Optimized Correlation of Geophysical And Geotechnical Methods In Sinkhole Investigations: Emphasizing On Spatial Variations In West-Central Florida

Abstract Sinkholes and sinkhole-related features in West-Central Florida (WCF) are commonly identified using geotechnical investigations such as standard penetration test (SPT) borings and geophysical methods such as ground penetrating radar (GPR) and electrical resistivity tomography (ERT). Geophysical investigation results can be used to locate drilling and field testing sites while geotechnical investigation can be used to ground truth geophysical results. Both methods can yield complementary information. Geotechnical investigations give important information about the type of soil, groundwater level and presence of low-density soils or voids at the test location, while geophysical investigations like GPR surveys have better spatial coverage and can resolve shallow stratigraphic indicators of subsidence. In GPR profiles collected at 103 residential sites in covered-karst terrain in WCF, sinkhole-related anomalies are identified using GPR and SPT methods. We analyze the degree to which the shallow features imaged in GPR correlate spatially with the N-values (blow counts) derived from SPTs at the 103 residential sites. GPR anomalies indicating sinkhole activity are defined as zones where subsurface layers show local downwarping, discontinuities, or sudden increases in amplitude or penetration of the GPR signal. Low SPT values indicating sinkhole activity are defined using an optimization code that searched for threshold SPT value showing optimum correlation between GPR and SPT for different optimal depth ranges. We also compared these criteria with other commonly used geotechnical criteria such as weight of rod and weight of hammer conditions. Geotechnical results were also used to filter the data based on site characteristics such as presence of shallow clay layers to study the effectiveness of GPR at different zones. Subsets of the dataset are further analyzed based on geotechnical results such as clay thickness, bedrock depth, groundwater conditions and other geological factors such as geomorphology, lithology, engineering soil type, soil thickness and prevalent sinkhole type. Results are used to examine (1) which SPT indicators show the strongest correlations with GPR anomalies, (2) the degree to which GPR surveys improve the placement of SPT borings, and (3) what these results indicate about the structure of sinkholes at these sites. For the entire data set, we find a statistically significant correlation between GPR anomalies and low SPT N-values with a confidence level of 90%. Logistic regression analysis shows that the strongest correlations are between GPR anomalies and SPT values measured in the depth range of 0-4.5 m. The probability of observing a GPR anomaly on a site will decrease by up to 84% as the minimum SPT value increases from 0 to 20 in the general study area. Boreholes drilled on GPR anomalies are statistically significantly more likely to show zones of anomalously low SPT values than boreholes drilled off GPR anomalies. We also find that the optimum SPT criteria result in better correlation with GPR than other simple commonly used geotechnical criteria such as weight of rod and weight of hammer. Better correlations were found when sites with poor GPR penetrations are filtered out from the dataset. The odds ratio showed similar result while the result varied with the depth range, statistics and threshold SPT value (low N- value with optimum correlation), with a maximum observed odds ratio of 3. Several statistical results suggest that raveling zones that connect voids to the surface may be inclined, so that shallow GPR anomalies are laterally offset from deeper zones of low N-values. Compared to the general study area, we found locally stronger correlation in some sub-regions. For example, the odds ratio found for tertiary hawthorn subgroup were 25 times higher than the odds ratio found for the general study area (WCF)

Improved 2D and 3D resistivity surveys using buried electrodes and optimized arrays: The multi-electrode resistivity implant technique (MERIT)

