Site Characterization of an Abandoned Gold Mine Using the Multi-Channel Analysis of the Surface Waves Method

Geophysical methods and tools are becoming more widely used in the geotechnical and engineering arena. The non-invasive nature and capability to survey significant areas in a rapid and cost-efficient manner typically make them an attractive complementary tool to conventional drilling and invasive investigations. The multi-channel analysis of surface waves (MASW) is one method that has seen growing application in the geotechnical world. We present a case study from an assessment of a historic gold mine site in Colorado. The site was extensively mined in the late 19th and early 20th centuries and left a legacy of collapsed shafts and shallow, typically poorly documented, mine workings. The objective of the survey was to investigate the upper 30 to 50 feet of the subsurface to characterize potential collapsed or backfilled mine workings in advance of the construction of a new mill pond. A total of 40 MASW profiles were collected, along several 1000 foot sections of the targeted mineral veins across the site. While there are numerous surface expressions of old or collapsed shafts across the site, there was limited additional subsurface information to ground truth the geophysics. A significant number of targets in the subsurface were identified in the MASW profiles. A hazards map of the site was produced highlighting potential weak zones or void space in the subsurface for the ensuing earthworks and construction activities.