Shallow geophysical surveying may prove a valuable component for an ongoing Department of the Interior and Office of Surface Mining Reclamation and Enforcement program for abandoned coal mines in coal-producing states, including Colorado. A need for geophysical methods exists that can be used to determine areas where hazardous and economically damaging subsidence incidents could develop. We show here that by using a combination of the self-potential (SP) and DC resistivity methods, we can identify existing and possibly potential subsidence locations in an area of abandoned coal mines. The data from a survey utilizing SP and resistivity presented results that agreed with existing maps and also indicated areas that require further investigation.
Historically, there has been heavy coal mining in the Boulder and Weld Counties, Colorado as noted in Figure 1 (USGS, 2001). The majority of the coal was mined using the room and pillar method. With this method, working areas, or “rooms” are carved into the coal seams with “pillars” of coal left intact to support a roof structure during the mining. As the coal seam end is met, the mining equipment and personnel retreat towards the entrance, extracting coal from the pillars as they retreat. Over time, the rock in the ceiling and above the cavities can collapse into the voids. Without sufficient overburden, this caving can reach the surface of the ground in the form of subsidence or a sinkhole. Depending on the permeability and hydrology of the area, this subsidence may be dry and filled with rubbish or the sinkhole could be filled with water. During periods of drought or diversion of water resources, the hydraulic head pressure exerted on the cavity ceiling lessens, and may drop to where it cannot support the overburden, allowing the ground surface to subside. Much of the old mining area shown in Figure 1 has been developed for residential and commercial use. Poor documentation of coal mine locations in the past and current safety concerns have lent urgency to assessing subsidence potential in historical mining areas. Safety and economical damage prevention are of paramount issue in these areas, as evidenced in Figure 2 (courtesy of Colorado Geological Survey archives). With such tenuous subsurface areas, it was determined that any survey methods utilized for this assessment must be as non-intrusive as possible, due both to residential concerns and personnel safety. This ruled out deep and shallow seismic surveys. Ground-penetrating radar was also eliminated because of the suspected high electrical conductivity of the clay-rich materials in the general area subsurface. SP is a non-intrusive, passive geophysical method that measures the electrical potential distribution at ground surface with respect to a fixed electrode located somewhere in the field (Revil et al, 2010). The existence of selfpotential anomalies implies a source of subsurface electrical current, often associated with groundwater flow. These are called streaming potentials and can be used to map flow directions. SP has been previously utilized to find cavities in abandoned marl pit areas in northwest France (Jardani et al, 2006a).
