Crosshole seismic tomography for the detection of abandoned underground lignite mine galleries

Summary This work presents the results of crosshole seismic tomography geophysical survey at selected positions along the extension of Athens subway line #2, Ilion-Anthoupolis area (Greece). This geophysical survey aimed at: 1) the detection of abandoned underground lignite mine galleries at selected positions along subway line, 2) the mapping of geological formations and possible faults at the studied sites and 3) providing information for optimal design of tunnel path. Eleven (11) seismic tomograms of 225.3m total length and 520m total depth were generated between twenty (20) boreholes. The borehole source BIS-SH was utilized to produce highly repeatable compressional (P) waves at 2 m intervals and a hydrophone streamer consisted of 12 receivers at 1 meter intervals and a 12-channel seismograph were employed for seismic wave recording. First arrivals picking and travel time tomography were performed using SeisImager2D and ReflexW, respectively. Low velocity anomalies were found at 2 seismic sections, which probably are associated with abandoned underground lignite mine galleries or intact lignite layers. Moreover, indications for an unmapped fault presence revealed and a new tunnel path was proposed in order to avoid crossing the fault zone.