used to better understand the ground-water flow regime in the Biscayne aquifer. In this study, digital optical borehole image logs were compiled for test coreholes that penetrate the rocks of the Biscayne aquifer. The borehole image data were then processed to map the 3-D distribution of macropores and rock matrix present on the borehole walls using Stanford geostatistical software (SGeMS). The SGeMS program was used to compute variograms that were used as input for a computer simulation. The simulation results provided virtual 3-D renderings of the complex karst macropore network of the Biscayne aquifer that statistically replicate the borehole wall image data. These renderings provided 3-D visual records of areas of the aquifer that are composed of a carbonate eogenetic macropore system dominated by centimeter-scale vugs produced by fossil molds and voids associated with trace fossils. The vugs can coalesce over broad areas in the Biscayne aquifer to form laterally persistent zones of preferential ground-water flow. Lattice Boltzmann methods (LBMs) were used to measure the intrinsic permeability of the 3-D aquifer renderings. When using LBMs the rock matrix was assumed to be a nonporous media, thus permeability was only measured within the network of macropores. Comparison of LBM-derived permeabilities to those obtained from conventional laboratory techniques show that the measured permeability of whole-core samples are substantially challenged in areas where centimeter-scale vuggy macroporosity is present and for this type of porosity, LBMs are preferred. The results obtained using LBMs closely conform to the analytical solutions for pipeflow, providing the impetus and justification for its use in obtaining intrinsic permeability values for virtual macropore systems. LBMs were also used to simulate 3-D fluid flow through the renderings of macropores and rock matrix. LBMs were especially useful for simulations of inertial (non-Darcian) fluid flow, which may dominate flow in macroporous zones in parts of the Biscayne aquifer. The methods being developed in this study are providing a means for estimating and correlating the permeability of macroporous zones, as well as determining whether flow is primarily laminar or turbulent.
NS23A-04
High-Resolution X-ray Computed Tomography of Macroporous Karst for Permeability Measurement and Non-Darcian Flow via Lattice Boltzmann Models
The permeability of macroporous karstic rocks is extremely difficult to measure in a laboratory setting due to flow- rate limitations of the measuring apparatus, and issues related to the maintaining and measuring the extremely small gradients needed to sustain Darcian flow regimes. High-resolution X-ray computed tomography (HRXCT) can provide digital reconstructions of porous media that are essential for detailed pore-scale flow modeling. The HRXCT provides rendering data for lattice Boltzmann calculation of permeability in samples with well-connected macropores. Samples representative of biomoldic and trace-fossil related macroporosity were collected from the Fort Thompson Formation and Miami Limestone of the Biscayne aquifer in southeastern Florida and scanned using HRXCT methods that provided resolutions on the order of 0.3 mm per pixel. Permeability measurements were conducted using lattice Boltzmann techniques applied to seven renderings of samples created from the HRXCT scans. Non-Darcian inertial flows were avoided by applying extremely small gradients while making the permeability measurements. The lattice Boltzmann method was verified against analytical solutions for pipe and conduit flow. Measured permeabilities derived from lattice Boltzmann methods correspond to hydraulic conductivity values ranging from 0 m/s (for a sample lacking well-connected macroporosity) to 167 m/s (vertical), and are as much as five orders of magnitude greater than the largest values typically reported by testing laboratories. Non-Darcian effects are of considerable interest if they occur under field-scale conditions and lattice Boltzmann models permit investigation of the potential significance of non-Darcian effects. For one limestone sample from the Biscayne aquifer with extremely high, well-connected macroporosity, it is concluded that non-Darcian behavior due to inertial flow under field-scale gradients could effectively reduce the apparent hydraulic conductivity by nearly 50 percent.
