STRATIGRAPHIC MAPPING OF THE UPPER IOWA RIVER WATERSHED USING AIRBORNE GEOPHYSICS
A major groundwater issue in the UIRW stems from a limestone-rich karst layer that is locally overlain by relatively thin unconsolidated sediments. The combination of fractured karst and thin cover means surface contaminants can enter the water system and spread rapidly in the subsurface, making it difficult to predict and manage groundwater quality. The airborne electromagnetic (AEM) data are well suited to map out areas that are vulnerable to contamination. Although AEM data cannot directly image voids and fractures in the karst, an underlying clay-rich aquitard is clearly delineated. Mapping this interface, especially where it nears the surface, is important to refining groundwater flow estimates.
The incorporation of lithologic data from 768 drillholes allowed us to develop a resistivity stratigraphy which can be applied to resistivity cross-sections throughout the UIRW. In addition, several wells had down-hole resistivity logs, which were used to confirm which stratigraphic picks are identifiable based solely on resistivity. Using this approach we are able to estimate the configuration and spatial variability of several aquifers and aquitards. Such information provides a high-resolution framework that can be directly input into hydrologic models.
Prominent interfaces recovered from our resistivity stratigraphy include the tops of the Ordovician Maquoketa Formation, Galena Group, Decorah shale, and St. Peter sandstone. The top of the Cambrian Eau Claire Formation is also clearly imaged, and defines a proxy for the minimum depth to the Precambrian. These depth estimates are being examined together with those derived independently from the airborne gravity gradiometry and magnetic data sets.