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International Journal of Coal Geology 87 (2011) 190–196 Contents lists available at ScienceDirect International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o Magnetometer measurements to characterize a subsurface coal fire Taku S. Ide a,⁎, Nigel Crook b, F.M. Orr, Jr. c a Energy Resources and Engineering, Stanford University, Green Earth Sciences Building, Room 65, 367 Panama St, Stanford, CA, 94305-2220, United States hydroGEOPHYSICS, Inc., 2302 N Forbes Blvd., Tucson, AZ, 85745, United States c Energy Resources and Engineering, Stanford University, Precourt Institute for Energy, The Jerry Yang & Akiko Yamazaki Environment and Energy Building, Mail Code 4230, Room 324, 473 Via Ortega Stanford, CA 94305-4230, United States b a r t i c l e i n f o Article history: Received 10 February 2011 Received in revised form 9 June 2011 Accepted 11 June 2011 Available online 12 July 2011 Keywords: Coal fire Cesium vapor magnetometer Magnetometer survey Subsurface characterization Monitoring a b s t r a c t Underground coal fires pose a threat to the environment and the health of those living in their proximity and can result in economic losses if these fires occur at mining areas. Design of methods to extinguish these fires requires that the extent of the subsurface fire be delineated. A conceptual picture of the workings of a subsurface coal fire near Durango, CO is presented first, which shows how the overburden above the burning coal seam can become heated. In high temperature and low O2 conditions, the heating of the overburden leads to the formation of magnetite, and its presence and the alignment of magnetic moments can be detected by a magnetometer. Magnetometer surveys allow high resolution areal mapping that differentiates among previously burned, currently burning, and unburned coal seam areas. The current and previous locations of the subsurface fire regions that are delineated by the magnetometer survey conducted at a fire on the Southern Ute Indian Reservation are consistent with various supporting data such as gas composition, temperature, and snowmelt data. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Underground coal fires occur world-wide, including India, China, U.S., Australia, Indonesia, and South Africa, among others (Stracher and Taylor, 2003). These fires emit toxic gases including carbon monoxide and hydrogen sulfide and many others (Hower et al., 2009). In addition, greenhouse gas emissions from these fires may be significant (Ide and Orr, 2011; Kolker et al., 2010; Kuenzer et al., 2005). Design of methods to extinguish subsurface fires requires delineation of the subsurface extent of the fire. Surface expressions of a fire include hot fissures, snowmelt boundaries, surface deformation, and elevated surface temperatures. Measurements of these can only be used to approximate the areal extent of the fire. Attempts have also been made to relate surface temperature expressions to the subsurface temperature distribution using empirical functions (Prakash and Berthelote, 2007). Subsurface measurements can provide additional insight into the regions affected by the fire, but with limitations. Measuring subsurface temperatures using thermocouples in boreholes can corroborate surface features, though high resolution of the active combustion area would require many boreholes. In addition, high subsurface temperatures can exceed drilling temperature limitations in the active combustion zone. Other geophysical methods that were ⁎ Corresponding author. Tel.: + 1 650 868 6575 (mobile), + 1 650 725 0801 (office). E-mail addresses: [email protected] (T.S. Ide), [email protected] (N. Crook), [email protected] (F.M. Orr). 0166-5162/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2011.06.007 employed at the NCF, such as shallow seismic, electrical resistivity, and ground penetrating radar, encountered depth and resolution limitations (de Ridder et al., 2010). Automated detection and demarcation of coal fires using remote sensing methods are useful when areas affected by coal fires are extensive (Chatterjee et al., 2007). Satellite approaches, however, may have limited applicability for smaller coal fires on the O(10 2) m, due to limitations in resolutions. 