Evaluation of MASW techniques to image steeply dipping cavities in laterally inhomogeneous terrain

Abstract This study evaluated the potential of using Multi-channel Spectral Analysis of Surface Wave (MASW) seismic techniques to image steeply dipping cavities in laterally inhomogeneous geological terrain. The primary test site was the Montague District, a historical gold mining area in Nova Scotia, Canada with numerous narrow steeply dipping mine workings, actively subsiding or collapsed crown pillars, and laterally inhomogeneous bedrock. Seismic surveys were conducted at two test sites using portable components and an effective geophone spacing of 1 m. A complimentary gravity survey was also conducted at one of the sites. Pre-processing focused on techniques which highlighted and then isolated the direct Rayleigh Waves for each waveform. Two MASW techniques were applied to the resulting pre-processed seismic sections; time delay mapping and shear velocity imaging. Both techniques showed the presence of time-delay or velocity anomalies where mine workings were mapped or inferred from surface subsidence patterns. Time-delay mapping was effective at determining the lateral position of the cavities, however, velocity imaging was able to generate images in both lateral distance and depth and thus has the greater potential to image both the shape and position of the cavities. Reduced velocity regions on the shear velocity images were consistent with the relative thickness of overburden at both test sites and the weakening of crown pillars at one site. Gravity surveying agreed with both the time-delay and velocity images and highlighted the use complimentary geophysical techniques to image near-surface cavities. Recommended future work includes additional field trials with rigorous ground truthing, improvements in the waveform processing methodology to reduce the processing time and the accuracy of depth resolution, and continued work in a related research program to quantify the relationships between seismic attributes and rock mass quality, subsidence potential, etc…