Bastien Affeltranger

Elmar Berger

Gertraud Peinel

Thomas Rose

Radu-Adrian Mlesnita

IT for risk and emergency management: Consolidating methodologies for user-centred design

Managing risks and emergencies caused by natural and technological hazards is a complex task for local authorities, emergency services, and their partners in risk management. When making risk-related decisions, processing available data and information can be arduous. In that context, information and communication technologies (IT) can help. This has been demonstrated in several research projects funded under the FP6/IST framework and programmes. Several risk and emergency management tasks appear particularly demanding. These are: monitoring risk levels in a multi-hazard context; organising emergency response at municipality level; disseminating warning messages to the population; communicating risk-related information to the public. ERMA (Electronic Risk Management Architecture), an FP6/IST project, aims at developing a system and a set of tools that will help authorities conduct these tasks. Building ERMA relies on a user-centred methodology that capitalises upon participatory approaches developed in the field of Information Technology (IT) and Risk Management (RM). This paper describes the rationale of the ERMA project; the methodology followed; the functions and modules of the ERMA architecture.

Introduction: local authorities confronting risks and emergencies

1.1

A situation with multiple hazards and risks With communities exposed to a variety of natural, technological and other man-made hazards, as well as to severe public health issues, local authorities must deal with a great load of data and information. Two types of situations should be considered:

• In usual situation, local authorities are usually presented with the following duties or tasks:

• Ensure monitoring of existing hazards and risks • Avoid additional exposure of human and material assets to hazards

• Provide community members with information on hazards, vulnerabilities and risks • In emergency situations, local authorities are confronted with the following duties or task:

• Identify when monitored hazards overpass a pre-defined threshold • Issue pre-warning to staff in charge of risk management, and/or issue early warning • Issue general warning to the affected population and/or community subgroups • Implement emergency management measures Understandably, the effective implementation of risk and emergency (RM) duties and tasks often suffers from a lack of accurate information on ongoing hazards and risks. This is particularly the case with highly dynamic hazards, or hazards with high kinetics -such as flash flooding for instance. In that case, up-to-date data is often missing. In other cases, it can be difficult to manage or optimise the available large quantity of information. Last but not least, a problem is when available data or information is characterised by a high level of uncertainty.

Information and communication technologies: a role to play

As shown in other projects of the FP6 / IST programme (e.g.: RIMS; OSIRIS etc.), information and communication technologies (IT) can help decision-makers and risk experts or managers to make more appropriate decisions -this includes making decisions more easily or faster. IT tools or systems considered here include: hardware and software; modelling; mobile communication devices; grid computational facilities; computer -based mapping; automated messaging etc. Risk and emergency management functions supported by IT tools and solutions include:

• Access to background data and information for risk characterisation • Hazard monitoring and mapping, including real-time transfer of monitored data • Issuing of pre-warning and general warning, possibly automated • Monitoring and management of emergency situation (updates; staff; supplies etc.)

1.3

Involving users in the design of IT solutions Experience shows that application of IT to risk and emergency management (RM) is neither always easy, nor successful. Expectedly, the development of IT applications to RM should follow a user-based approach, whereby end-users' needs are considered early in the design process of considered tools or systems. The present paper therefore consolidates lessons learned in IT applications to RM, and indicates expected added-value from the ERMA architecture.

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IT solutions for risk and emergency management

2.1

A growing interest in IT applications to risk management In the last years, the " Information Society" has increasingly become a leitmotiv on the application of Information Technologies (IT) to environment-related issues [START_REF] Di Castri | Environment in a global information society[END_REF]. In the field of natural hazards for instance, this trend could be witnessed by the number of activities developed during the UN-voted International Decade for Natural Disaster Reduction (IDNDR) 1 . Rapid developments in information technology and modelling have emerged, helping to cope up with the complexity of disasters and to deepen the understanding of natural hazards (Herath, 1999). These include:

• Hazard mapping for land-use planning;

• Provision of timely warning at community level;

• Knowledge management and E-learning on natural and technological hazards;

Another example is the IST Programme of the EC-funded Framework Programme 6, that did support several projects -all aiming at an application of IT solutions and techniques to disaster risk reduction 2 . As part of these initiatives however, several key questions did emerge:

• What are the hazards and risk management decisions that should be considered?

• What are the relevant technologies to be selected?

• Is it realistic to consider that all data needed for IT systems will be available?

• Is it realistic to consider that IT systems can be fail-proof and/or fail-safe?

• To what extent is it possible to anticipate the behaviour of IT users?