1 Satellites that can detect thermal anomalies are frequently utilized to detect coal fire regions based on surface temperature anomalies at coal fires (Jing et al., 2005; Kuenzer et al., 2007, 2008; Mansor et al., 1994; Mishra et al., 2011; Prakash and Gupta, 1998; Prakash et al., 1995; Zhang, 1998). Thermal satellite images can pose difficulties detecting deeper thermal anomalies (Zhang, 1998), and the true extent of the subsurface combustion region may not be determined solely from this remote sensing technique. Infrared remote sensing methods can detect coal fire locations based on surface signatures (Kuenzer et al., 2007; Mishra et al., 2011) but cannot see into the subsurface. Interferometric synthetic aperture radar (InSAR) can detect subtle surface deformation over time (Massonnet and Feigle, 1998). However, the subsidence may or may not always correspond to an active fire region, and the expected pixel resolution is on the order of 20 to 100 m (Burgmann et al., 2000). The InSAR technique has been applied most frequently at coal fires in Northern China, although its application has been limited due to low resolution issues and loss of image coherence 1 of’. The notation O(value) unit is used throughout this manuscript to indicate ‘order T.S. Ide et al. / International Journal of Coal Geology 87 (2011) 190–196 due to complex surface breaking (Hoffman et al., 2003; Jing et al., 2005). Some researchers have proposed integrating multiple types of satellite data to detect and demarcate areas affected by coal fires (Voigt et al., 2004; Zhang et al., 1999). In this paper, we show that magnetometer surveys can help overcome some of the challenges encountered by these conventional coal fire characterization methods. A pack-mounted magnetometer was used to delineate in detail the subsurface fire boundary of a coal fire by collecting subsurface magnetic anomaly data. Characterization efforts were concentrated at one of four coal fires, on the Southern Ute Indian Reservation, located near Durango, CO termed the North Coal Fire (NCF). Several investigators have used magnetometer surveys to characterize underground coal fires (Bandelow and Gielisch, 2004; Gielisch, 2007; Hooper, 1987; Schaumann et al., 2008; Sternberg, 2004; Sternberg et al., 2008). The study presented here improves upon the spatial resolution of previous magnetometer surveys, which have resolutions on the order of 10 m or else is not reported. The spatial resolution of the magnetometer anomaly data presented is on the order of 1 m or less. In the sections that follow, we present a conceptual picture of mechanisms at work in the coal fire, describe mechanisms for changes in magnetic anomalies due to combustion, and consider approaches for filtering the magnetic anomaly data acquired. We then present a detailed magnetic survey result from the NCF and compare it with other observations at that site including magnetic susceptibility of rock samples, well-logs, driller's logs, a core 191 sample, surface observations, gas composition results, fissure mapping, and subsurface temperature measurements. 2. Geological setting and a conceptual diagram of the North Coal Fire The burning coal seam at the NCF is a thick (~ 8 m) and continuous layer, part of the Fruitland Formation. It is located along the Hogback Monocline of the natural gas bearing San Juan Basin. The strike of the seam is about 25° NE, and the approximate dip is 12° to the SE (Ide et al., 2010). It is overlain by an average of 15 m of overburden that is composed of shale and sandstone. Details of the site such as the diagram of the San Juan Basin, a satellite image of the area with latitude/longitude coordinates, a geological cross section of the coal fire area based on a USGS survey, stratigraphic columns of the lithology of the area, and other geological features were presented by Ide et al. (2010). Fig. 1 is an adaptation of a map of the San Juan Basin that was originally presented by Lorenz and Cooper(2003) and reproduced in Ide et al. (2010). The NCF is located along the Hogback Monocline, and its location is indicated by the red box. Fig. 2 is a satellite image of the area in the vicinity of the NCF (Ide et al., 2010). The area affected by the site is again indicated by a red dotted box. Trees over the NCF site were removed in 2000 to prevent forest fires. The cross-section taken across the NCF site, denoted by A–A′, is shown below the satellite image. The lithology information Fig. 1. A picture of the San Juan Basin adapted from Lorenz and Cooper(2003). The NCF is located along the Hogback Monocline near the Four Corners. It is indicated by the red dotted box. 192 T.S. Ide et al. / International Journal of Coal Geology 87 (2011) 190–196 Fig. 2. (Top) A satellite picture of the region around the NCF. (Bottom) A cross-section and the lithology at the NCF. Kl = Lewis Shale, Kp = Pictured Cliffs Sandstone, Kf = Fruitland Formation, and Kkl = Kirtland Shale. was obtained from a USGS survey of the area. In the cross-section, the lower coal seam is overlain by the Fruitland Formation, Kf, and underlain by the Pictured Cliff Sandstone, Kp, and the Lewis Shale, Kl. Down-dip of the coal fire (to the southeast, toward the Central San Juan Basin), the Kirtland Shale, Kkl, caps the Fruitland formation. The NCF likely started at the lower coal seam