We were asked to locate six abandoned mine shafts on a site which was undergoing active reclamation.  Mine shafts were thought to exist after being located on old maps, but no surface expression could be detected and the old maps were the only evidence that a potential problem existed.  It was too dangerous and expensive to shut down the remedial effort and go looking for them with an excavator.  To make matters worse, the surface topography had changed since the maps were made and the maps' accuracy was unknown.

Ground penetrating radar did not work well on the site because highly conductive mine tailings covered the surface. We decided to use 3D electrical resistivity because of the sharp contrast in conductivities between the bedrock, the tailings, and the void space.  The method was quite successful and we were able to flag all six shaft locations prior to leaving the site.  The project was completed in two days and did not interrupt the site activity.

This is a horizontal depth slice from one of the 3D grids. The shaft cap is shown here 2 meters below the ground surface. The shaft dips west at about a 45 degree angle, exiting the grid by the fourth depth panel.

This vertical slice through the 3D model shows the shaft going deeper and to the west.  The panels are shown starting in the east and progressing westward.