Date of this Version
Szopinski, P. 2019. Geophysical Analysis of the Midcontinent Rift’s Subsurface Structure in Southeastern Nebraska. Undergraduate Honors Thesis. University of Nebraska-Lincoln.
The Midcontinent Rift System (MCRS) is a 1.1 billion-year-old failed rift system that spans much of the North American continental interior. The MCRS is exposed at Lake Superior and is buried in the subsurface along its southwest-extending arm through southeastern Nebraska. Due to the presence of buried volcanic rocks, the MCRS has characteristic highly-pronounced potential field anomalies (gravity and magnetic). Despite these large anomalies, not much is known about the subsurface faulting associated with the rift zone in the Midwest. The goal of this project is to attempt to use integrated analysis of collected geophysical data from multiple methods to image one bounding fault of the MCRS.
This study examines a predicted bounding fault of the MCRS in Saunders County near Yutan, NE, by utilizing gravity, magnetic, and refraction/reflection seismic methods. In total, 16 gravity and 42 magnetic measurements along the profile were collected by researchers from the Lincoln and Omaha campuses of the University of Nebraska. Three seismic records were collected over the predicted fault. In addition to the fieldwork performed in Yutan, the data from 18 exploration wells from the Nebraska Oil & Gas Conservation Commission (NOGCC) were used to constrain the model, which was also correlated with a published cross-section of southeastern Nebraska.
All necessary gravity and magnetic corrections were applied to the potential field data, allowing for the derivation of the gravity and magnetic anomalies. These anomalies, along with seismic interpretations and prior published information about the subsurface, were used for the development of a cross-sectional model that attempted to match the measured potential field anomalies. The model consisted of several layers, constrained by seismic and well data; however, the depth of penetration from the seismic results was too shallow to image the fault. The deepest boundary interpreted from the seismic data was the base of the unconsolidated sediments, the thickness of which agrees with published well data. Based on interpretations of the gravity and magnetic anomalies, two possible locations for the subsurface fault were modeled, one of which corresponds to a published predicted location; however, there was not enough data to resolve between them to determine a unique interpretation. The conclusion was that more geophysical data must be collected in order to more accurately determine the fault’s location. Despite the fact that values from measured gravity surveying differed from previously published studies, this project served as a test for the UNL geophysics team to practice acquiring data in the field using multiple instruments. The findings and experiences from this project will be used for future fieldwork.