outcrops (cf. Fig. 2). This conclusion is based on the presence of ash, thermally altered rocks, and the largest surface subsidence at the outcrop. Data collected at the NCF supports the notion that the fire subsequently burned into the formation (Ide et al., 2010; Ide and Orr, 2011). The presence of the fire has been detected up to about 200 m down-dip, away from the outcrop. The coal seam beyond that is intact and unaffected by the fire. A conceptual picture of the workings of the NCF is presented in Fig. 3. Fig. 3 is divided into four zones. In Zone 1, some of the cool air is drawn into the fractured zone in the subsurface immediately above the affected coal seam. The air is heated by the residual energy trapped in the overburden from the coal fire that existed there in the past. In Zone 2, O2-char combustion may occur. If O2 is present with the volatiles, it is more likely that the O2 will be consumed in combustion reactions with the combustible volatiles. This is because the homogeneous gas phase combustion reactions occur at rates that are orders of magnitude larger than the heterogeneous and exothermic char combustion reactions. The volatile species are present at the NCF either due to the coal devolatilization/pyrolysis process or the native CH4 that flows up-dip to the site from the Central San Juan Basin. Both the char-O2 and volatiles-O2 reactions are exothermic, and the energy released heats the overburden above the coal seam. The energy produced will also cause more volatiles to evolve from the coal in Zone 3, helping the propagation of combustion reactions downstream. In Zone 4, some of the combustible volatiles that did not react in the Fig. 3. A conceptual picture of the coal consumption process at the NCF. In Zone 1, the incoming O2 is heated by the residual energy trapped in the overburden. In Zone 2, exothermic reactions heat up the overburden. In Zone 3, new volatile species are formed, and finally in Zone 4, additional exothermic reactions take place inside of the fissure, heat up the overburden near it. T.S. Ide et al. / International Journal of Coal Geology 87 (2011) 190–196 subsurface can mix with the O2 inside of the fissures and ignite. This set of exothermic reactions also adds energy to the overburden as heat. More detailed description of the coal consumption process at the NCF can be found in Ide (2011) and Ide and Orr (2011). 193 coal fires provide ideal environments for magnetite formation. The low permeability and the low porosity of these rocks limit the diffusion of O2 into the matrix. 4. Methods 3. Magnetometer detection of subsurface combustion regions The energy transferred to the overburden as heat due to the reactions in Zone 2 and Zone 4 can alter both magnetic susceptibility and magnetic moments of the rocks in the overburden. Magnetic susceptibility measurements of the thermally altered overburden samples at the NCF showed positive magnetic susceptibilities that exceeded natural, unaltered susceptibility values by one to two orders of magnitude. Natural sandstones and shales found over the NCF site possess positive magnetic susceptibilities of O(10 − 5), based on laboratory measurements. A positive magnetic susceptibility of a rock can result from two sources: the presence of iron, nickel, or cobalt bearing minerals in the rocks, and the degree of alignment of the magnetic moments within these minerals (Hooper, 1987). The enhancement of the susceptibilities in the thermally altered rock is likely due to the formation of ferrimagnetic magnetite (Fe3O4) in the shales that have been affected by the heat from the coal fire. Although amounts of magnetite present were not directly measured in the laboratory, the low O2 and high temperature environment in some regions (downstream side of Zone 2 and also Zone 3 in Fig. 1) support magnetite formation (Hooper, 1987). The heat from the burning coal seam leads to the thermal decomposition of iron-rich clay in shales and siltstones, resulting in magnetite in these rocks (Hooper, 1987). Magnetic susceptibility is defined by the equation k = M = H; Magnetic data were collected using a pack-mounted Geometrics G859 Cesium Vapor (CV) magnetometer with a built-in GPS unit. The GPS data, after post-processing, yielded a positional accuracy better than 0.5 m. Data were recorded by traversing the area over the NCF with a measurement frequency of 5 Hz. The survey was conducted over a 600 m × 200 m region, and approximately 363,000 points were collected. While the grid was not uniform, the data density yielded a sub-meter grid resolution on average. To remove both diurnal fluctuations and the background magnetic field, a base station, a G856 Proton Precession magnetometer, was sited over an area of the NCF where the coal seam was unburned. Data were collected every 30 s