Magnetqmetry As A Tool To Map Subsurface Conditions In An Abandoned Iron Ore Mining District In New Jersey

Abandoned underground coal and metal mines present a number of environmental risks to communities.<br>Ground subsidence resulting from the collapse of mine workings ranks high among these risks.<br>Collapse or subsidence of the ground surface can result in safety hazards to residents, property<br>damage, and loss of land use. Field methods that provide geotechnical information about the<br>underground mine configuration and stability of the mine overburden are needed. In the past, a number<br>of site specific geophysical tools such as seismic techniques, gravity surveys, and electromagnetic<br>methods have been used in an attempt to acquire subsurface mine and overburden rock mass<br>information. This study investigates the use of magnetic mapping as a tool to delineate shafts and<br>underground workings responsible for ground instability in an abandoned metal-mining district of New Jersey.<br>The Township of Mine Hill, New Jersey has been plagued by subsidence for decades due to<br>unpredictable failure of portions of more than 20 abandoned iron ore mines which riddle the subsurface<br>of this community. Historical records indicate that mining started during the early 1700’s and continued<br>until 1966 with the closing of the last mine. Ore production was from high grade magnetite deposits<br>intruded into Precambrian crystalline rocks. Due to the high magnetic susceptibility contrast between<br>the ore and country rock, and a rather uniform orientation of the deposits, a unique opportunity was<br>available to delineate the abandoned mine workings using ground magnetic surveys.<br>Several abandoned mines in the Township of Mine Hill were selected for investigation. Historical mine<br>maps were used to design a gridded station array, and a proton precession magnetometer was used to<br>conduct a ground survey over each mine. Modeled anomalies computed from historical mine maps and<br>other available subsurface information were compared to anomalies observed in the field. Results<br>suggest that magnetometry is useful in delineating the condition and location of mine workings. This<br>information could be useful for subsequent remediation and land use planning for undermined areas.