SEISMIC REFRACTION FOR CHARACTERIZATION OF THE CHATTANOOGA FEN SITE, COLORADO

摘要

Iron fens are low pH, peat-forming wetlands that support an acid-tolerant flora unique to Colorado. Groundwater circulating though highly mineralized rock masses acquires dissolved ferrous iron via dissolution of sulfide minerals. Upon emergence at the surface, ferrous iron is oxidized, forming iron oxyhydroxide terraces and creating the conditions for the natural ecosystem. The Chattanooga Fen site is located about 6 miles north of Silverton, CO in the Mineral Creek basin. Understanding groundwater recharge source and pathways are critical as potential development in the basin could adversely affect the fens hydrology. In highly-fractured bedrock masses, in-situ groundwater chemical tracing and age-dating are valuable tools in understanding hydrogeologic conditions. Seismic surveys were performed to identify basin features such as depth to bedrock and transitions within the overburden. This information was used to aid in identifying locations and depths of monitoring wells for use in collecting groundwater samples and for monitoring of water levels. Seismic refraction data were acquired along three profile lines ranging from 380 to 710 feet in length. A sledge hammer was used as the source with 4.5 Hz geophones at 10-ft intervals along each line. Data were processed with the Rayfract software package. Surface wave analysis was also performed on portions of the data with SurfSeis to allow comparison of p-wave and shear wave sections. The seismic data indicated deeper bedrock than was anticipated, with a shallow fat clay layer producing a high-velocity zone. Seismic sections correlated well with subsequent sparse borings.