Determination of Aquitard and Crystalline Bedrock Depth Using Time Domain Electromagnetics

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 is the development of a conceptual model that includes hydrostratigraphic units, e.g., shale aquitards and sandstone aquifers. The depth, thickness, and extent of these units are usually determined from geologic logs, but in locations were the logs are sparse or nonexistent, the modeler is left with the difficult choice of deciding stratigraphic placement at depth with little information. This crucial first step is often “set in stone”. That is, the stratigraphy of the model is unlikely to be altered unless it is later found to obviously be in error. For this reason it is important to place the hydrostratigraphic units properly at the time of original model creation. The proposed research will provide the groundwater modeler in Wisconsin an additional tool for determining the placement of the hydrostratigraphic units. We propose to use a geophysical method, the Time Domain Electromagnetic (TDEM) method, to delineate the shaley facies of the Eau Claire formation, the Eau Claire shale, as well as to determine the depth to crystalline bedrock. This geophysical method was chosen because of its ease of deployment, and because it can be easily scaled to image the depths of interest. We chose to use the Eau Claire shale as our test formation because of its importance as a regional aquitard in much of southern Wisconsin, and because preliminary tests and modeling indicate that the TDEM method should be successful in imaging this important aquitard. Although an isopach map of the Eau Claire shale has been created for Dane county and portions of Sauk and Dodge counties, there has not been a compilation of all the well logs in Wisconsin to determine the statewide extent of the aquitard. Our first step in this research would be to create such a compilation. The resulting map would guide our investigation of the Eau Claire shale by providing points of known geology to calibrate our geophysical measurements. The map would also provide us with information on where a lack of data on this important regional aquitard exists. At those points we will employ our calibrated geophysical method for determination of the depth, and possibly, the thickness of the Eau Claire shale. The end product of this research will be an assessment of the TDEM method as a hydrostratagraphic mapping tool, as well as a revised map of the extent and thickness of the Eau Claire shale throughout southern Wisconsin.