and were subtracted from data collected by the CV magnetometer to isolate the anomalies induced by the heat of the coal fire. Removing the diurnal fluctuations – which could sometimes be as large as 60 nanoteslas (nT) over the course of the day – was important, because the anomalies that were mapped over in the overburden was in the range of 50 to 100 nT. Diurnal fluctuations could easily overshadow subtle changes in the overburden caused by the small amounts of magnetite that are formed due to the heat from the fire. Additional filters were applied to the magnetic anomaly data in both the physical domain and the Fourier domain. In the physical domain, spikes and dropped signals were removed. The appropriate range of magnetic anomaly data to retain was determined by ð1Þ I = 2πkF: where M (amp/m) is the induced magnetization in the material, H (amp/m) is the applied magnetic field strength, and k is the magnetic susceptibility. The magnetic susceptibility of rocks in the overburden above the NCF was measured using a kappa bridge system. The rock samples collected from boreholes at the site were crushed into fine particles and loaded into non-magnetic containers. When rocks are crushed, in-situ orientations of the magnetic moments are destroyed; thus any contribution to the magnetic susceptibility values due to preferential alignment in the remnant magnetization bearing minerals was not reflected in the measurements. When the overburden is heated to temperatures approaching the Curie temperature of the magnetic minerals, 585 °C (1085 °F) for magnetite, any natural magnetic alignment of the magnetic moments – and thus magnetization – that existed in the area is randomized. Thus areas that are currently hot have reduced magnetic anomalies. While magnetite formation may result at high temperatures, the newly produced magnetite in the overburden would not immediately enhance the magnetization in the region if the temperatures remain above the Curie temperature. Only when the combustion region migrates away from an area and the overburden cools, magnetic moments in the overburden become aligned to the Earth's ambient magnetic field, enhancing the magnetization of the region. The enhancement is attributed to both the new preferential alignment of magnetic moments and the newly formed magnetite in the overburden. Hematite (Fe2O3) formation, which is antiferromagnetic and thus does not enhance the magnetization of a region upon formation, can also result from heating shales and siltstones. The equilibrium between magnetite and hematite formation is a strong function of O2 availability at temperatures typically observed at coal fires (200 °C– 1000 °C). For these temperature ranges, low O2 concentrations lead to magnetite formation, while high O2 concentrations lead to hematite formation (Hooper, 1987). Rocks such as siltstones and shales above ð2Þ In Eq. (2), I is the induced magnetic anomaly, k is the magnetic susceptibility of the overburden rock that has been heated by the coal bed fire, and F is the ambient magnetic field. The relationship in Eq. (2) holds for a semi-infinite slab (Breiner, 1999). This assumption is warranted for the NCF geometry, because the length scale of the area affected by the coal fire is much larger than the depth at which it occurs. At the NCF, the range of k was between 5 × 10 − 5 and 5 × 10 − 4 for overburden that had been thermally altered. This was an O(10– 10 2) magnitude increase in magnetic susceptibility compared to the same overburden layers from an unburned zone. The ambient magnetic field, F, was approximately 52,000 nT. Using these values, I above regions affected by coal fires should have values O(10 2) nT. Magnetic anomaly data that fell outside of this range by large margins likely resulted from steel structures. Data spikes were encountered close to large metallic objects at the surface, such as a steel water tank at the site. Readings of magnetic anomalies in its vicinity exceeded 3000 nT above the measured ambient magnetic field. In the Fourier domain, magnetic anomaly data were reduced to the north magnetic pole to remove the asymmetries (Blakely, 1996) that arise due to the fact that the induced and ambient magnetic fields at the NCF do not point vertically downwards as they would at the north magnetic pole (Blakely, 1996; Breiner, 1999). Pole reduction transforms the existing dataset into a dataset that would have been obtained had the NCF been located at the magnetic north pole. Average values of the two magnetic properties at the NCF necessary for this transformation – the magnetic inclination and declination – were 63.685° and 10.281°, respectively, during the period of the survey in October 2009 based on the International Geomagnetic Reference Field. Pole reduction removed lateral shifts and asymmetries caused by the local orientation of the magnetic field. While in the case of the NCF results the reduction to pole did not make a significant difference, sign reversals and large shifts were evident on the magnetometer results 194 T.S. Ide et al. / International Journal of Coal Geology 87 (2011) 190–196 collected at two other fire sites on the Reservation. Even without significant changes to the original data, reducing the data to pole removes one layer of data distortion from the results, making the interpretation of results more straight forward. 