Precambrian Basement Surface Estimation using Coupled 3D Modeling of Gravity and Aeromagnetic Data in Fond du Lac County and Southeastern, Wisconsin

Background/Need

Increased concerns about groundwater resources in Wisconsin have brought about the need for better understanding of the subsurface geologic structure that lead to developing conceptual hydrogeologic models for numerical simulation of groundwater flow. Models are often based on sparse data from well logs usually located large distances apart and limited in depth. Model assumptions based on limited spatial data typically requires simplification that may add uncertainty to the simulation results and the accuracy of a groundwater model. This research provides another tool for the groundwater modeler to better constrain the conceptual model of a hydrogeologic system. The area in southeastern Wisconsin near the Waukesha Fault provides an excellent research opportunity for our proposed approach because of the strong gravity and aeromagnetic anomalies associated with the fault, the apparent complexity in fault geometry, and uncertainty in Precambrian basement depth and structure. Precambrian basement surface throughout Fond du Lac County is known to be undulated and this uneven basement topography controls water well yields and zones of stagnant water. Therefore, an accurate estimation of the basement topography in Fond Du Lac County is vital to determining ground water flow and quality of groundwater in this region.

Objectives

The objectives of this research are to improve the current understanding of the subsurface Precambrian basement topography in southeastern Wisconsin and in Fond Du Lac County. Results from coupled modeling of gravity and aeromagnetic data along profiles (Skalbeck et al., 2007) in this area show that the estimated bedrock surface is uneven on both sides of the Waukesha Fault. Although, this modeling greatly improved our understanding of Precambrian bedrock topography in southeast Wisconsin, detailed estimation of this surface is limited by the 10 km spacing between profiles. The 3D modeling of gravity and aeromagnetic data from this study provides an even better definition of the Precambrian bedrock surface topography and the fault geometry because the model grid density is much greater (1 km grid) relative to the profile separation. The second objective it to provide a better estimate of the uneven Precambrian basement topography that has been documented throughout Fond du Lac County (Smith, 1978; Newport, 1962). Because basement surface relief is dramatic over short lateral distances in Fond du Lac County, 3D modeling of gravity and aeromagnetic data is particularly well suited for this area. Study results yield highly constrained subsurface Precambrian elevation maps for southeastern Wisconsin and Fond du Lac County that may be valuable for refining existing numerical groundwater models.

Methods

Three dimensional (3D) models of the Precambrian basement were developed by modeling existing gravity and aeromagnetic data using computer software GMSYS-3D and Oasis Montaj. The models are constructed with 1000 m grids for each data set and each geologic unit. Initial density and magnetic susceptibility values for the layers were obtained from modeling results in southeastern Wisconsin (Skalbeck et al., 2007). Blocks were assigned constant density and magnetic susceptibility or internal variance of these physical parameters calculated by GM SYS 3D. The forward modeling option of GM-SYS 3D is used initially to calculate the model anomaly and it statistics relative to the observed anomaly. The inverse modeling option is used for the remaining model runs to adjust the geologic model surface elevation and the block density or magnetic susceptibility values o optimize the model calculated anomalies to the observed gravity and magnetic anomalies. We employed a modification of model acceptance criteria from previous studies (Skalbeck, 2001; Skalbeck et al., 2005; Skalbeck, 2007) by using percent standard deviation ([% SD]; SD/ anomaly range]).