The seismic refraction method was used underground in a Saskatchewan potash mine to monitor the thickness of the salt formation throughout the mine. The statistical random error in the interpretation, which was due primarily to noise created by the mining machines, ranged from + or - 4-15 feet for typical thicknesses of about 130 feet. Seismic techniques aided in locating a nearly completed mine shaft with respect to existing underground workings. By using the relative time differences between geophones, the position of the center of the shaft was estimated to be 340 + or - 2 feet northeast and 235 + or - 10 feet northwest of a reference point. This compared with the actual position of 342.2 feet northeast and 228.5 feet northwest of the reference position determined upon completion of the shaft connection.