5. Magnetometer survey results A map of the magnetic anomalies was constructed based on the method described above. The data shown in Fig. 4 were collected in October, 2009. In Fig. 4, paths taken over the NCF to create the map are shown. The data are not interpolated in between the survey lines. Fig. 4 shows induced magnetic anomalies at the site after they have been filtered to remove spikes and data asymmetries. The two parallel black lines in the figures indicate the approximate location of the coal outcrop. The line closest to the active fire is the top of the outcrop, and the second line marks where the bottom of the coal intersects the eroded surface. The seam dips at about 12° transverse to the outcrop to the southeast. In Fig. 4, the blue region indicates areas where the magnetic anomalies are enhanced with respect to ambient magnetic anomalies present in the area, and the red regions show where the magnetic anomalies are reduced with respect to natural conditions. We argue that the blue regions show locations of the overburden that was heated and subsequently cooled, the red regions are where some portion of the overburden is above the Curie temperature of the magnetite today, and the light green regions are where the overburden is unaffected by the coal fire. The magenta lines that are superimposed on both of the figures are snowmelt boundary lines taken in January of 2009, one week after a snowstorm. Inside of the magenta lines, the snow had melted due to the heat conducted from the subsurface. Outside of the snowmelt boundaries, snow remained unmelted. The colorbar in Fig. 4 is in nT, which measures the degree of magnetic anomaly over a specific location. Fig. 4. a) Map of magnetic anomaly data after diurnal fluctuations, ambient magnetic field, spikes, and data asymmetries have been removed. Colorbar in nT. Fig. 5. Fissures, gas composition, red rock regions, and snowmelt plotted on the same scale as the magnetometer results in Fig. 4. The results of the magnetometer survey were corroborated using supporting data shown Fig. 5: a map of fissures above the NCF categorized by their temperatures (blue and red lines, mapped in 2007), gas compositions of samples taken at the boreholes (shown at each of the black triangles, measured in 2008), and the locations of red, thermally altered rocks along the outcrop (green lines along the outcrop, mapped in 2007). The mapped fissure locations over the NCF agree well with the magnetometer results. Fissures only exist over areas where the coal seam was previously burned or is currently hot, presumably due to combustion, according to Fig. 4. Where the magnetometer results indicate that the coal is unburned there are no fissures present at the surface. This is consistent with the idea that surface fissures form only over areas where subsurface collapse occurs due to coal combustion or gasification (Ide et al., 2010). Furthermore, fissures over the previously burned zones (blue zones, or where magnetic anomalies are enhanced) in Fig. 4 are at ambient temperatures, and most of the fissures that are over the currently burning regions in Fig. 4 are at elevated temperatures. The distribution of gas compositions shown in Fig. 5 is also consistent with magnetometer results. Gas compositions over areas that are previously burned in Fig. 4 have compositions close to that of air, while gas compositions over active fire regions have combustion gas compositions. These gas composition data are consistent with the idea that air is entering from fissures that are located in previously burned zones, and combustion gases are being emitted from fissures above active fire areas. At boreholes drilled farther down dip from the active fire regions native gas compositions that consist of approximately equal mole fractions of CO2 and CH4 were recovered. Regions that are far away from the fire are not in the pathway of the incoming air, and thus the gas compositions in those regions are native San Juan gas compositions. Finally, the locations of the red, thermally altered sandstone align well with where the previously burned zones (blue regions) are located in Fig. 4. For example, consider the two previously burned locations, x = 200 m, y = 280 m, and x = 250 m, y = 350 