This thesis presents a novel resistivity method called Multi-Electrode resistivity technique (MERIT) that is used for high resolution imaging of complex geologic features at depth and near the edges of survey lines. The MERIT electrodes are especially shaped and designed to be self-driven using a robust-direct push technique. Measurements are taken using optimized arrays that are generated using a modified version of the “Compare-R” optimization algorithm. This work focused on both two-dimensional (MERIT2D) and three-dimensional (MERIT3D) applications of the buried array and show the relevance of the additional information gained by the addition of deep electrodes especially in sites with limited survey area. Numerical and laboratory studies are used to test and develop the technique and are later applied to image complex subsurface geologic structures on the field. The configuration of MERIT arrays brings some additional problems in terms the sensitivity of the deep MERIT arrays to a problem of non-uniqueness, mis-information, geometric error and resolution break between the two layers of electrodes. Multiple vertical resolution characteristic curves (RC curves) are analyzed to study the effect of array type, resistivity contrast, target resistivity and implant depth on the above-mentioned problems. Results show that MERIT measurements taken using standard dipole -dipole and wenner arrays along the surface and deep electrodes will strongly suffer from the problem of non-uniqueness or ambiguity while measurements taken using optimized arrays is suitable for MERIT configuration and will not suffer from any problem of ambiguity or non-uniqueness. Based on our result, a procedural guideline is developed to determine optimal MERIT implant depth and resolution cutoff that can be used for successful field implementation and for controlling misinformation during data interpretation. Numerical studies involving simple shapes and complex geometries mostly based on actual geological cross-sections from karst environments were used to compare the effectiveness of MERIT2D in terms its high depth resolution and is compared in detail with traditional 2D and 3D surface resistivity methods of equal foot prints. Similar comparison was made between MERIT3D technique and 3D surface resistivity measurements. Results show that both methods achieve high depth resolution compared to their equivalent traditional resistivity methods. Laboratory experiment conducted using a complex analogue model mimicking actual sinkhole structure is used to test MERIT2D. Also laboratory experiment involving a 3D printed plastic cave model mimicking an actual cave was conducted using MERIT3D approach. Both results show the promise of MERIT approach to image and solve complex geological structures or problems. Finally, the method is applied to collect field data in three case study sites involving complex karst related sinkhole structures and an old landfill site. The result shows the promising capability of the MERIT technique to study challenging geologic conditions with high depth resolution

Improved 2D and 3D resistivity surveys using buried electrodes and optimized arrays: The multi-electrode resistivity implant technique (MERIT)

This thesis presents a novel resistivity method called Multi-Electrode resistivity technique (MERIT) that is used for high resolution imaging of complex geologic features at depth and near the edges of survey lines. The MERIT electrodes are especially shaped and designed to be self-driven using a robust-direct push technique. Measurements are taken using optimized arrays that are generated using a modified version of the “Compare-R” optimization algorithm. This work focused on both two-dimensional (MERIT2D) and three-dimensional (MERIT3D) applications of the buried array and show the relevance of the additional information gained by the addition of deep electrodes especially in sites with limited survey area. Numerical and laboratory studies are used to test and develop the technique and are later applied to image complex subsurface geologic structures on the field. The configuration of MERIT arrays brings some additional problems in terms the sensitivity of the deep MERIT arrays to a problem of non-uniqueness, mis-information, geometric error and resolution break between the two layers of electrodes. Multiple vertical resolution characteristic curves (RC curves) are analyzed to study the effect of array type, resistivity contrast, target resistivity and implant depth on the above-mentioned problems. Results show that MERIT measurements taken using standard dipole -dipole and wenner arrays along the surface and deep electrodes will strongly suffer from the problem of non-uniqueness or ambiguity while measurements taken using optimized arrays is suitable for MERIT configuration and will not suffer from any problem of ambiguity or non-uniqueness. Based on our result, a procedural guideline is developed to determine optimal MERIT implant depth and resolution cutoff that can be used for successful field implementation and for controlling misinformation during data interpretation. Numerical studies involving simple shapes and complex geometries mostly based on actual geological cross-sections from karst environments were used to compare the effectiveness of MERIT2D in terms its high depth resolution and is compared in detail with traditional 2D and 3D surface resistivity methods of equal foot prints. Similar comparison was made between MERIT3D technique and 3D surface resistivity measurements. Results show that both methods achieve high depth resolution compared to their equivalent traditional resistivity methods. Laboratory experiment conducted using a complex analogue model mimicking actual sinkhole structure is used to test MERIT2D. Also laboratory experiment involving a 3D printed plastic cave model mimicking an actual cave was conducted using MERIT3D approach. Both results show the promise of MERIT approach to image and solve complex geological structures or problems. Finally, the method is applied to collect field data in three case study sites involving complex karst related sinkhole structures and an old landfill site. The result shows the promising capability of the MERIT technique to study challenging geologic conditions with high depth resolution

