MAGNETIC SURVEYS OVER CLINKERS AND COAL SEAM FIRES IN WESTERN NORTH DAKOTA

Burning coal seams alter overlying sedimentary rocks, including clinker formation, enhancing magnetic susceptibility and causing acquisition of a thermoremanent magnetization.  Magnetic anomalies are generated at the edges of clinkers due to contrasting magnetization with the adjacent rocks, and over clinkers due to inhomogeneous magnetization.

Magnetic surveys were run in three areas of western North Dakota.  Eleven profiles were run at the Coal Vein Trail (CVT), Theodore Roosevelt National Park (South Unit), where a coal seam burned from 1951 to 1977.  Five profiles were run at the Burning Coal Vein (BCV), Little Missouri National Grassland, near Amidon, where a coal seam burned from before white settlers first arrived until around 1990.  Ten magnetic profiles were measured over coal seam fires (Cummings A and Rodeske burns) in the Little Missouri National Grassland, McKenzie County, that have been burning since December, 1999.  Oriented blocks and cores from 15 sites were collected at CVT and BCV.  Unoriented and unconsolidated samples were collected from the actively burning areas.

All oriented samples had a median natural remanent magnetization of 0.82 A m^-1, and a median Koenigsberger ratio of 1.1.  Samples from a clinker chimney at CVT had the highest remanences and high susceptibilities.  One unoriented sample from the Rodeske burn had the highest susceptibility, over 1200 x 10^-5 m³ kg^-1.

The along-profile magnetic anomaly peak-to-peak amplitudes at CVT had a median value of 2200 nanoteslas (nT), 1400 nT at BCV, 660 nT at Cummings, and 77 nT at Rodeske.  Anomalies along CVT profiles correlate with the location of clinker outcrop, most of which is older than the recent burn.  Edge effects gave strong anomalies.  Anomalies at BCV coincide with clinkerized outcrop, but are absent over non-clinkerized areas of subsidence.  Smaller anomalies occur over the currently burning coal seams where surface alteration is obvious, but no clinker is evident.

For the burning coal seams, simple straight-slope estimates of depths to the anomaly sources yielded values of 2 and 4 meters below the surface for two anomalies at Cummings A, and 1 meter for each of three anomalies at Rodeske.  This raises the possibility of using magnetics for determining approximate depths to burning coal seams quickly and noninvasively for mitigation strategies.