m. These red rocks result when overburden sandstones and shales are exposed to high temperatures. The existence of these thermally altered rocks T.S. Ide et al. / International Journal of Coal Geology 87 (2011) 190–196 along the outcrop also supports the argument that the coal fire originated at the outcrop, before migrating into the formation. There are additional supporting data that were not plotted in Fig. 5. Subsurface temperatures that were measured using thermocouples deployed at over 40 boreholes in the NCF region matched well with the snowmelt outline. Rock cuttings, well logs, and a recovered coal sample also agreed well with magnetometer results. In regions where the fire previously burned, thermally altered rocks were recovered from the boreholes, and many fractures were encountered during the drilling. Fractures were encountered at most of the boreholes that were drilled in the blue region in Fig. 4. In one of the boreholes, a downhole video camera that was lowered into the hole showed that the fractures had apertures on the order of 10 − 2 to 10 − 1 m. Well logs showed low density readings in the overburden, indicative of fractures. Where the magnetometer indicated the presence of an active combustion region, boreholes could not be completed to the coal seam due to temperatures exceeding the operational limit of the drill bit. An unaltered, 8 m coal core was recovered from an area where the magnetometer showed no combustion. Most of these supporting features were mapped or measured one to two years before the magnetometer survey was performed. However, they agree well with the magnetometer results. This correspondence suggests that the NCF has not moved significantly over the past two years. This slow moving nature of the combustion front at the NCF was confirmed by conducting a second magnetometer survey five months after the initial survey. Data were collected over the northern portion of the NCF where red fissures are present (cf. Figs. 4 and 5). Fig. 6A is from the survey conducted in October of 2009, and Fig. 6B is from the survey conducted in March of 2010. Diurnal fluctuations, ambient magnetic fields, data asymmetry, and erroneous data have been removed in both panels a and b in Fig. 6. The repeat survey lines traversed in March 2010 are similar but not exactly the same as those traversed in October 2009. In Fig. 6, two features of interest are highlighted by a dotted circle and a dotted line. A comparison of the region encompassed by the circle shows that a zone that was burned and cool in October, 2009 (Fig. 6 (left)) subsequently reheated sometime in between the 195 surveys. The dotted lines in Fig. 6 separate the previously burned (blue region) and currently burning (red region) regions. Some portions of the line advanced to the NE on the O(1) m during the five month period, but the differences are small. 6. Discussion Repeat magnetometer surveys performed at the NCF (first one in October, 2009 and the second one in March, 2010) showed slow migration of the fire front. The observed rate of coal consumption is consistent with the three independent rate coal consumption estimates presented by Ide and Orr (2011). The three methods formulated were based on surface subsidence, CO2 flux measurements, and a natural convection chimney model (Ide and Orr, 2011). A key difference between the two surveys was the reheating of a previously cooled zone. This reheating may be due to an opening of a fissure nearby that reoriented the flow in its proximity. For the zone to reheat, it must be that the entire thickness of the coal seam was not consumed when the combustion front first swept through the area. Rock cuttings from some boreholes indicate that it is possible for only a top fraction of the coal seam to burn while leaving a significant fraction unaltered. The consistency of the two magnetometer surveys shows that magnetometer surveys can track subsurface fire front advancements. Magnetometer surveys that were conducted at two other fires on the Southern Ute Indian Reservation also distinguished among previously burned, currently burning, and unconsumed coal seams. Despite the advantages of magnetometer surveys over other coal fire characterization methods, the use of a magnetometer will not be applicable at all coal fire sites. First, metal surface objects such as water tanks, train tracks, power lines will significantly distort results, often overshadowing small anomaly changes that are associated with coal fires. Signals from metallic objects can be O(10 2) times larger than magnetic anomalies resulting from coal fires. Buried metallic objects will have similar effects. Coal fires that occur in abandoned mines may not be properly characterized by magnetometers if there are metallic objects – such as tracks for the carts – left behind. Fig. 6. (Left) A magnetometer survey result that was collected in October, 2009, and (right) a survey collected over the same area in March, 2010. Regions of interest are highlighted with black dotted lines. Colorbar in nT. 196 T.S. Ide et al. / International Journal of Coal Geology 87 (2011) 190–196 A second limitation of magnetometers is the requirement for the presence of magnetic minerals in the overburden, such as magnetite. Magnetite preferentially forms in low O2 environments at elevated temperatures. Typical temperatures reported for coal fires in China, India, and the U.S. are high enough for magnetite formation in the overburden rocks, but high O2 concentrations in some cases may preclude magnetite formation. High O2 concentrations can be expected at outcrop fires, where the fire has not yet made significant advancements into the formation. Hematite formation would be preferred over magnetite formation at such fires. In addition, there must be enough overburden in which magnetite can form, such that changes can be detected by the CV magnetometer. Magnetometer surveys performed at coal fires where only small amounts of overburden exist – such as at coal heap fires – are less likely to be successful. At the NCF, neither of the limitations was present. The NCF is a naturally occurring coal seam fire that likely started at the outcrop and propagated into the formation. Thus there are no metallic objects that may be present abandoned mines. The gas compositions measured at the NCF indicate that there are hot subsurface temperature zones with low O2 concentrations. These conditions are suitable for magnetite formation. 7. Summary and conclusions Comparison of magnetic anomaly data for an active subsurface coal fire with results of a variety of measurements at the fire leads to the following conclusions: 1. Spatial variations in magnetic anomaly can be used to determine with relatively high resolution the locations of burned and cooled, actively combusting, and unburned regions. 2. Filtering observed magnetic anomaly data to remove the effects of diurnal variations in the Earth's magnetic field, effects of metal objects, and effects of alignment with the direction of the magnetic pole allows more straightforward interpretation of the results. 3. Magnetometer observations at the NCF are consistent with all the other lines of physical measurements, and they offer much improved resolution of the burned and burning zones than do other available methods. 4. Repeat surveys at the NCF indicate that results obtained are repeatable and that monitoring of movement of the combustion front is possible. 5. Use of magnetic anomaly measurements requires that the rocks heated by a subsurface fire contain sufficient magnetite. Acknowledgments This project was made possible by the Southern Ute Indian Tribe, which provided access to the site as well as financial and considerable technical support for the field portions of this project. The authors especially thank Bill Flint and Kyle Siesser of the Southern Ute Indian Tribe Department of Energy for their help and insights and many helpful discussions. The Global Climate and Energy Project at Stanford provided support for the authors. References Bandelow, F.K., Gielisch, H.F., 2004. Modern exploration methods as key to fighting of uncontrolled coal fires in China. 2004 Denver Annual Meeting, Denver, CO. Blakely, R., 1996. Potential Theory in Gravity and Magnetic Applications. Cambridge University Press, Cambridge, U.K. Breiner, S., 1999. Applications Manual for Portable Magnetometers. Geometrics Inc., San Jose, CA. Burgmann, R., Rosen, P., Fielding, E., 2000. Synthetic aperture radar interferometry to measure Earth's surface topography and its deformation. Annual Review of Earth and Planetary Sciences 28, 169–209. Chatterjee, R.S., Wahiduzzaman, M., Shah, A., Raju, E., Lakhera, R., Dadhwal, V., 2007. Dynamics of coal fire in Jharia Coalfield Jharkhand, India during the 1990s as observed from space. Current Science 92, 61–68. de Ridder, S., Crook, N., Haines, S., Ide, T.S., 2010. Seismic investigation of underground coal fires: a feasibility study at the Southern Ute Nation Coal Fire site. Symposium on the application of geophysics to engineering and environmental problems. Gielisch, H., 2007. Detecting concealed coal fires. Reviews in Engineering Geology 18, 199–210. Hoffman, J., Roth, A., Voigt, S., 2003. Detecting coal fires in China using differential interferometric synthetic aperture radar (InSAR). Proceedings of FRINGE 2003 Workshop, Frascati, Italy. Hooper, R.L., 1987. Factors affecting the magnetic susceptibility of baked rocks above a burned coal seam. International Journal of Coal Geology 9, 157–169. Hower, J.C., Henke, K., O'Keefe, J., Engle, M., Blake, D., Stracher, G., 2009. The