Delineation of an old coal mine in an urban environment with surface wave seismics using a landstreamer and laterally constrained inversion

Prior to the site investigation for a tunnel below Helsingborg, southern Sweden, a surface wave seismic investigation was made to delineate an old coal mine. The mine as described in old literature has an area of about 6 acres and each layer of coal has a height of less than one 1 m; however, the exact location and status is unclear. The sedimentary geological setting consists of fill, quaternary deposits, shale, coal and sandstone. The mine, or alternatively the coal, is found at 10 m depth between a layer of shale and a layer of soft sandstone. The seismic measurements were made along two crossing profiles, located on the walkways covered with gravel, in the area where the mine is expected. The measurement system was a landstreamer with 244.5 Hz geophones, a Geometrics Geode and a shotgun. The v{sub s} models clearly show increasing velocities with depth with a low velocity layer at 10 m depth. The results correlate well with the expected geology and results from geotechnical drillings that indicate an open mine in parts of the area; however, the low velocity layer is mainly due to the soft sandstone and does not seem to be strongly affected bymore » the presence of the open mine.« less