Seismic Reflection and Gravity Constraints on the Bedrock Configuration in the Greater East Missoula Area

Author

Frank Janiszewski, The University of Montana

Year of Award

2007

Document Type

Thesis

Degree Type

Master of Science (MS)

Other Degree Name/Area of Focus

Geophysics

Department or School/College

Department of Geosciences

Committee Chair

Steven Sheriff

Commitee Members

James Sears, Jesse Johnson

Keywords

depth to bedrock, gravity, inversion, milltown, seismic reflection

Abstract

The greater East Missoula, MT area is the site of numerous studies to track possible groundwater contamination from the EPA Superfund Site at the Milltown Dam. The accuracy of these groundwater models depends on many factors, one of which is the accuracy to which the bedrock topography is mapped. Currently, a map based heavily on a gravity survey provides the most detailed map of the bedrock. The accuracy of this map may be improved through the use of seismic reflection techniques, better estimates of the density contrast used in the gravity modeling, and by extending the gravity survey to include more data and a broader area. The seismic reflection technique used to supplement the gravity data is the optimum offset technique. This method simplifies field collection of the data and processing of the data. The final result of this method is a seismic section showing the depth to different reflectors in the subsurface, one of which is the bedrock. In order to improve the estimate of the density contrast used in the gravity modeling, the homogeneity of the valley fill was tested. This was done by comparing the results from two different modeling programs, one of which let the density contrast vary, to see if there was an improvement in the final result. The gravity survey was also extended to incorporate a larger area and more data. The results show that seismic reflection can be used to improve the depth estimate in the valley where the depth is shallow and that the density contrast is most likely homogeneous. The extended gravity survey provided more data to work with and the final result is a map of the bedrock topography for the greater East Missoula Area that incorporates all currently known data and provides a sufficiently accurate estimate of the depth to be used in groundwater models.

Recommended Citation

Janiszewski, Frank, "Seismic Reflection and Gravity Constraints on the Bedrock Configuration in the Greater East Missoula Area" (2007). Graduate Student Theses, Dissertations, & Professional Papers. 1219.
https://scholarworks.umt.edu/etd/1219