Tip Top coal-mine fire, Kentucky: Preliminary investigation of the measurement of mercury and other hazardous gases from coal-fire gas vents. International Journal of Coal Geology 80, 63–67. Ide, T.S., 2011. Anatomy of subsurface coal fires: a case study of a coal fire on the Southern Ute Indian Reservation. Ph.D. dissertation, Stanford University. Ide, T.S., Orr Jr., F.M., 2011. Comparison of methods to estimate the rate of CO2 emissions and coal consumption from a coal fire near Durango, CO. International Journal of Coal Geology 86, 95–107. Ide, T.S., Pollard, D., Orr Jr., F.M., 2010. Fissure formation and subsurface subsidence in a coalbed fire. International Journal of Rock Mechanics 47, 81–93. Jing, L., Voigt, S., Kuenzer, C., Bo, Y., Jianzhong, Z., Yianmin, Z., Bing, K., Songmei, Z., 2005. The progress in detecting of coal fire on remote sensing: the first result of the joint Sino-German research project on innovative technologies for exploration, extinction, and monitoring of coal fires in North China. Proceedings 2005, Dragon Symposium Mid-term results. Kolker, A., Engle, M., Olea, R., Hower, J.C., Hower, J.M., O'Keefe, J., Heffern, E., Radke, L., Stracher, G., Prakash, A., ter Schure, A., 2010. An integrated approach to determining atmospheric emissions from coal fires. ICCFR2, Berlin, Germany. Kuenzer, C., Voigt, S., Morth, D., 2005. Investigating land cover changes in two Chinese coal mining environment using partial unmixing. Gottinger Geographische Abhandlungen 113, 31–37. Kuenzer, C., Zhang, J., Li, J., Voigt, S., Mehl, H., Wagner, W., 2007. Detecting unknown coal fires: synergy of automated coal fire risk area delineation and improved thermal anomaly extraction. International Journal of Remote Sensing 28, 4561–4583. Kuenzer, C., Hecker, C., Zhang, J., Wessling, S., Wagner, W., 2008. The potential of multidiurnal MODIS thermal band data for coal fire detection. International Journal of Remote Sensing 29, 923–944. Lorenz, J.C., Cooper, S.P., 2003. Tectonic setting and characteristics of natural fractures in Mesaverde and Dakota reservoirs of the San Juan Basin. New Mexico Geology 25, 3–14. Mansor, S.B., Cracknell, A.P., Shilin, B.V., Gornyi, V.I., 1994. Monitoring of underground coal fires using thermal infrared data. International Journal of Remote Sensing 15, 1675–1685. Massonnet, D., Feigle, K.L., 1998. Radar interferometry and its application to changes in the Earth's surface. Reviews of Geophysics 36, 441–500. Mishra, R., Bahuguna, P., Singh, V., 2011. Detection of coal mine fire in Jharia coal field using Landsat-7 ETM+ data. International Journal of Coal Geology 86, 73–78. Prakash, A., Berthelote, A., 2007. Subsurface coal-mine fires: Laboratory simulation, numerical modeling, and depth estimation. Reviews in Engineering Geology 18, 211–218. Prakash, A., Gupta, R.P., 1998. Land-use mapping and change detection in a coal mining area: a case study in the Jharia coalfield, India. International Journal of Remote Sensing 19, 391–410. Prakash, A., Saraf, A.K., Gupta, R.P., Sundaram, R.M., 1995. Surface thermal anomalies associated with underground fires in Jharia coal mines, India. International Journal of Remote Sensing 16, 2105–2109. Schaumann, G., Siemon, B., Yu, C.C., 2008. Geophysical investigation of Wuda Coal Mining Area, Inner Mongolia: Electromagnetics and Magnetics for coal fire detection. In: UNESCO, Mera (Ed.), Spontaneous Coal Seam Fires: Mitigating a Global Disaster. International Research for Sustainable Control and Management. ERSEC Ecological Book Series, 4. Tsinghua University Press and Springer, Beijing, pp. 336–350. Sternberg, R., 2004. Magnetic surveys over clinkers and coal seam fires in Western North Dakota. 2004 Denver Annual Meeting, Denver, CO. Sternberg, R., Sparks, A., Knutson, O., 2008. Magnetic surveys over burning and remediated coal seam fires in Western North Dakota. Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems: SAGEEP, 21. doi:10.2113/1.2963279. Stracher, G., Taylor, T., 2003. Coal fires burning out of control around the world: thermodynamic recipe for environmental catastrophe. International Journal of Coal Geology 59, 7–17. Voigt, S., Tetzlaff, A., Zhang, J., Kuenzer, C., Zhukov, B., Strunz, G., Oertel, D., Roth, A., van Dijk, P., Mehl, H., 2004. Integrating satellite remote sensing techniques for detection and analysis of uncontrolled coal seam fires in North China. International Journal of Coal Geology 59, 121–136. Zhang, X.M., 1998. Coal fires in Northwest China — Detection, monitoring, and prediction using remote sensing data. Ph.D. dissertation, International Institute for Geo-Information Science and Earth Observation (ITC). Zhang, X.M., Cassells, C., van Genderen, J., 1999. Multi-sensor data fusion for the detection of underground coal fires. Geologie en Mijnbouw 77, 117–127.