For the protection of wetland and water resources it would be beneficial to understand when collapse conduits function as recharge points to the underlying aquifer. Inexpensive, noninvasive methods to detect recharge are desirable. Previous studies show negative self-potential (SP) anomalies over sinkholes that correspond to the expected electrokinetic effects of groundwater flowing downward through a conduit. SP surveys are less labor-intensive than high-resolution 3D GPR and resistivity, and continuous long-term monitoring is possible. However, before SP surveys can be reliable indicators of flow, SP contributions from ET, conductivity changes, redox reactions, thermoelectric effects, cultural noise, and lateral flow must be understood. A year of continuous SP monitoring was combined with high-resolution 3-D GPR surveys, and intermittent water table monitoring over two small covered-karst sinkholes in Tampa, Florida. Positive and negative SP anomalies episodically manifested over conduits, suggesting that conduit flow is dynamic, not static. Three distinct SP flow regimes in the conduits are postulated: fast flow open to the aquifer, slow flow open to the confining layer through the collapse conduit walls, and a conduit, plugged high enough to behave like the rest of the confining layer. SP responses after rain events also appear to measure the effects of two moving Gaussian wetting front curves, one striking the monitoring electrode, one the reference. viii The wetting front volumes are differently dispersed by traveling different distances. By comparing curve shapes for all possible pairs of electrodes, it may be possible to establish surficial infiltration and flow patterns. Temporal SP response clearly shows SP is also affected by soil conductivity, rainfall history, and cultural noise. Ultimately, SP changes too frequently to rely on measurements many hours or days apart. Over the course of the year, the electrodes became less responsive to rainfall and more erratic. Extremely wet and dry conditions seemed to affect responses. The porous faces of the electrodes or the bentonite clay gel used to enhance contact may decline. It appears a better design for electrodes and electrode contact needs to be developed. To test the intermittent behavior hypothesis, more conduits need to be studied, and moisture and SP must be studied concurrently. Several reference electrodes placed in various topographic, vegetative, geologic, and climatic settings could help distinguish groundwater flow from other SP sources. SP is a valuable research tool; however external complexities such as cultural noise, sinkhole lithology, and the state of the unsaturated zone make SP data difficult to interpret without ancillary information.

Predicting infrastructure damage and economic impact of sinkholes requires high accuracy mapping distribution and development. The study mapped sinkholes and sinkhole hotpsots in Johnson City, TN using LiDAR-derived Digital Elevation Model (DEM) and a database of known sinkholes which were matched to LiDAR-derived depressions. For all matched depressions (n = 404), three metrics were calculated: circularity index, ratio of length to width of the Minimum Bounding Rectangle (MBR) and percent coverage of the MBR by the depression, and 3,634 new sinkholes were identified. Newly developed hotspots were identified in north Johnson City and other areas in the south near the Johnson City Medical Center. The methodology developed can be applied to identify hotspots in other small metropolitan cities and the hotspot map produced can be employed in hazard mitigation planning, resource allocation, and made available publicly to property owners and insurance companies.

No description available.

The prospect of using U.S.G.S. Digital Elevation Models (DEM's) to locate sinkholes has been investigated. Three quadrangle maps, those of Blacksburg, Ironto and Newport, were selected as these areas are dotted with sinkholes. An algorithm was developed to extract sinkholes from DEM's. Sinkholes were also digitized from the topographic maps to check the accuracy of those extracted by the program from DEM's. This was done by overlaying pits extracted from DEM's with digitized sinkholes. Discriminant Analysis was run to compare the characteristics of sinkholes identified by the program from those that were not. It was found that few sinkholes could be located using the available integer elevation data of 30m resolution. Noise in the data, even after running a filter, precluded satisfactory results. It was concluded that elevation data of finer resolution and in real format should be used for this analysis. / Master of Science