Imaging of Deep Sinkholes Using the Multi-electrode Resistivity Implant Technique (MERIT) Case Studies in Florida

Surface geophysical methods have been extensively utilized for sinkhole investigations. While surface geophysical methods can penetrate to depth where sinkhole development occurs the resolution is typically poor. A detailed understanding of deep raveling zones into sinkhole throat through a new and novel geophysical technique was developed by the authors. The authors performed over 750 sinkhole investigations on residential properties over a five year period of time, in each case geophysical methods of Ground Penetrating Radar (GPR) or Electrical Resistivity (ER) were performed. Over 1500 confirmatory Standard Penetration Test (SPT) drillings were performed of the geophysical anomalies. In a very large percentage of the geophysical surveys performed, the location, size and depth of the raveling zones into the sinkhole throat could be clearly identified. The authors developed a novel geophysical technique called The Multi-Electrode Resistivity Implant (MERIT) to address the need to image deeper into karst formations to help identify the location of deep raveling zones and sinkhole throats. The purpose of this paper is to present case studies of the application of MERIT technology. Three case studies are presented in this paper. The first case study focuses on the first application of MERIT at the Bordeaux Village in Tampa, Florida where a sinkhole swallowed a car in 2010. The MERIT survey was able to image the car in the sinkhole throat. This case study demonstrates the ability of the MERIT technique to identify the location of the sinkhole throat by identifying the depth and location of the car, a large conductive ER anomaly. The second case study focuses on a pipeline in Orlando, Florida being threatened by sinkhole development on the adjacent property. MERIT was able to identify size and depth of raveling of soils into the sinkhole throat near the pipeline. The results of the MERIT image were critical in engineering design to address the treatment to the pipeline. In the third case study MERIT technology was applied to a proposed roadway through an extensive karst region of Lake County, Florida. Initial geotechnical investigation indicated a potentially large and deep sinkhole feature. MERIT was able to provide a concise geologic structure including the identification of the locations sinkhole throat at 52m deep. MERIT has been shown to identify details of the complex geology and geometry of karst formations. In particular the techniques ability to provide improved image capabilities of the raveling zone and sinkhole throats has significant engineering applications for assessment, risk analysis, and remediation of sinkholes

Imaging of Deep Sinkholes Using the Multi-electrode Resistivity Implant Technique (MERIT) Case Studies in Florida

Surface geophysical methods have been extensively utilized for sinkhole investigations. While surface geophysical methods can penetrate to depth where sinkhole development occurs the resolution is typically poor. A detailed understanding of deep raveling zones into sinkhole throat through a new and novel geophysical technique was developed by the authors. The authors performed over 750 sinkhole investigations on residential properties over a five year period of time, in each case geophysical methods of Ground Penetrating Radar (GPR) or Electrical Resistivity (ER) were performed. Over 1500 confirmatory Standard Penetration Test (SPT) drillings were performed of the geophysical anomalies. In a very large percentage of the geophysical surveys performed, the location, size and depth of the raveling zones into the sinkhole throat could be clearly identified. The authors developed a novel geophysical technique called The Multi-Electrode Resistivity Implant (MERIT) to address the need to image deeper into karst formations to help identify the location of deep raveling zones and sinkhole throats. The purpose of this paper is to present case studies of the application of MERIT technology. Three case studies are presented in this paper. The first case study focuses on the first application of MERIT at the Bordeaux Village in Tampa, Florida where a sinkhole swallowed a car in 2010. The MERIT survey was able to image the car in the sinkhole throat. This case study demonstrates the ability of the MERIT technique to identify the location of the sinkhole throat by identifying the depth and location of the car, a large conductive ER anomaly. The second case study focuses on a pipeline in Orlando, Florida being threatened by sinkhole development on the adjacent property. MERIT was able to identify size and depth of raveling of soils into the sinkhole throat near the pipeline. The results of the MERIT image were critical in engineering design to address the treatment to the pipeline. In the third case study MERIT technology was applied to a proposed roadway through an extensive karst region of Lake County, Florida. Initial geotechnical investigation indicated a potentially large and deep sinkhole feature. MERIT was able to provide a concise geologic structure including the identification of the locations sinkhole throat at 52m deep. MERIT has been shown to identify details of the complex geology and geometry of karst formations. In particular the techniques ability to provide improved image capabilities of the raveling zone and sinkhole throats has significant engineering applications for assessment, risk analysis, and remediation of sinkholes

