the use of infrared thermographic and gps topographic surveys to monitor spontaneous combustion of...

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Applied Thermal Engineering 25 (2005) 2677–2686

The use of infrared thermographic and GPS topographicsurveys to monitor spontaneous combustion of coal tips

O. Carpentier, D. Defer *, E. Antczak, B. Duthoit

Laboratoire d’Artois de Mecanique Thermique Instrumentation (LAMTI), FSA—Universite d’Artois,

Technoparc Futura, 62400 Bethune, France

Received 9 February 2004; accepted 30 November 2004

Abstract

Aerial infrared thermography is one of the methods most widely used for monitoring coal tips. Althoughit offers the advantage of providing wide coverage of the site and clearly displaying areas at risk on the ther-mal image, it is costly, sensitive to atmospheric conditions and cannot be used to monitor combustion reac-tions. It is for these reasons that the LAMTI, in liaison with the Charbonnages de France Group, hasdeveloped a ground-level method based on a combination of GPS topographic and infrared thermographicsurveys that is intended to be more precise and less expensive and may eventually enable combustion reac-tions to be monitored.� 2004 Elsevier Ltd. All rights reserved.

Keywords: Infrared thermography; Coal tips; Digital terrain model; Monitoring

1. Introduction

Coal tips (Fig. 1) are conical or truncated-cone hills formed by piling up the waste from miningoperations. Some of them still possess 5–15% of residual coal, the percentage varying according tothe sorting techniques used. The oldest tips thus often contain more residual coal. The porous

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* Corresponding author. Tel.: +33 3 21 63 71 55.E-mail address: [email protected] (D. Defer).

mailto:[email protected]

Fig. 1. An example of conical coal tips.

2678 O. Carpentier et al. / Applied Thermal Engineering 25 (2005) 2677–2686

nature of the tip enables air and water to circulate, causing the coal to oxidise. Any iron sulphidessuch as marcassite and pyrites present in the tip will tend to catalyse the exothermal oxidationreaction, causing spontaneous combustion of the residual coal [1]. In north of France, 10% of coaltips (above 30) are undergoing a process of spontaneous combustion, the effects of which are asfollows [2]:

• Slipping of the tip, accompanied by clouds of inflammable dust.• Formation of cavities.• Emission of noxious gases.• Creation of water–gas, a compound that explodes in air.

Most failures occur during heavy rains, with penetrating water having the following additionaldirect effects:

• Increase in the density of the materials.• Drop in shear strength.

Fig. 2. Slips effects of spontaneous combustion in a conical coal tips.

O. Carpentier et al. / Applied Thermal Engineering 25 (2005) 2677–2686 2679

• Increase in pore pressures.• Downward movements, creating destabilising forces.

The effects can be particularly catastrophic. Slips (Fig. 2) are not classic rotational ones but, onaccount of the high water content in the schists, are more similar to mud slides; these can bedeadly, as experienced in Virginia in the 1940s. There is also the added problem of major atmo-spheric pollution due to the release of sulphurous fumes [3]. As it is very expensive to fight coal tipfires, attempts have been made since the second half of the 20th century to prevent such fires dur-ing the formation of coal tips by taking precautions, such as for example not creating them abovecarbonate materials or spreading them out in order to reduce internal pressures. In the case of tipsformed before this period, spreading them can be dangerous and costly if high-risk areas are notknown, and monitoring then appears to be a necessary step.

2. Monitoring of coal tips

Today there aremanymethods formonitoring soils, such as radarmethods or electrical scanning.These have been applied to coal tips in order to draw up surface maps or profiles that link resistivityto soil stability. Themain drawback of thesemethods is that they involve working in high-risk areas,as cavities formed at the surface can collapse under the weight of a man. Optical fibre methods [4]have also been used, but they appear to be more suited to detecting spontaneous combustion in

Fig. 3. The monitoring of coal tips by aerial infrared thermography (Charbonnages de France Group files).


mining galleries, which have amore slender profile thanmassive coal tips, where a surface approachis more appropriate. It is a known fact that the state of a soil subjected to stress (thermal, mechan-ical, etc.) can be observed by infrared methods [5]. This is especially true of rocks and coal deposits.Spontaneous combustion in coal deposits has already beenmodelled [6] as well. Here, on the basis ofexternal information, the internal development of the combustion phenomenon is determined.Infrared thermographicmonitoring can therefore display the internal activity in a coal tip on its sur-face. The Charbonnages de France Group uses aerial infrared thermography (Fig. 3) to monitorcoal tips. Moreover, these are not the only igniting masses to be monitored. Fermenting publicdumps, in which the formation of internal heating can be compared to that of coal tips, are alsomonitored by aerial infrared thermography [7], while the usefulness of ground-level monitoringfor checking anomalies is also recognised.When conducting aerial monitoring surveys, it is very dif-ficult or even impossible to take into account certain data such as the topography, the relativehumidity or CO2 level, the influence of the atmosphere, or the directional aspect of emissivity.