2.2

Challenges of IT applications to RM Lessons learned from daily practice of disaster risk reduction, and emergency management at community level, show that local authorities often confront many difficulties. These add to threats posed by natural, industrial and man-made hazards and include:

• Lack or limitation of knowledge on existing hazards and risks • Need to manage large amount of data and information • Diversity of sources collecting, processing and disseminating data • High expectations from the population in terms of safety level In addition to the above, the development of IT-based options (systems, tools and services) for risk and emergency management can be made more difficult, or even jeopardised by several key features. Municipalities and local authorities in cha rge of risk and emergency management are often ill-prepared to use IT solutions for conducting RM tasks and duties. Reasons include:

• Cost of IT systems: not affordable for many small and medium sized communities 1 See http://www.unisdr.org 2 Visit the online CORDIS database at http://cordis.europa.eu/search/index.cfm --

• Value of assets exposed: might be considere d insufficient to justify investment in IT • Municipality services often ill-prepared for IT systems (e.g.: low computer literacy) • "Real-life" decision often features adhoc processes for which IT might be too rigid • Social acceptability of IT systems and tools is not guaranteed

The above-listed elements -only to mention a few -pose many difficulties to the feasibility of an information system, decision-support tool like ERMA.

ERMA: a service-oriented architecture for risk management

ERMA in a few words

The ERMA project develops a comprehensive risk management platform which is based on a SOA orchestration of relevant systems. Interfaces to other systems will augment the ERMA service portfolio where needed. Figure 1 shows the ERMA architecture with different modules to be integrated and the respective information flow. Optional components (presented in light grey in the figure) will be linked by SOA (Service Oriented Architecture) to establish loosely coupled and interoperable services, which can be integrated or deselected for individual requirements. The decision support system (DSS) serves as core component integrating the application logic and scheduling other functions when demanded. The ERMA components are described in the following sections. Wherever available, screen captures are provided, that give an indication of the possible functions to be offered by the ERMA components.

• Component 1: ERMA Decision Support System (DSS): The DSS of ERMA will perform the following tasks:

• collection of sensor data, and assessment by key indicators,

• comparison with criticity thresholds,

• visualisation of measured data in specific region, deduced state and actions, and finally,

• display of process models for corresponding tasks (figure 2), and

• editing and processing of process models. The process management system will detail the proposed key indicator actions and guide users in executing the necessary steps. These steps in turn have been documented before and the target community has the possibility to adapt existing process models to its own needs in advance or directly in the crisis situation (ad-hoc processes). The exchange and adaptation of process models and instances will be possible, which allows the re-use of models across different organisations.

Each reference process describes certain patterns of actions to be taken depending on the event at hand. As such, predefined process models collect experience, organisational and administrative knowledge about how specific actions are to be undertaken, like e.g. evacuations, securing of installations, mounting of flooding dams. Ad-hoc processes allow one to plan and execute not yet modelled series of actions in specific occasions in order to customise pre-defined patterns to event-specific requirements. Once defined and completed, they can be adapted, stored, and reused later for similar situations. Moreover, the use of the process workbench will ease the definition of complex scenarios, so that each step and respective information exchange can be modelled. In case of an emergency, involved staff can concentrate on extreme and unusual events while routine jobs are guided by quality-assured process models.

• Component 2: Customer Relationship Management (CRM) and Citizen Relationship Management (CiRM). Customer relationship management (CRM) covers methods and technologies used by companies to manage their relationships with clients.

-Information stored about existing customers (and potential customers) is analyzed and used to this end. Typical CRM solutions support the following features (example from CAS Software AG, 2006): address Management ; complete view of the customer's dossier; calendar, appointments and tasks ; phone calls; correspondence and vouchers; project management and project files; marketing operations and mailing lists; ERP data integration; prsonalisation; etc. -CRM is not just a technology, but rather a holistic approach to an organisation's philosophy in dealing with its customers. In companion with the introduction of egovernment applications, the interest of the public sector in the application of CRM rises. Citizen Relationship Management (CiRM) [START_REF] Schellong | CRM in the public sector: towards a conceptual research framework[END_REF] systems focus on providing citizens with timely, consistent, responsive access to government information and services using multiple channels like web access or mobile communication devices. CiRM promises to strengthen the links and co-operation between government and its citizens, realising operational efficiencies.

-Concerning ERMA, the functionality provided by CRM systems builds the base for the citizen relationship management. Most of existing commercial systems are far too complex for small communities to be suitable in the context of ERMA. Easy adjustable and manageable systems are required that also incorporate the different needs of the private and public sector. One aim of ERMA is to adjust an existing system to the requirements of citizens and small communities in the context of risk management. As such, the CRM/CiRM module of ERMA will support more collaborative work among risk management partners -for instance for exchanging data (e.g.: risk levels) or information (e.g.: emergency plans). -Part of the CiRM will also be a citizen information portal, which furthers communication --with the public and also channels citizen feedback and calls to the responsible position.

