MODELING OF GRAVITY AND AEROMAGNETIC DATA TO ESTIMATE PRECAMBRIAN BASEMENT TOPOGRAPHY IN SOUTHEASTERN WISCONSIN

Depth to Precambrian crystalline basement rock southeast of the Waukesha Fault (down-thrown block) exceeds the total depth of water wells with one exception (a USGS well in Zion, IL) due to normal vertical displacement along the fault. Unknown basement depths in this area increases uncertainty of groundwater flow models built to address numerous groundwater resource issues in southeastern Wisconsin. 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. Strong gravity and aeromagnetic anomalies associated with the fault, the large number of water wells documenting subsurface geology, and a reasonable database of density and magnetic susceptibility values for geologic units provides an excellent research opportunity for highly constrained potential fields modeling.

Coupled modeling of gravity and aeromagnetic data was performed to estimate subsurface structure along eight NE-SW profiles (perpendicular to the fault) and one N-S profile (Tie Line). Well log data constrains depth to basement for the profile portions northwest of the fault on the up-thrown block and depth to Cambrian Mount Simon Formation on the down-thrown block. Only the southern most profile (A-A') contains data to constrain the depth to basement on the down-thrown block. Subsurface models units were assigned density and magnetic susceptibility values based on published data and values calibrated from Profile A-A'. Model fits were judged acceptable when the cumulative error between observed and calculated values divided by the anomaly range was <5% for gravity and <10% for aeromagnetic. Basement depths from the profile models were used to obtain a 3D representation of the Precambrian basement topography. The study results show the Waukesha Fault as a high angle normal fault dipping to the southeast. Model topography of the Precambrian basement surface appears complex on both sides of the fault with maximum vertical displacement of 560 m.