Download Magnetometer measurements to characterize a subsurface coal fire

International Journal of Coal Geology 87 (2011) 190–196
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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.
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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
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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.
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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.
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