3. Combined topography and thermography

Combining thermography and topography (Fig. 4) means that it is possible to work over a largesurface area and quickly. Once anomalies have been determined precisely, they can be monitoredwith regard to changes in temperature and displacement of the source. Information gathered inthe field indicates the environmental conditions. Subsequently, the site can be monitored and itwill be possible to tell whether temperature measurements indicate internal activity within thetip or are rather the direct result of environmental conditions.

Three stages are involved:

• A topographic and thermographic survey in the field.• Processing of the topographic data, pixels and infrared images to produce a 3D thermogram.• Post-processing of the temperatures, incorporating the environmental components.

3.1. On the field

An AGEMA 570 FPA monitoring camera (Fig. 5) was used with a resolution of 320 · 240 pix-els working in the 8–14 lm wavelength band, which is suited to low temperatures. The point set is

Fig. 4. Combination of IR and GPS methods.

Fig. 5. IR camera on the field.


gathered with a Trimble type GPS accurate to within a centimetre. The environmental data arecollected with a portable probe.

The topographic survey must be precise enough to offer a satisfactory reconstruction of the siteso that the projection error between the images and the DTM (Digital Terrain Model) is as smallas possible. The thermal camera must be treated as both a monitoring tool and a theodolite. It isfirst necessary to carry out an investigation of the site as areas subject to slips (Fig. 6) can some-times be identified with the naked eye (previous slips, fumes escaping, absence of vegetation).

In this case, the thermographic survey will help especially to detect any movement of the sourceand locate future risk areas. Fig. 6 clearly shows the slip area, due to spontaneous combustion,over a wide strip running from the summit eastwards, but it is also possible to detect areas of heat-ing to the south and north-east that are not visible to the naked eye and that perhaps correspondto movement of the source or the opening of a vent from the source to the surface. Surface tem-peratures collected by the heat probe also confirm that these apparently inert areas have surfacetemperatures of 50 �C and temperatures reaching 90 �C at a depth of 20 cm.

Fig. 6. Landslide due to spontaneous combustion of residual coal.


During the field investigation stage, the camera must be levelled as a topographic instrumentsince the quality of the reconstructed image will depend on the orthogonality of the images.All points of the survey are determined by global positioning system (GPS), including the camerastations and image references. It is the link between the surveyed reference determined by GPSand simultaneously displayed by the camera that will enable thermographic information to beadded. With regard to the actual monitoring, coal tips undergoing combustion generally providegood contrasts. However, work should not be carried out when it is raining as this tends to makesurface temperatures uniform, or in strong sunlight, since the addition of atmospheric radiation isa source of error in surface temperatures. Lastly, low angles should be avoided, as the directionalnature of emissivity means that its value decreases with distance from the normal angle of obser-vation. The optimum conditions for measurements of this type are generally found at night as thisis when the best contrast is obtained and the effect of atmospheric radiation is much lower. Sur-face temperature measurements made with a probe can nevertheless be used to correct the resultsobtained with a thermal camera. The raw data obtained in this way must therefore be processed,and this can only be done by collecting information on the site.

3.2. Plotting a 3D thermogram

In the area of research and application, the DTM has become a common surveying techniquefor identifying forms of terrain. Infrared thermographic scanning creates specific geometric prob-lems. Backing up imagery with topographic data (DTM) is a way of achieving a better under-standing and higher degree of accuracy [8]. To prepare a 3D thermogram, a topographicmethod (GPS survey) is used with a view to forming a base for future modelling. The locationof coal tips and their simple nature overcomes the main problems arising with the GPS, namelymultipath and mask phenomena. A GPS survey is collected in the form of a set of points. As thethermographic survey is combined with the topographic survey, it is possible to obtain georefer-enced images and pixel coordinates can therefore be defined as 3D polar coordinates in relation tothe camera, within a system chosen by the user (either national or local).

Two successive grids will be used to obtain the DTM, namely a triangular grid obtained by a so-called Delaunay triangulation, which will be used for the actual topographic model, and a quad-rangle grid that will transpose the quality of the thermogram reconstruction into three dimensions.In the case of the Delaunay triangulation, the associated convex envelope is covered by a set of ele-ments, in this case triangles. This method is based on the Voronoi diagram [9]. The point set con-sists of a set of vertices P containing n points Pi with (x,y) coordinates. A Voronoi polygon is thenassociated with each point, each polygon being associated with a site Pi of region Vor(Pi), andeach region being all the points (x,y) closest to point P such that the nearest site to each pointofP isPi. Unifying all the Voronoi regions produces the Voronoi diagram on which the reconstruc-tion is based. All defined vertices are grouped into triplets. Each triangle is defined by its vertices[Ki = (Pi,Pi+1,Pi+2)]. After triangulation in the plane, the sides are elevated along Z [Zi = f(Pi,Pi+1,Pi+2)]. The quadrangle grid is superimposed on the triangle grid (Fig. 7), providing a workingbase for adding the thermographic information. (x and y axis in Figs. 7, 9, 10 are in Lambert na-tional topographic system, 1 · 10�3 variation of coordinates on graph represents 100 m on field.)

