Preliminary results from coupled modeling of gravity and aeromagnetic data in the Waukesha Fault area of southeastern Wisconsin

Increased concerns recently about the quantity and quality of groundwater resources in Wisconsin have brought about the need for better understanding of the subsurface geologic lithology and structure that controls groundwater flow. Concern centers on excessive drawdown in the deep sandstone aquifer system throughout southeastern Wisconsin and its relation to increasing levels of total dissolved solids and radium in some municipal supply wells. It is possible that preferential flow paths influence the circulation of groundwater to these wells and their vulnerability to contamination. The largest feature that could channel vertical and horizontal flow at the regional scale is the northeast trending Waukesha fault zone that intersects the main pumping center at Waukesha. The normal vertical displacement along the Waukesha Fault produces strong gravity and aeromagnetic anomalies coincident with the northeast trend of the fault. Previous gravity modeling studies associated with the fault are generally unconstrained yielding results with wide variation in fault geometry and vertical fault offset. Reasonable model fits were obtained with fault dip ranging from 10^o to 85^o with vertical offset ranging from 500 to 700 m. This study provides well-constrained results from coupled modeling of gravity and aeromagnetic (potential fields) data and incorporation of lithologic depths from the lithology database used to construct a regional numerical model for southeastern Wisconsin. We also utilized depth information from the only well (USGS Test Well, Zion, IL) that penetrates the entire Cambrian-Ordovician aquifer system to reach the Precambrian basement on the down-thrown (southeast) block of the fault. Preliminary modeling of gravity and aeromagnetic data along two profiles suggests vertical offset of Precambrian basement from 700 to 800 m along a normal fault with a 60^o southeast dip. Based on thickness of the Cambrian Mount Simon sandstone, preliminary results also suggest that most of the offset occurred during the Cambrian. The results of this study can be used to refine the representation of heterogeneity in the regional groundwater model and improve its value as a management tool.