Integrated geophysical methods to characterize urban subsidence in Butte, Montana, USA

摘要

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 imaging (ERI), 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 phenomenon. Geophysical measurements provide lateral and vertical variations of electrical resistivity in the subsurface to a depth of 6 m while also contouring the streaming potential to help characterize the site-specific groundwater flow components. Least-squares 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. ERI 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 ERI and further characterized the depth and thickness of the clay. The SP results indicated surficial seepage associated with subsidence locations. A borehole was hand-augered. We did a sieve test, atterberg limits test, and ASD Terraspec Halo shortwave-infrared (SWIR) mineral analysis. The findings showed a fines ration from 36.1% to 38.1%, and clays are dominantly kaolinite PX, montmorillinite, and some ferryhydrite. This work sets a baseline