THE APPLICATION OF MULTICHANNEL ANALYSIS OF SURFACE WAVES TO IDENTIFY ABANDONED MINE TUNNELS

Multichannel Analysis of Surface Waves (MASW) is a geophysical method which has become a valuable tool for investigating the near-surface. MASW relies on the generation of dispersive surface waves, which historically have been considered noise on a seismic record, to extract shear wave velocities through an inversion process. Rayleigh waves (R-waves) have some advantages over body waves when used to investigate the near-surface. R-waves are less sensitive to noise, have large amplitudes when compared to body waves, and are relatively easy to produce using an impact seismic source.

Mine subsidence is a problem that Western Pennsylvania has historically faced. Public records on the locations and extent of inactive mines are poor. As such, many structures in Western Pennsylvania have been built above these unknown mine workings and are at risk of damage due to mine subsidence. A non-invasive surface method such as MASW has the potential to be used to locate these abandoned mines quickly and with a much lower cost to engineers than traditional bore-hole methods.

A survey site in Butler County, PA was chosen to employ MASW techniques in an attempt to locate abandoned mine tunnels. Based upon historical documents, mine tunnels are known to exist beneath portions of the campus of Slippery Rock University. Twenty-four 28 Hertz geophones with a 1 meter spacing were used as receivers and the source was a 10 pound sledgehammer struck on a steel plate with a 6 meter offset distance. The survey employed a roll-along data acquisition with a moving window of 2 meters and covered a survey distance of 64 meters. Preliminary processing shows two low shear wave velocity anomalies at a uniform depth and size separated by 2 meters. These low velocity shear wave zones are interpreted as mine tunnels with an area demonstrating high shear wave velocity separating the two. This area of high shear wave velocity displays a width of correct size to indicate a supporting pillar.