Increased stormwater runoff due to urban development in the northern Shenandoah Valley (NSV) region of Virginia has prompted local officials and representatives to consider Low Impact Development (LID) as a stormwater management technique. LID is based on infiltrating stormwater runoff at the source through practices such as bioretention, rain gardens, and grass swales. The karst terrain that underlies the Shenandoah Valley presents a major barrier to the use of LID. Infiltration of surface runoff in karst landscapes may threaten groundwater quality and the stability of the bedrock. In 2004 the Center for Geospatial Information Technology (CGIT) at Virginia Tech developed an LID site suitability model for the NSV region incorporating karst as a key component in distinguishing unsuitable from suitable conditions for LID. But, due to the difficulty of mapping karst, the karst layer used in the site suitability model is very coarse in resolution, based primarily on carbonate versus non-carbonate rock. This study uses a 1:24,000 scale sinkhole map derived from sinkhole boundaries identified by geologist David Hubbard (1984) of the Virginia Department of Mines and Minerals (DMME) to develop a more detailed karst map for a sub-watershed of the NSV region. The analysis uses geospatial techniques to determine the relationship between sinkhole distribution and four major landscape factors: bedrock type, soil depth to bedrock, proximity to geologic faults, and proximity to surface streams. The analysis identified three major trends in sinkhole occurrence: (1) sinkholes are more abundant in relatively pure carbonate rocks of Ordivician age; (2) sinkhole occurrence increases with proximity to fault lines; and (3) sinkholes are sparse near streams, most abundant 600 to 1400 feet away from surface streams. Based on these findings a sinkhole susceptibility index was produced using weighted overlay analysis in ArcGIS. The sinkhole susceptibility index provides a more detailed karst layer for the LID site suitability maps and can be used by the NSV region as a predictive tool for future sinkhole occurrence. / Master of Science

Full Waveform Inversion (FWI) is a powerful seismic imaging technique used to reconstruct high-resolution velocity models of the subsurface. It relies on the inversion of seismic data acquired from multiple sources and receivers to estimate the mechanical properties of geologic materials and can be used to detect anomalous subsurface conditions. The accuracy of FWI results is influenced by various factors related to the workflow used for its implementation. This includes the survey parameters, the mathematical framework of the inversion, and the complexity of the subsurface conditions modeled during the inversion process. Therefore, it is crucial to have a fundamental understanding of the interplay between these factors and their impact on the accuracy of the reconstructed model, particularly given the effects of these factors on computational costs. This is an area that has been understudied within the context of near-surface geotechnical applications for anomaly detection, which is an application that presents unique challenges relative to seismic exploration for hydrocarbons where FWI has been more fully developed. One key aspect that has not received sufficient attention is the impact of survey parameters on the accuracy of FWI results. The lack of formal research in this topic may lead to near-surface FWI studies that use more seismic sources than required for subsurface feature reconstruction, which results in data collection and computational inefficiencies. The selection of misfit function and starting model are also essential factors influencing the reliability of the reconstructed model. The physics employed for forward modeling can also affect the ability to simulate wave propagation in the domain of interest. These factors have significant implications for near-surface applications of FWI, and further research is required to explore their interplay and improve FWI workflow.Given the gaps in the current implementation of FWI for geotechnical applications, this research will explore the role of parameterization and workflow on FWI results when applied to anomaly detection in karst conditions. This will include selection of an FWI workflow that can improve the feasibility of fieldwork and reduce the processing time. The research will investigate four key factors of the FWI workflow (i.e., survey design, initial model, misfit function, and forward modeling physics) for detection of sinkholes using numerical and field testing in different subsurface conditions. Overall, the outcomes of this research will help practitioners with more appropriate choices in the FWI process and consequently promote its high potential in near-surface applications. / Civil Engineering

<p>Dams that comprise broadly and widely graded glacial materials, such as moraines, have been found to be susceptible to internal erosion, perhaps more than dams of other soil types. Internal erosion washes out fine-grained particles from the filling material; the erosion occurs within the material itself or at an interface to another dam zone, depending on the mode of initiation. Whether or not internal erosion proceeds depend on the adequacy of the filter material. If internal erosion is allowed, it may manifest itself as sinkholes on the crest, increased leakage and muddy seepage once it surfaces, which here is called surfacing internal erosion (i.e. internal erosion in the excessive erosion or continuation phase). In spite of significant developments since the 1980s in the field of internal erosion assessment, the validity of methods developed by others on broadly graded materials are still less clear because most available criteria are based on tests of narrowly graded granular soils. This thesis specifically addresses dams that are composed of broadly graded glacial soils and investigates typical indicators, signs and behaviors of internal erosion prone dams. Based on a review of 90+ existing moraine core dams, which are located mainly in Scandinavia as well as North America and Australia/New Zealand, this thesis will show that not only the filter’s coarseness needs to be reviewed when assessing the potential for internal erosion to surface (i.e., erosion in the excessive or continuing phase); in addition, the grading stability of the filter and the core material as well as non-homogeneities that are caused by filter segregation need to be studied. Cross-referencing between these aspects improves the assessment of potential for internal erosion in dams of broadly graded soils and furthermore it provides aid-to-judgment.</p> / QC 20100715

