Magnetic Signatures of Rocks and Soils Affected by Burning Coal Seams

This chapter provides an overview on the magnetic susceptibility of soil, the formation of pyrometamorphic rocks, and magnetic anomalies that help to detect coal fires. Magnetics can be very useful to locate the edge of a clinker burn and the boundary with the unburned portion of the coal seam. This would be helpful for better outlining the hazard of a burning coal seam, as well as estimating unburned coal resources. The high resolution of ground magnetics could allow tracking of hot spots over time to delineate and monitor the path of underground fires. The magnetic anomaly is intrinsically associated with the burned and altered material in the near subsurface, whereas thermal and aromatic anomalies might be at least partially controlled by fractures that affect diffusion of hot gases. These clinker anomalies allow the geophysical delineation of older coal burns and can locate the transition from coal seams, used as an energy resource, to the less valuable clinker, used as an aggregate. Because the magnetic signatures are generated under unusual geologic conditions of low pressure, high temperature, and high oxygen fugacity, these soils and rocks might also be used to understand magnetization acquired under these conditions. Magnetic measurements taken along a profile can be represented as simple x – y plots. A few examples show the results that typify measurements over clinkers and burning coal seams. Modeling of anomalies, using known magnetizations, could also allow the separation of different magnetic sources, such as older clinkers and younger surface burns. The surveys described in the chapter all used standard procedures for magnetic surveying.