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

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