Date of Award
Master of Science (MS)
Geophysical investigations, designed to characterize unique subsidence features of unknown natural origin, provide imaging analysis of potential causes of subsidence. Recent subsidence in the alluvial plain in Butte, Montana is unrelated to historic mining in the area. This study aims to utilize a combined application of shallow electrical resistivity tomography (ERT), self-potential (SP), and frequency-domain electromagnetic (FDEM) methods in order to develop a better understanding of the particular set of hydrogeological and environmental conditions that contribute to this unique phenomena. Geophysical measurements provide lateral and vertical variations of electrical resistivity in the subsurface to a depth of 10 m while also contouring the streaming potential in each site to help characterize the site-specific groundwater flow components. Least-square inversion resistivity models and conductivity from electromagnetic data are compared to known well lithologic information to identify general variations of sediments with depth as well as delineate the extent of the known subsidence features. Site investigations indicate that the subsidence features in basin fill sediments are spatially associated with electrical and electromagnetic signatures of water seepage and sharp contacts between resistive and conductive sediment layers. ERT results showed a circular resistive anomaly in place of known subsidence locations and delineated lithologic heterogeneity in each site, suggesting a clay contact at shallow depth. The FDEM results complemented ERT and further characterized the depth and thickness of the clay lens. The SP results indicated surficial seepage associated with subsidence locations. This work sets a baseline site characterization and analysis on the origin of these subsidence features, where the subsidence is expected to be associated with volumetric changes in clay and porous media during surficial seepage.
Prudhomme, Kristen, "Integrated geophysical methods to characterize subsidence in Butte, Montana, U.S.A." (2018). Graduate Theses & Non-Theses. 179.