A multimethod approach for deep-tunnel detection using near-surface electrical geophysics at a gold-mine remediation site

Pendrigh, Nicole (Zonge International, Inc.) | Sirles, Phil (Zonge International, Inc.) | Carlson, Norman (Zonge International, Inc.) | LaBrecque, Douglas (Multi-Phase Technologies) | Ivancie, Paul (AMEC)


At the Captain Jack Superfund site located four miles southwest of Ward, Colorado, acid mine drainage is trickling in to Left Hand Creek, which flows in to Boulder, Co (Figure 1). The water needs to be treated and monitored. The proposed solution is to use the mine tunnels for in-situ water treatment. Therefore, there is a need to determine the location of the mine tunnel system, if possible.

There is limited information regarding the extent of mineworkings and bedrock fractures through the reservoir zone of the Big Five tunnel. While these areas are accessible for tracked drilling rigs, some tunnel segments lie 400 to 500 feet underground. Because the historical maps of mine/tunnel workings have no survey data, locating the tunnel "target zones" either involves drilling holes on 5 to 10-footspacings across multiple transects, or utilizing geophysical methods to detect/locate the tunnel voids prior to drilling. Surface applications of geophysics such as electrical resistivity (ER) are recommended to obtain subsurface “imaging” of the tunnel locations. The benefit of ER methods is that they minimize the number of drill locations and per-foot drilling expense, as well as resulting surface disturbance requiring reclamation.

Surface and crosshole geophysical investigations, including time-domain dipole-dipole resistivity and frequency domain Mise-a-la-Masse (MALM) surveys, were conducted on the Captain Jack Project

Since the geophysical surveys were completed in 2012, the site has undergone downhole Electical Resistivity Tomography (ERT) and Magnetics monitoring and modelling. The bulkhead and remediation pumping system is in place as of summer, 2017. Surface and borehole monitoring equipment is in place, and in the near-future, the valve will be closed and in-place monitoring will begin.

Presentation Date: Thursday, October 18, 2018

Start Time: 8:30:00 AM

Location: 204A (Anaheim Convention Center)

Presentation Type: Oral