Statistical Analysis of GPR and SPT Methods for Sinkhole Investigation in Covered Karst Terrain, West-Central Florida, USA

pg(s) 247-25

Monitoring and Modeling of Sinkhole-Related Subsidence in West-Central Florida Mapped from InSAR and Surface Observations

Sinkholes in Florida cause millions of dollars in damage to infrastructure each year. Methods of early detection of sinkhole-related subsidence are clearly desirable. We have completed two years of monitoring of selected sinkhole-prone areas in west central Florida with XXX data and analysis with XXX algorithms. Filters for selecting targets with high signal-to-noise ratio and subsidence over this time window (XX-2015-XX-2017) are being used to select sites for ground study. A subset of the buildings with InSAR-detected subsidence indicated show clear structural indications of subsidence in the form of cracks in walls and roofs. Comsol Multiphysics models have been developed to describe subsidence at the rates identified from the InSAR analysis (a few mm/year) and on spatial scales observed from surface observations, including structural deformation of buildings and ground penetrating radar images of subsurface deformation (length scales of meters to tens of meters). These models assume cylindrical symmetry and deformation of elastic and poroelastic layers over a growing sphering void

Monitoring and Modeling of Sinkhole-Related Subsidence in West-Central Florida Mapped from InSAR and Surface Observations

Sinkholes in Florida cause millions of dollars in damage to infrastructure each year. Methods of early detection of sinkhole-related subsidence are clearly desirable. We have completed two years of monitoring of selected sinkhole-prone areas in west central Florida with XXX data and analysis with XXX algorithms. Filters for selecting targets with high signal-to-noise ratio and subsidence over this time window (XX-2015-XX-2017) are being used to select sites for ground study. A subset of the buildings with InSAR-detected subsidence indicated show clear structural indications of subsidence in the form of cracks in walls and roofs. Comsol Multiphysics models have been developed to describe subsidence at the rates identified from the InSAR analysis (a few mm/year) and on spatial scales observed from surface observations, including structural deformation of buildings and ground penetrating radar images of subsurface deformation (length scales of meters to tens of meters). These models assume cylindrical symmetry and deformation of elastic and poroelastic layers over a growing sphering void

Complex relationships between surface topography, ground motion, and cover sediments in covered karst, west-central Florida, USA

Sinkhole processes can be more complicated than vertical drainage or collapse of sediments into an underlying limestone void. To better understand the relationships between surface and underlying karst structures, geodetic and geophysical methods were applied to high-resolution mapping of active sinkhole features in covered karst, west-central Florida, USA. Cracks in a pool house at the Sandhill Scout Reservation prompted surface and subsurface investigations in a grassy open field with a distinct ~60-m diameter topographic low west of the pool area. Beneath the smooth topographic low, ground-penetrating radar (GPR) with limited penetration (up to 6 m depth) shows incongruent smaller-scale (~5–20 m) variability in a horizon draping the limestone surface. Electrical Resistivity Tomography (ERT) profiles provide a broader overview of the underlying karst system (to depths ~25–36 m) and show possible voids in the limestone bedrock beneath a local topographic high. Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) analysis of ~2 yr of TerraSAR-X satellite data from two corner-reflectors installed in the topographic low reveals a 1 mm/yr subsidence rate on the flank of the topographic low but stability in its center. This suggests that subsidence has halted in the central topographic low and may be occurring on smaller scales elsewhere within the survey area. The data suggest that non-vertical fluxes of sediment significantly smooth surface topography relative to underlying heterogeneities and that activity migrates within complex systems. Our results also illustrate the benefits of corner reflector installations for resolving subsidence in vegetated environments. The 1-mm/yr rate of motion on the grassy field could not be resolved with InSAR before reflector installation