Imaging steeply dipping near-surface mine workings using surface seismic waves

CIM Bulletin, Vol. 98, No. 1089, 2005

S.D. Butt, C. Xu, M.D. Vance and G.C. Corbett

Surface subsidence and sinkhole development over abandoned and collapsing underground mine workings are major environmental and public safety hazards in some areas. Small-scale undocumented workings are particularly problematic because their presence is often unknown until infrastructure has been developed over them and there is an unexpected subsidence event, often requiring expensive site stabilization or abandonment. Surface wave imaging is a relatively new seismic technique that generates subsurface depth images without having to drill boreholes and is becoming a standard tool in non-destructive geotechnical characterization. The objective of this investigation was to evaluate the capability of surface wave seismic techniques to image narrow steeply dipping abandoned workings at the Montague Gold Mines district near Dartmouth, Nova Scotia. This district has numerous abandoned workings from historical mining activity and has actively subsiding and collapsing crown pillars. The steeply dipping geology, and narrow workings (often less than 1 m) at the Montague site were conditions not typically encountered for routine surface wave imaging. These conditions were addressed in the imaging system developed for these field trials.Two locations at the Montague district were selected for field trials following a preliminary site survey. At each site, mine records and surface subsidence patterns indicated the presence of two parallel steeply dipping workings. The seismic data recorded at both sites were processed to generate seismic velocity versus depth images and a comparative gravity survey was also conducted at one site. On the velocity-depth images, narrow vertical zones of zero velocity are prominent at the same lateral positions where vertical mine workings were expected. This is shown in the figure where both the zero velocity zones are present where workings were mapped, and where the gravity survey shows prominent negative anomalies. Overall, the results of the study indicate that surface wave imaging techniques have good potential to image shallow steeply dipping mine workings. However, there are several key areas for future work in this area, including: 1) additional field trials where ground truthing, either by drilling boreholes or excavation of the site, is conducted to rigorously evaluate the accuracy of the seismic images; 2) improvements in the waveform processing methodology to reduce the processing time and improve the depth resolution; and 3) continued work in a related research program to quantify the relationships between measured seismic attributes and mechanical rock mass quality, subsidence potential, etc.