UTILIZATION OF THE SQUARE ARRAY EARTH RESISTIVITY METHOD FOR CHARACTERIZING ANISOTROPY IN FRACTURED SEDIMENTARY ROCK
Open Access
- Author:
- Yoxtheimer, David
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 17, 2019
- Committee Members:
- Andrew Arnold Nyblade, Dissertation Advisor/Co-Advisor
Andrew Arnold Nyblade, Committee Chair/Co-Chair
Derek Elsworth, Committee Member
Peter Christopher La Femina, Committee Member
Anthony Robert Buda, Outside Member
Mark E Patzkowsky, Program Head/Chair - Keywords:
- earth resistivity
square array
anisotropy
fractured bedrock
karst
hydrogeology
paradox of anisotropy - Abstract:
- Fractured bedrock aquifers are key sources of potable groundwater globally. Therefore it is important to characterize the presence and orientations of subsurface fractures that impact groundwater flow in order to develop, manage and protect these critical water resources. A significant challenge in characterizing groundwater flow in a fractured bedrock aquifer is determining if anisotropic conditions exist. The square array method provides a means to estimate geo-electrical anisotropy, which can be useful when conducting hydrogeologic investigations including mapping fracture orientations, siting water supply wells, conducting source water protection programs, or mapping contaminant plumes. However, this method has not been previously tested to characterize the hydrogeology of folded and fractured carbonate bedrock overlain by conductive layers of soil and epikarst, nor for characterizing shale formations. In this study, the square array method is used to measure the change in electrical resistivity of the subsurface with respect to azimuth at six locations in the Cambrio-Ordovician carbonate bedrock aquifer of Spring Creek watershed and the shale formation underlying the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), both located within the Appalachian Valley and Ridge Province of central Pennsylvania. The carbonate bedrock aquifer is mantled with residual soils consisting primarily of silt and clay loams of variable thickness (0 to greater than 10 meters) below which occurs an epikarst system that plays a significant role in shallow groundwater flow. The square array is used to characterize the carbonate aquifer’s bedrock strike- and fracture-related anisotropy and provide estimates of secondary porosity. The results show that the square array apparent resistivity data correlates well with known bedrock structure, in particular the resistivity minima for the deeper measurements (40- and 50-meter a-spacings) are coincident with the northeast-southwest orientation of bedrock strike and/or mapped fractures, where present. In addition, estimates of secondary porosity from the square array’s 40- and 50-meter a-spacings (range of 0.7-4.4% with a mean of 3.1%) generally compare favorably to independent estimates of bedrock structure and secondary porosity from outcrop measurements and groundwater level/streamflow recession data (1-5%). The results of this study demonstrate that the square array method can be used effectively in complex, fractured carbonate bedrock settings to characterize bedrock anisotropy and secondary porosity, which were field-validated based on bedrock outcrop structure and fracture geometry measurements. Previous square array studies have detected anisotropy and estimated secondary porosity in both carbonate and crystalline bedrock, however this research further validates the method by comparing field measurements to the square array data, and thus advances the method’s application. Geo-electrical anisotropy associated with inclined bedding planes and fractures in bedrock is often not factored into apparent resistivity results, much less data inversion, which can lead to misleading model results. In particular, the “paradox of anisotropy” occurs where collinear resistivity data are collected in areas with inclined bedding planes or fractures and longitudinal apparent resistivity is greater than transverse apparent resistivity, which is the converse of when true resistivity values are considered. The SSHCZO study expands critical zone research by evaluating the effects of anisotropy on earth resistivity measurements in a fractured shale bedrock setting using both square and Wenner arrays. The square array can be used to determine the magnitude and orientation of anisotropy, as it is not subject to the paradox of anisotropy, whereas collinear arrays are, including the Wenner array. In fractured shale bedrock the anisotropy effects can be significant, including the paradox of anisotropy, which can lead to significantly inaccurate models if not factored into the input data. In this study the square array was used to evaluate site anisotropy at variable depths, including through the soils and bedrock profile, including fractured, weathered and unweathered bedrock intervals. The square array’s anisotropy coefficient was then factored into apparent resistivity data from strike parallel and perpendicular 2-D Wenner arrays to correct for the paradox of anisotropy. The corrected 2-D resistivity data were then inverted to obtain model results where longitudinal resistivity values were lower than transverse resistivity values, as would be expected. In addition, a series of ten parallel 2-D Wenner arrays were run and used to create “pseudo” 3-D models of the site’s resistivity distribution using anisotropy corrections from the square array data for comparison to 3-D models without this correction. This sequence of steps resulted in a 3-D resistivity model that provided useful insights into shallow groundwater interflow at the SSHCZO site. Ultimately this study provides a method that can be applied to geophysically characterize the groundwater flow mechanisms in the critical zone and allow investigators to design subsurface monitoring programs to further advance hydrologic and hydrogeologic research. The combined results of these studies show the square array can be utilized in fractured carbonate and shale bedrock settings to ascertain geo-electrical anisotropy, which correlates well to bedrock and fracture strike. For the fractured carbonate aquifer the square array was validated to provide reasonable estimates of secondary porosity based on both outcrop fracture measurements and groundwater level recession analysis. In the shale bedrock setting, the square array provided a useful correction factor for the coefficient of anisotropy in 2-D collinear resistivity arrays, which then yielded useful insights into shallow groundwater flow conditions via 3-D modeling developed from the corrected 2-D resistivity arrays.