• Team collaboration software allows sharing of short textual information, forms, movies and images, documents and the like between rescue organisations and/or authorities. Additional features like chat functions, notifications on changes and upload, calendars, address and contact lists supports information exchange between all concerned parties. • Alarm Component. Historically sirens have been used to alert population against war related threats like bomb attack or other major disasters. Lots have happened from the introduction of sirens in the mid thirties until now. The threats have changed and so has also the technology and the behaviour of the population. Today modern telecom technology enables new methods for efficient alert of population. Maturing of telecom infrastructure combined with the penetration of mobile devices among the population especially in Europe makes telecommunication channels ideal as means for alerting. The main components of a telecom based alert system are illustrated in figure 3. The ERMA system will feature a georeferenced, GIS-based warning system. The system shall enable users ( e.g. community's decision-makers and municipality staff) to develop and implement an appropriate warning strategy: sending the right message, in the right time, to the right recipient, and using the right channel. Based on this system, local authorities keep track of social dissemination and processing (i.e.: receiving; understanding etc.) of the warning messages.

--See figure 4 for an example of the user interface. • Connection to other information services via SOA: ERMA's implementation is based upon SOA (Service Oriented Architecture) in order to link all i ts components and to allow connections to other external risk management architectures and platforms like ORCHESTRA and OASIS. Also, external systems can be linked and their information displayed or processed. For example, weather information from a commercial or state provider will be integrated in the key indicator system to support the decision process. In this manner ERMA is flexible and extensible and such easy to customise to special requirements of end-users.

Involving end-users in the development phases of ERMA

The involvement of end user requirements in the development of ERMA includes the following activities:

• user requirement elicitation by field surveys with questionnaire and consultation workshops.

• elaboration of use case scenarios in cooperation with the 2 involved end users.

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• derivation of process models, fine tune again with selected emergency scenarios of users.

• installation, integration with existing systems and techniques, with training sessions.

• field trials, incorporation of user feedback, adaptation of the system.

• publication of results, public discussions, and organisation of a final event with more users.

All these activities will allow to incorporate early in the design phase user and domain specific requirements. In addition, the system can be tailored to existing emergency practices and such lower the reluctance barrier of the staff involved. Final test and user feedback will improve the prototype and therefore build a first step from prototype to commercial available system. Deve lopment of the ERMA system is realistic only, if there is an opportunity for end-users to contribute to the process of needs' assessment. Only a clear understanding of actions taken and decisions made by users will allow the ERMA system to be useful for risk and emergency management. Consequently, two end-users are members of the ERMA project consortium3 . These are: the Community Council of Targu Lapus (Romania); the Santander Port Authority (Spain). Each end-user presents a different profile. In Romania, the end-user is formed by elected local authorities, municipality council and staff, and emergency management services (e.g.: fire forces). In Spain, the end-user is formed by the administrative and technical authority responsible for an international port area facing the Atlantic Ocean. Accordingly, site-specific hazards, exposed assets, risk management procedures and technical-organisational resources should be considered. This diversity will therefore be an opportunity to develop an ERMA system that is adaptive to very different end-user contexts.

Conclusion

ERMA started in September 2006 with a two year perspective. It will provide a first prototype to be tested at two user sites in 2007. Final findings about the performance of the platform and its cus tomisation prospects are expected by the end of 2008. Currently the user requirement state is finalised with first mock-ups which have been pr esented to the user sites. Next step is the definition of emergency scenarios as use cases which will be translated to process models. Mock-ups and use cases will be presented in the ISESS 2007 conference in order to trigger a discussion about user-centred design and process support for the risk management domain..

Acknowledgement

The presented work has been supported by research funding from the European Commission within the Community's Sixth Framework Programme (IST project ERMA, IST-2005-034889). This document reflects only the authors' views, and that the Community is not liable for any use that may be made of the information contained herein.

Figure 1 :

 1 Figure 1: The ERMA platform and components

Figure 2 :

 2 Figure 2: Mockup of process management component: display of processes

Figure 3 :

 3 Figure 3: Telecom-based warning system. Source: UMS company (Norway). Website: http://www.umscom.com/)

Figure 4 :

 4 Figure 4: Possible interface for the GIS-based warning system developed by the UMS company (Norway). Source: http://www.umscom.com/)

For more, up-to -date information on the development and achievements of the ERMA project, please visit: http://www.erma-project.org


Claire Petitfrère

Christophe Proust

Ineris Ineris

ANALYSIS OF IGNITION RISK ON MECHANICAL EQUIPMENT IN ATEX

Keywords: ATEX, Mechanical ignition source, Nonelectrical Equipment

Until the implementation of the ATEX directive 94/9/CE, the certification of equipment intended to explosive atmospheres was only dedicated to electrical equipment. Since the 1st July 2003, the non-electrical sources of inflammation are also to be looked at before putting an ATEX equipment on the market (pumps, couplers, reducing gears, …).

