The Use Of Shallow Seismic Techniques To Target Subsurface Mine Voids For Installation Of Monitoring Wells

Abstract

A Superfund waste disposal site in western Pennsylvania was the location of a recent shallow seismic survey conducted with the objective of detecting subsurface mine voids for the installation of groundwater monitoring wells. Waste was disposed between 1950 and 1964 along benches of an abandoned strip mine; the highwall of this mine also contained drift openings into an underground mine. The waste contained mobile organic contaminants which entered the underground mine and migrated down gradient from the disposal site. The remediation design for the site required the installation of monitoring wells drilled to the underground mine voids; these wells would be used to determine the extent of the organic contaminants and to monitor the efficacy of the cleanup efforts. Site stratigraphy was determined from pre-existing monitoring wells and consists of 3-5.5 m (lo-15 ft) of unconsolidated overburden overlying Pennsylvanian-aged interbedded shale, siltstone, sandstone, and coal. Although mine maps were not available, information from the existing monitoring wells suggested the mine workings in the Pittsburgh coal were at a depth of 35 m (100 ft). Excavation of a nearby underground mine indicated that mine entries could be as little as 3.5 m (10 ft) wide and 1.5 m (5 ft) high. A seismic survey conducted at the site used the common offset technique and a 24-channel seismograph. Anomalies in the seismic records marked by loss of signal coherency were identified as representing mine voids. On the basis of these results, seven target sites were selected. Three monitoring wells were drilled at three of these sites and all wells encountered mine voids.