Eighty-nine caves and sinkholes were investigated in the Edwards Aquifer recharge zone in Bexar County, Texas. The study examined their hydrogeologic and topographic origins and distribution, relationships to major fracture traces, quantity of recharge into the aquifer and degree of sensitivity towards degradation of the aquifer’s water quality. Groundwater traces were attempted to determine aquifer flow routes, time of groundwater travel, groundwater volume within conduits, and the aquifer’s capacity for dilution and dispersion of recharged contaminants. Trends in water quality were examined to quantify the volume and variety of contaminants recharged into the aquifer and to determine the effects of urbanization upon the Edwards Aquifer. The Edwards recharge zone was hydrogeologically assessed to rate the sensitivity of its areas. Socio-political impacts on recharge zone development were also examined. Based on the results of the above outlined research method, the conclusions of this investigation are that caves and sinkholes contribute substantial recharge into the Edwards Aquifer, rapidly transmit that recharge to the aquifer and are sensitive sites for potential contamination. The entire recharge zone was determined to be very sensitive to contamination. No significant differences were found between areas within the recharge zone to scale their degree of sensitivity. Major conduit flow networks were found to exist within the aquifer and their groundwater flow paths could be traced. Urban development of the Edwards recharge zone was shown to decrease the volume of recharge and degrade the aquifer’s water quality. No significant detrimental effects on the aquifer were observed. The volume of diminished recharge and the concentration of recharged contaminants that were necessary to produce significant adverse effects on the aquifer were not determined due to lack of precipitation during the study period and inconclusive groundwater tracings. It was recommended that further development of the recharge zone be suspended until the effects of urbanization are quantified.

A hierarchical and standardized environmental disturbance index, specifically designed for karst landscapes, was created by van Beynen and Townsend (2005). To assess the applicability of the index and provide recommendations for its refinement, the index was applied in four west-central Florida counties and interviews were conducted with local and state officials, community planners, and land resource managers. The karst disturbance index consists of 30 indicators contained within five broad categories: geomorphology, hydrology, atmosphere, biota, and culture. Data was readily available for most environmental indicators used to construct the index. Overall, levels of disturbance vary between the counties due to the level of urbanization, with the highly populated Hillsborough and Pinellas Counties having higher degrees of disturbance than less developed Pasco and Hernando Counties. While this result may seem obvious, the measure of disturbance using many indicators provides benchmarks of levels of disturbance that can be reassessed with time and highlights those aspects of the environment most in need of attention. Several minor issues arose during the testing: the need for broader indicator descriptions that encompass a variety of scenarios, a new water quality indicator, obsolete sinkhole data, and a lack of data for biota indicators. The lack of data for certain indicators suggests where future research efforts can be directed.

This study assesses the capability and practical applications of quasi 3-Dimensional (3D) electrical resistivity surveying (ER) for mapping air-filled karst conduits. Vadose zone caves within the Brooksville Ridge of West Central Florida are relatively similar in architecture, with N-S elongation, and do not consist of an interconnected network of conduits. A high resolution quasi-3D ER survey was performed over two mapped cave systems on the Brooksville Ridge. The resultant survey verified the general effectiveness of quasi-3D ER in locating the two known near-surface cave features. Several other locations in the survey show similar or stronger resistivity anomalies trending in a N-S direction; these are interpreted to represent previously unknown voids. The quality of inversion results were tested by comparing results against the known void locations and by computing cross-over errors from surveys conducted at the same point with different orientations. Our results show that 3D inversions of multiple adjacent parallel profiles produces higher quality results (lower cross-over errors, better fit to cave locations) than 2D inversions. The data indicate that no single value of resistivity can be used as a reliable indicator of the presence of a void, presumably due to variable void size and the complexity in resistivities in the host rock. With sinkholes continuing to be of concern to residential and urban development in West Central Florida, the results of this research show the necessity of applying geophysical techniques in order to reduce the potential hazards posed by karst terrain.