Among these sources, mechanical friction and impacts are a main cause of ignition of explosive atmosphere. The risk analysis of the equipment consists in considering the failures, which can lead to ignition. If the failure involves a friction between two parts or an impact, we shall estimate if this friction or impact dissipates a sufficient amount of energy to ignite the surrounding explosive atmosphere.

As part of the European program MECHEX, we have studied the process of degradation of the mechanical energy into heat during friction and impacts and we have examined the mechanisms of ignition at the contact zone. An extensive experimental program is presented and some "simple" modelling is proposed on purpose of practical applications. For frictional situations, a critical rubbing power is calculated without any limitations as for a potential lower boundary concerning the rubbing velocity. For "impacts", the relevant parameter for ignition is not the kinetic energy of the projectile but its velocity and the nature of the materials.

I. INTRODUCTION

Since the implementation of the ATEX directive 94/9/CE [1] in July 2003, the non-electrical sources of inflammation shall also be examined before putting an ATEX equipment on the market (pumps, couplers, reducing gears, …).

European harmonized standards have been developed by CEN/TC 305 WG2 to give requirements for non-electrical equipment. The basic standard for non-electrical equipment is the EN 13463-1 [START_REF]Non-electrical equipment for potentially explosive atmospheres -Part 1 : Basic method and requirements[END_REF], which requires an ignition hazard assessment of the equipment.

Concerning mechanical equipment, this assessment often underlines the possibility of mechanical friction or impacts as an ignition source during foreseeable or rare malfunction. When such a source appears, the manufacturer shall estimate if the friction or impact dissipates a sufficient amount of energy to ignite the surrounding explosive atmosphere.

The European program MECHEX was dedicated to the study of the process of ignition during friction and impact.

In the first part of this paper, the actual requirements for mechanical equipment are given. The main results concerning the ignition by friction and impact are presented in the second part and some practical implications in the last section.

II. REQUIREMENTS AND HAZARD ASSESSMENT

A. Requirements of Directives and Standards

Non electrical equipment are dealt with in directive 94/9/CE when placed on the market for use in hazardous areas (concerns firstly the supplier) and by directive 1999/92/CE [START_REF]EC of the European Parliament and of the Council of 16 December 1999 on minimum requirements for improving the safety and health protection of workers potentially[END_REF] when intended to be installed in hazardous area (concerns firstly the user). The essential health and safety requirements of the directive 94/9/CE demand among others that the manufacturer take measures to prevent the ignition of explosive atmospheres, taking into account the nature of every electrical and nonelectrical sources of ignition. Some supplementary requirements are specific of the category of equipment :

• For category 1 (equipment intended to zone 0) :

Equipment must be so designed and constructed that sources of ignition do not so become active, even in the event of rare malfunctions.

•

For category 2 (equipment intended to zone 1) : Equipment must be so designed and constructed as to prevent ignition sources arising, even in the event of frequently occurring disturbances or equipment operating faults, which normally have to be taken into account.

•

For category 3 (equipment intended to zone 2) : Equipment must be so designed and constructed as to prevent foreseeable ignition sources which can occur during normal operation.

The directive 1999/92/CE demands that :

• The employer shall ensure that a document, called 'explosion protection document', is drawn up and kept up to date. This document shall contain the demonstration that work equipment is designed, operated and maintained with due regard for safety. To comply with the requirements of the Directive 94/9/CE we may generally use the harmonized standard EN13463-1. We can also use the frame of this standard to carry the hazard assessment of the equipment that is already installed and to include this assessment to the explosion protection document demanded by the directive 1999/92/CE.

The standard EN 13463-1 is based on the assessment of the ignition hazards of equipment depending on the category of this equipment. It can be used as a stand-alone standard but for mechanical equipment the standard EN 13463-5 [START_REF]Non-electrical equipment for potentially explosive atmospheres -Part 5 : Protection by constructional safety "c[END_REF] is generally used in addition. This standard defines the protection concept "constructional safety" with marking "c". The principle of this type of protection is to apply constructional measures so as to protect against the possibility of ignition from hot surfaces, sparks and adiabatic compression generated by moving parts. The requirements are based on good engineering principles, so that risk of mechanical failures likely to create incendive temperatures or sparks, are reduced to a very low level.

B. Ignition Hazard Assessment

The ignition hazard assessment consists in reviewing all potential ignition sources of the equipment and in distinguishing normal operation, expected malfunction and rare malfunction. Then will be considered the measures applied to prevent each source becoming effective.

Concerning mechanical equipment with moving parts the hazard assessment will often underline the risk of friction or impact as an expected or rare malfunction. That could be the case for instance for pumps when there is no more fluid and where parts could enter in contact in rare malfunction. We can also think to the bearings or to the friction of a roller on a rail.

When the malfunction introducing a risk or friction or impact is identified, the problem is then to determine if this friction or impact could become an effective ignition source or not. The type of source could then be a hot surface or sparks but it is difficult to know if the phenomena are sufficient to ignite the explosive atmosphere.

