An Integrated Geophysical Approach for a Karst Characterization of the Marshall Space Flight Center

The Marshall Space Flight Center is a NASA facility located in a karst prone area of northern Alabama. The geology consists of Mississippian age Tuscumbia Limestone underlying clay residuum and an epikarst zone of variable thickness. Groundwater flow is affected by karst conduits, fractures, and structural trends through the site. In an effort to characterize karst features on a local and regional basis, geophysical measurements were integrated with existing well and boring data along with regional geologic information. Geophysical measurements included microgravity, multichannel analysis of surface waves (MASW), and geophysical logging. The use of multiple geophysical methods provided supporting datasets and overlapping depths of investigation. The measurements were focused on 14 contaminant Source Areas to provide a detailed characterization of the top of rock, the epikarst zone, and anomalous zones within the residuum and upper limestone. The data were also used to map the thickness of the weathered zone in the upper limestone that may contain karst conduits for preferential groundwater flow. Additional reconnaissance measurements were made throughout the site to provide a regional assessment of karst conditions. Models of the geophysical data generally correlate well with each other and with existing boring data. The regional data also show a good correlation with geologic structure maps of the area. The integrated geophysical approach to the karst characterization allowed multiple geologic variables to be assessed simultaneously with a greater level of spatial sampling than borings alone. The results of the investigation will be used to efficiently place new borings and develop a more complete understanding of the karst hydrogeology.