As the images are georeferenced, they can be superimposed on the DTM (Fig. 8) by consideringeach pixel of each image taken during the monitoring survey. However, optical deformation by

Fig. 7. Coal tips DTM.

Fig. 8. IR data link to DTM.


the camera lens introduces constant angular errors that distort the image in proportion to the dis-tance from the central pixel. Part of the thermographic image must therefore be eliminated. Thepart to be eliminated will be determined by the tolerance applied to the angular distance in rela-tion to an image that has not been distorted in any way. As an example, an error of one pixel at100 m produces a difference of 13 cm. A tolerance of 20 cm applied to the plotted points elimi-nates about 15% of the pixels from the image.

This is therefore not thermal imagery but a thermographic data base that can be processed asrequired, in particular for views (Figs. 9 and 10), but also and especially for temperatures, whichare collected by infrared camera and determined on the basis of the directional luminance of blackbodies in a spectral band [10].

LCNðk;T ;hÞ ¼

Z kb

ka

c1 � k�5

expðc2=kT Þ � 1� dk ðWm�2 sr�1Þ

Fig. 10. Aerial view simulation.

Fig. 9. East view of 3D thermogram with localisation of combustion areas.


k: wavelength (m),c1 and c2, Stephan constants,c1: 1.191062 · 10�16 W m�2 sr�1,


c2: 1.4388 · 10�2 m K,T: temperature (K).

This apparent temperature is not the temperature of the object. Real objects are not Lamber-tian bodies, and therefore the directional emissivity of the object is taken into account. This maybe defined as follows:

e0ðk;h;T Þ ¼R kbka

c1�k�5

expðc2=kTmÞ�1� dkR kb

kac1�k�5

expðc2=kT rÞ�1� dk

Tm: measured temperature (K),Tr: real temperature (K).

As raw data were measured on site, the parameters can be included in the radiometric equationfor the camera [11].

Lmes ¼ s � e � Lobj þ ð1� eÞs � Lenv þ ð1� sÞLatm ðW m�2Þ

Lmes: measured luminance,Lobj: object luminance,Lenv: environment luminance,Latm: atmosphere luminance,e: object emissivity,s: atmospheric transmission factor.

The three-dimensional aspect of the measurement means that the luminance values associatedwith the topography can be weighted on the basis of the directional emissivity curve of a material:

Lðx;y;zÞ ¼Pn

i¼1s � ðeihÞ2Li

mesPni¼1e

ih

þPn

i¼1Lienv � ð1� eihÞ

2

Pni¼1ð1� eihÞ

þ ð1� sÞ � Latm ðW m�2Þ

3.3. Post-processing of temperatures

Post-processing can take place after reconstruction, as the coordinates and temperatures ofall the points are known. This means, for example, that several emissivity values can be selectedfrom the same thermogram (coal tip, vegetation, etc.). The environment data must then beincluded. As the environment plays a predominant role in the in situ measurements [12], it wasnecessary to incorporate parameters such as the atmospheric transmission factor and the notionof thickness of the atmospheric layer into the measurement processes. These parameters are cur-rently being looked at in the laboratory in the context of a study of ground-level thermographicmonitoring specifically in relation to the 8–12 lm spectrum band. General nomographs [13] canalso be used. The atmospheric temperature and relative humidity can also be determined bythe probe. Measurements are taken from the camera without any internal compensation (e = 1


and RH = 50%). Compensation is carried out during post-processing of the data by includingenvironmental data.

4. Conclusion

Ground-level infrared thermographic monitoring has the advantage of being more rigorous,more flexible and less costly than aerial monitoring surveys. Characterising the emissive characterand modelling the micro-climate of the study environment are the keys for proposing a methodfor monitoring coal tip combustion reactions. It should be noted that an automatic system is cur-rently being developed; this should enable the two methods to be linked without there being anyneed for the processes to be simultaneous, and should also minimise the need for operators in thefield or even eliminate it if reconstruction is done on the basis of existing topographic data.

Acknowledgement

This work takes part of an Urban Engineering Concerted Research Action and authors aregrateful to the Charbonnages de France Group and to the Nord-Pas de Calais Region for sup-porting this work and allowing the publication of this results.

References

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