III. MECHANICAL FRICTION AND IMPACT

A. IGNITION RISK BY FRICTION

Friction between two solid bodies is a process through which mechanical energy is transformed into heat. Experience reveals that heat is produced in the rubbing zone where the material is severely stretched and diffuses outwards. It is well known that the amount of heat evolved is in proportion of the applied force and sliding speed [START_REF] Kragelskii | Friction and Wear[END_REF].

Further, if the friction is sufficiently intense, some material may be torn away resulting in fragments being expelled. This event is sometimes called "friction sparks" or "spark shower".

Experimental equipment

For a given heat power produced in the rubbing zone [START_REF] Proust | Toward modelling of frictional ignition Loss prevention and safety promotion in the process industries[END_REF], traditional thermodynamical laws tell that the maximum temperature may depend on some geometrical sizes (area of the contact zone, diameter of the machine,…) of the rubbing equipment. Mainly for this reason, two experimental set-ups have been used (figure 1). The configuration of a fixed slider rubbing onto a rotating wheel has been selected because a rather precise control of the rubbing parameter is amenable.

The larger device [START_REF] Hawksworth | Mechanical ignition hazards in potentially explosive atmospheres -EC project MECHEX[END_REF] is a 30 cm diameter wheel, 22 mm thickness rubbing against a 40 mm high, 25 x 25 mm square cross section slider for most tests. The sliding velocity can be adjusted between 0.2 and 20 m/s and the normal load between 0 and 5000 N.

The smaller device is a 10 cm wheel, 8 mm thick rubbing against a 15 mm high, 7 x 7 mm square cross section slider. The sliding velocity can be adjusted between 0.05 and 15.7 m/s and the normal load between 0.2 and 1000 N. Temperatures have been measured by optical methods and thermocouples. During the tests, the normal load, friction coefficient and speed were controlled [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF][START_REF] Hawksworth | Mechanical ignition hazards in potentially explosive atmospheres -EC project MECHEX[END_REF][START_REF] Hawksworth | Ignition of explosive atmospheres by mechanical equipment[END_REF]. The evolution of the rubbing temperature as function of the nominal power expressed in terms of normal load (N) times the sliding velocity (V) is shown on figure 2 for the various sliders used and the two machines. There is a clear trend of increasing the rubbing temperature as function of the nominal power. There is also a clear incidence of the nature and of the width of the slider and globally of the size of the rubbing machine. 1000 0,00 500,00 1000,00 1500,00 2000,00 2500,00 3000,00 3500,00 4000,00

N . V (W )

V for various sliders and the two machines (on the top, only with the small scale equipment, in the middle, only with the large scale equipment and on the bottom graph a comparison of the results with the two machines with a hard steel -mild steel rubbing configuration)

In some tests, flying fragments (sparks material) have been collected in order to evaluate the size of them and the wear rate. For steel rubbing against steel at a few m/s, the typical particle size seems of the order of a few hundred of mm and the wear rate, extremely dependent on the experimental conditions, is better scaled in mm 3 /s [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF].

Mechanisms of ignition

The analysis reveals that ignition during friction and impacts may result from three different processes [START_REF] Proust | Toward modelling of frictional ignition Loss prevention and safety promotion in the process industries[END_REF] :

• In direct contact with the hot rubbing zone whenever the local temperature is large enough according to the mechanism of the hot plate [START_REF] Carleton F | Prenormative Research on the Use of Optics in Potentially Explosive Atmospheres[END_REF]. The experimental investigation and some theoretical evidence shows that the critical parameter is the temperature of the hot zone, of the order of hundreds of °C;

• The power dissipated in friction may easily be of the order hundreds of Watts. If the friction is occurring in a sufficiently confined area, the mean temperature of the ATEX may rise and reach the autoignition point. This situation should well be represented by the standard ignition temperatures ; • The sparks may be a cause of ignition. However, since the initial temperature at the beginning of the flight should not be different from that of the rubbing zone, the sparks are likely to constitute a preferential cause of ignition only if, later in their flight, their temperature increases well above that of the rubbing zone. This may occur if the flying particles are able to burn. In this case, the ignition process may be linked to the spark ignition mechanism referring to minimum ignition energies.

Ignition around the heated surface

This ignition mechanism is believed to be the main cause for mechanical ignition [START_REF] Hawksworth | Mechanical ignition hazards in potentially explosive atmospheres -EC project MECHEX[END_REF][START_REF] Powell | Ignition of gases and vapors[END_REF] often invoked but, to our knowledge, never really proven.

