Determination of Aquitard and Crystalline Bedrock Depth Using Time Domain Electromagnetics

Background/Need
As groundwater needs and concerns have increased, larger and more complex groundwater flow models have been developed to address the problems associated with the exploitation of this precious resource. One of the first steps in creating a useful groundwater flow simulation for the Wisconsin region is the development of a conceptual model that includes hydrostratigraphic units, e.g., shale aquitards, sandstone aquifers, and streambed deposits in tills. The depth, thickness, and extent of these units are usually determined from geologic logs, but in locations where the logs are sparse or nonexistent, the modeler is left with the difficult choice of deciding stratigraphic placement at depth. Time domain electromagnetics is a geophysical tool that showed promise in filling in the gaps in the geologic record so that better flow models and understanding of geology can be realized. However, this tool needed further analysis to determine its accuracy under Wisconsin geologic conditions. Guidelines were needed to set boundaries on what structures the method is capable of resolving.

Objectives
The objective of this study was to provide an assessment of the Time Domain Electromagnetic (TEM) method as a hydrostratigraphic mapping tool and to delineate the shaley facies of the Eau Claire Formation.