RETTEW’s Geophysics team recently assisted a client with a tunneling project. The tunnel path was at a depth at which surface geophysics could not resolve potential small zones of concern, including low-stiffness weathered rock that might require special support, and water-bearing seams that could flood the opening. The required resolution could only be achieved using crosshole geophysical tomography—seismic P- and S-wave for material strength, and electrical resistivity imaging for water-bearing zones. To perform this highly specialized work, RETTEW acquired equipment and software that makes us the best-equipped—and now most-experienced at—crosshole geophysics firm in the United States. While both methods worked, the seismic tomography was superior.
A repeating pattern of alternating high- and low-velocity zones (grey and yellow on the cross section above) initially appeared to be a possible artefact. With checking and rechecking the data and processing, this pattern persisted, and was completely explained when we found a nearby outcrop in the same geologic formation that displayed alternating more- and less-deeply weathered bands with exactly the orientation we recorded in the seismic data. Knowing the locations and thicknesses of the deeply weathered zones allowed precise application of deep ground improvement to support the tunneling, leading to a successful project.