For friction, experiments performed with the large scale rubbing machine used in explosive atmospheres and filmed with high speed video [START_REF] Hawksworth | Mechanical ignition hazards in potentially explosive atmospheres -EC project MECHEX[END_REF] demonstrate clearly that ignition proceeds at the rubbing zone. It occurs as soon as some critical temperature threshold has been reached at the rubbing zone. These temperatures have been estimated for a number of atmospheres (hydrogen-air, methane-air, propane-air, ethylene-air, diethyl-ether-air, acetone-air,…) and compared to critical temperatures determined previously [START_REF] Carleton F | Prenormative Research on the Use of Optics in Potentially Explosive Atmospheres[END_REF] for hot surface ignition in explosive atmospheres (Tpcrit).It has been realized that both sets of values correlate extremely well demonstrating that "frictional" ignition is a "hot surface" ignition mechanism. It has been however noted that the critical hot surface temperature does not seem to correlate so well with the standard autoignition temperature nor better with the minimum ignition energy [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF], indicating that this might be a new type of ignition criterion.

Volumetric ignition

Experiments on the small scale rubbing machine with steel_steel configurationhave shown only a moderate increase of the average temperature throughout a volume in closing the wheel (40°C) and inside the spark shower as compared to the temperature reached at the rubbing zone (700°C). So the ignition should occur most probably at at the hot contact zone before any possibility of ignition throughout volume could arise [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF].

Sparks

There was no evidence of any ignition to be undoubtedly attributable to a flying spark in the present experiments [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF]. There was even no evidence of any fragment to burn in the surrounding atmosphere even in the extreme situation of aluminum rubbing intensively against hard steel. Is it to be said that only the situation of a light alloy rubbing against some rusty material and leading to "thermite reactions" [START_REF] Powell | Ignition of gases and vapors[END_REF] is to be considered for "spark" ignition during friction ?

To help and clarify this important point, explosive atmospheres were submitted to a small chemically inert and artificially heated object, simulating a fragment [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF]. Globally, the required temperature to produce ignition is well above the critical "hot surface" temperatures apart from the specific case of sulphur, which need to be further analysed. Thus, it is confirmed that the "spark" temperature need to be well above those of the rubbing zone to generate an increased risk of ignition suggesting that the fragments have to burn somehow.

Modelling

These findings have been analyzed and collapsed into a reasonable model [START_REF]Non-electrical equipment for potentially explosive atmospheres -Part 5 : Protection by constructional safety "c[END_REF]. It is postulated that ignition would occur at the hottest point, in the immediate vicinity of the rubbing zone (we thus leave apart the special case of « thermite » reaction). Following, the ignition parameter is the critical hot surface temperature defined before, depending on the nature of the atmosphere. We then have to derive the contact temperature from the frictional process and compare it to the former. The standard Coulomb-Joule relationship has been used to calculate how much heat is produced in the friction,. This amount of heat is driven out from the contact zone by thermal conduction and is modeled as a local heat source onto an semi-infinite solid [START_REF] Hawksworth | Ignition of explosive atmospheres by mechanical equipment[END_REF]. The final equation reads:

( ) (
) For the practical use of this expression within the frame of explosion safety, the following aspects need to be considered:

amb f T T R A V N f q - ⋅ ⋅ + ⋅ = ⋅ ⋅ =
• a proper choice of the critical ignition temperature must be made. For explosive atmospheres, the relevant parameter would be Tpcrit which is above but not directly related to the standard ignition temperature ; • the area of the contact zone depends on the geometry of the device and may be found with a detailed analysis of the system. It should not be smaller than the ratio of the normal load to the Brinell Hardness which gives a lower boundary for R ;

• the critical power dissipated in rubbing (q=f.N.V)
is the part lost in the friction which, in normal operation, is usually a very low fraction of the nominal power, typically a few percent for friction coefficients of the order of 0.001 to 0.01. Only in case of severe malfunction (a shaft broken, a fused bearing…), the friction power may reach the nominal power with a fraction coefficient between 0.1 and 1.. Some comparison between this simplified method and experimental data is shown on figure 4. Here, the rubbing situation typically corresponds to a severe malfunction as defined above with a friction coefficient equated to unity. The model gives a lower estimate of the critical power but is reasonable in trends and order of magnitude.

Figure 4 : performances of the simplified model for frictional ignition against existing data [7] and results issued from the present program

This physical analysis implies that the maximum temperature in the rubbing zone depends only on the friction power without any limitations about any lower boundary about the rubbing velocity. For instance, it has been possible to ignite propane-air, ethylene-air and hydrogen-air mixtures at rubbing velocities equal to or even smaller than 1 m/s, which has been sometimes given a "safe" limit [START_REF] Hawksworth | Ignition of explosive atmospheres by mechanical equipment[END_REF].

B. IGNITION RISK BY IMPACT

Experimental equipment

The experimental facility consists of a free fall vertical device. The target is a strong mild steel plate (5 mmx70 cmx45 cm) anchored on a rigid thick wood plate (11cm) impeding flexion during impact. The plate is inclined to favor a glancing blow. In most experiments the angle of 60°has been chosen. The projectile (diameter 18 mm of steel, aluminium, copper,…) is propelled by an air cannon on a precise point of the target at variable velocity between 5 to 50 m/s.