NS23A-05
Nuclear Magnetic Resonance Imaging of Ground Water Flow within Touching-Vug and Matrix Porosity in the Biscayne Aquifer of Southeast Florida
In this study, we use an innovative, non-invasive technology, nuclear magnetic resonance imaging (NMRI), to visualize the direction and magnitude of ground water flow in field samples of late Pleistocene limestone of the Biscayne aquifer. Specific goals of the first set of NMRI experiments are to map the advective velocity of water flowing at two rates of specific discharge (0.00025 and 0.00013 m/s) through a 10-cm diameter cylindrical, epoxy- resin model. The model interior accurately reproduces a well-connected maze of ichnologically influenced, centimeter-scale, touching-vug macroporosity common within preferred flow zones in parts of the Biscayne aquifer. A second set of NMRI experiments investigates the migration of freshwater into the matrix of permeable (gas minipermeameter mean 10-13.5 m2 on four samples) and porous (mean of 44% on four samples) peloid-oöid grainstone initially saturated with heavy-water (D2O). In the experiments on the physical model, we generate the velocity maps using phase-encoded, stimulated-echo imaging. In the experiments on the rock matrix, we visualize the progressive replacement of D2O in the rock matrix using sequential time-step images of NMRI signal strength. Results for the freshwater-D2O experiments reveal a substantial flux of freshwater into the matrix porosity with a simultaneous loss of D2O. Specifically, we measured rates (upward of 0.001 milliliters per hour per gram of sample (mL/hr-g) in static or non-flowing conditions, and perhaps as great as 0.07 mL/hr-g when freshwater continuously flows past a sample at velocities less than those found within stressed areas of the Biscayne aquifer. These experiments illustrate how freshwater and D2O, with different chemical properties, migrate within one type of matrix porosity found in the Biscayne aquifer. Furthermore, these experiments are a comparative exercise in the replacement of seawater by freshwater in the matrix of a coastal, karst aquifer, since D2O has a greater density than freshwater.
NS23A-06
Recent Developments in Karst Groundwater Flow Measurement in Southeastern Florida,USA
Groundwater seepage was first characterized in the early 1800's, when Henry Darcy determined that the flow of groundwater could be estimated from the head difference and the distance between two points. Since then, hydrogeologists have been struggling with ways to continuously measure groundwater flow in situ, and more recently have sought data in near-real time. Groundwater flow within aquifers that have relatively large head differences (several meters) are porous in nature and have low hydraulic conductivities, is linear in nature, and can be generally characterized by Darcy's solution. Prior to the research presented herein, it was assumed that aquifers within Miami-Dade County could also be characterized by Darcy's solution (with Reynolds numbers less than 10 or 20). The L-31N Canal lies on the eastern flank of Everglades National Park (ENP). In addition to conveying water to Florida Bay and Biscayne Bay, the canal's levees are intended to reduce surface-water sheet flow from ENP to eastern urban areas. In an effort to reduce groundwater seepage coming from ENP, the South Florida Water Management District (SFWMD) and the United States Army Corp of Engineers (USACE) have been tasked with evaluating the hydrogeology and the groundwater/surface-water interaction on the L-31N canal. This involved process of installation includes monitoring wells, recording automated water-level measurements, characterizing water-chemistry types and ages, and installation of instruments capable of measuring horizontal groundwater velocities and directions coming from ENP. The SFWMD initiated a cooperative agreement with the United States Geological Survey (USGS) for the geological and hydrogeological investigation and concurrently contracted the installation of borehole flowmeters in eight wells (two clusters). The USGS provided detailed core and sediment analysis, geophysical logging, in situ borehole flowmeter logging, and digital optical borehole imaging. In addition, the USGS produced a hydrogeologic cross-section using the new borehole data. The USGS delineated high-frequency cycles (HFCs) within the study area. The high-frequency cycles form the fundamental building blocks of the rocks composing the Biscayne aquifer. Vertical lithofacies successions, which have stacking patterns that reoccur, fit within the high-frequency cycles. An important observation is that a predictable vertical pattern of macroporosity and permeability commonly exists within the high-frequency cycles, thus preferential flow passageways can be constrained by the lower and upper cycle boundaries. In southeastern Florida, specific HFCs can contain relatively high hydraulic conductivities and vertical head gradients within centimeters of each other. This combination of high hydraulic conductivities (estimated at 1500 to 3000 m/d) and nearly flat water table gradients combine to convey large amounts of groundwater from ENP to the eastern urban areas, with the water ultimately discharging into Biscayne Bay. Horizontal groundwater flow velocity was measured with horizontal heat-pulse flowmeters installed in eight monitoring wells located on the western levee of the L-31N canal. Results show that flow velocity in the shallow wells (5.1 m in depth) is coupled to the surface water as measured by well water levels. A groundwater rise of about 0.5 m during the wet season of September-December 2007 led to a six-fold increase in horizontal groundwater flow rates.