Detection of tunnels and boulders using shallow SH-SH reflected seismic waves

We present three case studies on detecting buried glacial boulders, a sewage tunnel, and abandoned coal mine tunnels using shear-wave reflection methods. The seismic signature of such subsurface features is in the form of an isolated diffraction, distinctly recognized on seismic sections obtained from shallow seismic surveys using a transverse horizontal (H2) source and a multichannel landstreamer that consists of H2 geophones. We used H2 impulsive and vibrator sources with varying bandwidth. Based on field experiments with multicomponent recordings, we determined that the H2-H2 source-receiver configuration is the most optimal to generate downgoing horizontally polarized shear (SH) waves and upcoming SH reflected and diffracted waves. A shallow SH-SH image using a microvibe high-frequency sweep exhibits a wavelength between 1 and 2 m, which is comparable to that of a ground-penetrating radar image with the additional advantage of deeper penetration.