Figure 5 : Experimental impact test equipment

The impact was filmed with a high speed camera in order to estimate the impact speed and to detect the fragments. Simultaneously, the temperature in the contact zone was measured with a high speed monochromatic pyrometer [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF][START_REF] Proust | Toward modelling of frictional ignition Loss prevention and safety promotion in the process industries[END_REF].

A sample of those measurements is shown on figure 6. The impact results in a sharp temperature rise (up to hundreds of °C in a few tens of microseconds) followed from a rapid but more progressive decrease (in milliseconds). Usually a chip is produced which constitutes a unique fragment of mm size. Some data are shown on figure 7 for various experimental conditions. The impact temperature depends primarily upon the impact velocity and nature of the material and only very weakly on the size of the projectile. A further analysis of the same data shows that the impact temperature is not correlated to the incident kinetic energy [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF]. The physical analysis of the results [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF][START_REF] Proust | Toward modelling of frictional ignition Loss prevention and safety promotion in the process industries[END_REF] suggests that the efforts, deformations, fragment production… should be the consequence of the strain waves produced in the very first moments of the impact and propagated at the speed of sound in the materials. These waves are responsible for the rebound. The experimental evidence suggests also that heat is produced due to some intense but very short rubbing on the target during the time of the rebound. while the projectile slides against the target with the tangential component of the impact velocity. The amount of heat produced is then driven away by transient thermal conduction A model has been proposed elsewhere [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF] and the final equation reads:

cannon reservoir target compression chamber ( )L V K f T T m amb imp ⋅ ⋅ ⋅ = - 2
where L stands for the length of the projectile and V the impact velocity. Timp is the peak temperature reached during the impact. Km is a composite parameter dependant only on the intrinsic properties of the bodies like sound speeds, specific masses and heat capacities, thermal conductivity Km is typically of the order of 10 (20 for steel) in SI units. About ignition, both the fragment, if any, and the hot spot may contribute to ignition equally because of their similar size. It has been found also [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF], that the ignition process is closer to a hot spot mechanism as described before than to the standard spark ignition mechanism. Because of the short duration of the hot spot during an impact, the critical ignition temperature appears larger, by a factor between 1 to 2 depending on the atmosphere.

For the practical use of the preceding expression, a friction coefficient of 1 may be selected. √L may easily vary in more than a ratio of 10 whereas Tpcrit varies only in a ratio of 2 . It is then tempting to use this formula by setting Km to 20, a rather large value, and Timp to 550°C, one of the smallest practical ignition temperature, L being the remaining unique variable equal to the largest dimension of the projectile. It is finally very simple to find an order of magnitude of a critical impact velocity, capable of igniting an explosive atmosphere. A comparison with data from the literature [START_REF] Proust | Toward modelling of frictional ignition Loss prevention and safety promotion in the process industries[END_REF] is presented in figure 8.

Figure 8 : performances of the simplified model for impact ignition against existing data

The comparison is less favorable than for friction but impact ignition tests are difficult to perform because of a rather poor reproducibility. Nevertheless, this very simplified approach seems conservative apart from the specific situations of "thermite" reactions or large objects, mostly because, we believe, the exact geometry of the projectile is not precisely known.

As a final remark, the kinetic energy of the projectile does not appear, at any moment, neither in the data reduction of the experiments nor in the theoretical approach as Rasuo [START_REF] Rasuo | Development of device and method for testing of mechanical sparks which might ignite explosive mixtures[END_REF] noticed earlier The velocity of the impact appears as a more convenient parameter.

IV. PRACTICAL IMPLICATIONS

The foregoing simplified models are issued from a more sophisticated tool in which the various equations are solved numerically. The latter gives less conservative results than the former with exactly the same trend [START_REF] Proust | Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)[END_REF]. Both can be used for practical applications.

The basic parameters to feed into the models are the nature of involved material, the size of the equipment, the explosive properties of the atmosphere, the velocity (sliding velocity for friction and incident velocity for impact), the normal load and frictional area for friction. The temperature in the contact zone is calculated and compared to the critical hot spot temperature for the atmosphere. Some examples of applications are presented below.

A. Lifting screw

Within the scope of the CE certification, the case of a lifting screw devoted to the transfer of combustible powders was studied. The screw is a long helical steel wire turning inside a steel sleeve (inner diameter of the sleeve is about 150 mm, 10 mm wire diameter). The device is driven by a 5500 W electrical engine at 1500 rpm. Hazard assessment reveals a risk of friction of the wire against the sleeve. The explosive dusty atmosphere has a standard ignition temperature of 240°C suggesting a critical hot spot ignition temperature of about 700°C [START_REF] Carleton F | Prenormative Research on the Use of Optics in Potentially Explosive Atmospheres[END_REF].

