Application of Ground-Penetrating Radar to Map Stratigraphy of a Drained Carolina Bay and Aid Its Wetland Restoration

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Title: Application of Ground-Penetrating Radar to Map Stratigraphy of a Drained Carolina Bay and Aid Its Wetland Restoration
Author: Szuch, Ryan Paul
Advisors: C. W. Zanner, Committee Member
Rodney L. Huffman, Committee Member
Michael J. Vepraskas, Committee Co-Chair
Jeffrey G. White, Committee Co-Chair
Abstract: Carolina Bays (bays) are a wetland type found along the Atlantic Coastal Plain that occur as oval-shaped depressions. Knowledge of bay stratigraphy might improve inconclusive theories on bay formation and aid attempts at wetland restoration of drained bays. Ground-penetrating radar (GPR) provides high-resolution and continuous profiles of the subsurface but has seldom been used for large-scale investigations or in Carolina Bays. A GPR survey was performed at Juniper Bay, a 300 ha drained bay in Robeson County, North Carolina. The survey included 23.2 km of GPR transects and soil borings at 174 locations. The broad objective of the survey was to map Juniper Bay's stratigraphy, particularly to determine the depth, extent, and continuity of clayey horizons likely to act as aquitards. To prevent ambiguity in identifying the ground surface on GPR transects, a 'lift-test' was developed that delineated the surface. Spatial variation in wave velocity was addressed using a "reflector-interface matching" technique to determine velocity at multiple locations and at various depths within the bay. A linear calibration equation relating travel time of GPR waves to depth of soil interface was developed. The average deviation between observed and predicted depth to clayey horizons was 0.25 m (16% error). Error was mainly attributed to the survey's large scale, subsurface complexity, presence of organic soils, and depth of the horizons. The lift-test improved accuracy by 10%, and the use of multiple calibration points limited extrapolation of the calibration equation. These calibration methods should prove valuable for future study of GPR accuracy on large-scale, complex sites. Information obtained during the GPR survey and associated coring was used to describe Juniper Bay's stratigraphy and develop theories on its formation. The bay's stratigraphy consists of alternating layers of sands and clays. Clayey layers appear continuous over much of the bay except where truncated by features that seem to be paleochannels. Historic geomorphic events at Juniper Bay have varied spatially and temporally but have included repeated lacustrine deposition, fluvial deposition, and fluvial incision. The original bay probably expanded and incorporated a smaller bay and fluvial feature. Future GPR work and integration of geologic and hydrologic studies may aid our assessment of Juniper Bay's stratigraphy and evolution. Many bays have been drained for conversion to agriculture. Clayey subsurface strata commonly occurs in bays and act as aquitards, restricting vertical water flow. Modification to drainage systems could lead to restoration of wetland conditions. The North Carolina Department of Transportation (NCDOT) intends to restore Juniper Bay for wetland mitigation credit. Ground-penetrating radar interpretation found that clayey aquitards underlay most of the bay at an average depth of 1.64 m. An anomalous GPR reflection in the southeast corner of the bay was interpreted as a fluvial deposit that does not contain aquitards until 3 to 5 m. NCDOT should consider alternative restoration plans for this area. By comparing the depths of aquitards and drainage ditches, several areas were identified as likely locations of ditch-induced aquitard discontinuity. NCDOT should fill or line suspect ditches to prevent potential water losses. Hypothetical proposals by professional firms indicated that GPR could provide large volumes of data with cost and time efficiency. Ground-penetrating radar surveys are suggested as a useful tool for determining suitability of potential wetland restoration sites.
Date: 2005-01-14
Degree: MS
Discipline: Soil Science

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