Seismic reflections around a tunnel 'viewed' through waveguides - A case study

摘要

A two-lane tunnel on a busy highway will be expanded. Reflected seismic waves were used to map structural features in the surrounding rock from the inside of the tunnel. The results of seismic imaging, largely in accord with the geological investigation, provided 3D characterization of the rock mass useful for the expansion project. Parts of the concrete liner of the tunnel had separated from damaged rock. This posed a challenge to conducting the seismic ground characterization from inside the tunnel. The uniqueness of the seismic survey approach was not the use of a small, hand-held source driving a swept frequency signals into the rock which allowed conducting the survey without stopping the traffic; nor was it the use of high sensitivity, broad-band accelerometers for detecting seismic waves in the rock. It was the fiberglass waveguides (1 meter long), anchored to the rock via holes drilled through the liner that introduced the uniqueness. These waveguides provided reliable coupling to the rock mass for sources and receivers mounted inside the tunnel near the inner surface of the liner. The sources and receivers, in interchanging arrays, formed 14 sets of "directional seismic antennae" along the tunnel, each deployed around the liner circumference within the limits of having to leave one lane open to traffic. Each "antenna" was used for imaging its own section of the rock mass around the tunnel. Overlapping ground images were then joined to successfully map the whole length of the tunnel.