Abs. No. 50102

GEOPHYSICAL SURVEYS ALONG A LINE TRENCHED ACROSS THE ANIMAS RIVER VALLEY, COLORADO

Author(s): CAMPBELL, David L., U.S. Geological Survey, MS 964 Box 25046, Denver CO 80225, davec@usgs.gov; VINCENT, Kirk R., U.S. Geological Survey, 3215 Marine St., Boulder CO 80303.

Keywords: geophysics, radar, IP, trench, Animas

In August, 1998, BLM and USGS excavated a ~250m-long trench across the Animas River valley floor about a mile below the ghost town of Eureka, Colorado, in a glacially sculpted alpine valley. Because large amounts of ore-mill tailings were released to the stream early this century, the river is now braided, the flood plain nearly devoid of vegetation, and historical stream deposits contain tailings and elevated metal concentrations. Before trenching began, ground penetrating radar (GPR), spectral induced polarization (SIP), magnetic, and electromagnetic geophysical measurements were made along the trench line. The geophysical interpretations were then compared with the stratigraphy documented in the trench wall. The GPR profiles, made with 300 MHz antennas, showed waves reflected from different interfaces throughout the depth of the trench (~1.5m). These interfaces included most of the near-horizontal stratigraphic boundaries that were documented in the trench wall, but there were additional reflections from undocumented features that may represent textural boundaries. The GPR sections missed most near-vertical interfaces, but did show bright spots where horizontal beds were cut off by vertical contacts. The SIP work was done using 2m dipoles, so that its resolution was of that order. Therefore, SIP missed thin units, including practically all of the historical mill-tailings deposits. The SIP interpretations correctly showed the general locations of electrically conductive silt units and electrically polarizable deposits of gravel with MnO clast-coatings.

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