- Copyright © 2002 Society of Exploration Geophysicists
Preferential infiltration openings such as fractures, fissures, and soil pipes allow surface runoff to quickly enter the groundwater system and expose shallow aquifers to contamination. Geophysical techniques have the potential to noninvasively characterize these fractures or soil pipes, even when they are obscured by vegetation or thin soil cover. Electrical methods are particularly effective because they are sensitive to soil moisture variations associated with these features and may be deployed to image very shallow structures. Mine-induced subsidence fractures and soil piping above karstic bedrock are examined here to illustrate these applications.
Fractures above a collapsed mine
The longwall mining method removes coal from a long thin rectangular panel block located generally 30 to 150 m beneath the earth's surface. As the coal is removed, the roof of the panel collapses and a subsidence trough develops on the surface. High horizontal tensile strains and fracturing of near-surface materials occur just behind the panel face and along the panel edges (Figure 1). The central portion of the panel experiences compression and partial closing of fractures parallel to the mine face after the face passes. Along the panel margins, however, horizontal tensile strains persist in the “static tension zone,” and fractures may remain open for months or even years after the panel emplacement, providing openings through which surface contaminants may reach shallow water tables.
Electrical resistivity (ER) and electromagnetic (EM) conductivity surveys were conducted across static tension zone fractures for a panel in the southern part of the Illinois Basin. The shallowest water table in this area is 3–4 m deep. Geotechnical and other geophysical measurements suggest these fractures are at least 12 m deep, cutting loess, till, and glacial outwash deposits. The bedrock lies at a depth of 18 m and the longwall …