With a thermal conductivity of steel of the order of 15 W.m/K and a rubbing size of the order of 10 mm (diameter of the wire), the critical temperature could be reached for a frictional power as low as 400 W according to the simplified friction model presented above. This finding may be compared with the experimental data of figure 2 if we consider that this lifting screw is somewhat comparable to the experimental rubbing machines in size. At 700 °C, the friction coefficient is about 0.3 so that N.V for the lifting screw should be about 1500 W, in line with the data obtained with the small scale machine.

Although rather indicative, this result suggests that the power of the machine is larger enough to generate immediately a dangerous situation in case of steel-steel friction. The simultaneous presence of a dust atmosphere inside the sleeve should then be avoided by ensuring that the device is always full with bulk dust at full power for instance.

B. Turbine

We have examined a turbine with a mobile part in aluminium and an envelope in alloy of steel-aluminium. The diameter of the wheel was 340 mm. The nominal speed was of 3500 tr.min -1 and the peripherical speed was of 62 m/s. We have considered the case of a piece of alloy steelaluminium blocked between the rotor and the casing of the turbine generating a strength of 3,6 N. The use of the foregoing simplified expressions reveals that the rubbing temperature at the contact point will be about 416°C, which can be compared to the critical ignition hot spot temperature of an explosive atmosphere. In the practical studied case it has been demonstrated that there was non ignition risk.

C-Gasoline feeder

A gasoline pump supplier is concerned by the explosion hazard raised by a new type of connector to feed LPG in cars. The possibilities of an impact of the connector on some solid surface and of a friction due to the rubbing along a moving vehicle were identified. Impact and rubbing experiments have been performed. Both the connector and the rubbing/impact surfaces are mainly made of steel. The typical diameter of the connector is 5 cm. Given the geometry of the connector, a typical rubbing size of 10 mm was chosen.

For LPG, the critical ignition hot spot temperature is about 650°C. At this level of temperature, the steel-steel friction coefficient is about 0.3.

The use of the foregoing simplified expressions reveals that the critical impact velocity would be of the order of 23 m/s and the critical rubbing power of about 800 W (N.V = 800/0.3=2500 W). Experiments have shown that the peak impact temperature is about 630°C at 28 m/s and the rubbing temperature is close to 300°C with a rubbing power of 400 W (N.V = 1200 W). Recalling that the rubbing temperature has been observed to be approximately proportional to N.V, the experimental observations seem coherent with the predictions.

On the practical side, those values seem high with regard to the normal way of operating the device and the considered risk might only appear in case of rare events.

V. CONCLUSION

Since the implementation of the ATEX directive 94/9/CE in July 2003, the non-electrical sources of inflammation shall also be examined before putting an ATEX equipment on the market (pumps, couplers, reducing gears, …).

European harmonized standards have been developed by CEN/TC 305 WG2 to give requirements for non-electrical equipment. The basic standard for non-electrical equipment is the EN 13463-1, which requires an ignition hazard assessment of the equipment with the list of all potential ignition sources. Depending on the category of the equipment (1, 2 or 3), inducing the area where it can be installed (zone 0, 1, 3 or 20, 21, 22), the assessment has to be carried considering only normal use or also foreseeable or rare malfunction. The same work can be done to ensure that the equipment already in use fulfills the requirements of the directive 1999/92/CE and can be described in the explosion protection document.

Concerning mechanical equipment, this assessment often underline the possibility of mechanical friction or impacts as an ignition source during foreseeable or rare malfunction. When such a source appears, the manufacturer shall estimate if the friction or impact emits a sufficient energy to ignite the surrounding explosive atmosphere.

The European program MECHEX has permitted to study the process of ignition during friction and impact. The process of degradation of the mechanical energy into heat during friction or impacts and the mechanisms of ignition at the contact zone have been studied. An extensive experimental program is presented in this paper but many points have only been surveyed. It was found that, apart for the very special situation of "thermite" reactions, which has not been investigated in this program, the "sparks shower" resulting from the production of fragments in the contact zone is less dangerous than the high temperature of the friction or impact area. It has been noted that the critical parameter is the temperature of the hot surface, a property of the mixture, which does not seem to be closely linked to any known standard ignition parameters. Some "simple" modeling is proposed on purpose of practical applications. For frictional situations, a critical rubbing power is calculated without any limitations about any lower boundary concerning the rubbing velocity. For instance, it has been possible to ignite propane-air, ethylene-air and hydrogen-air mixtures at rubbing velocities equal to or even smaller than 1 m/s, which has been sometimes unduly given a "safe" limit. The main variable is the friction coefficient the prediction of which is not easy: it varies from 0.001 to 1 depending on the lubrication and materials. For "impacts", the relevant parameter for ignition is not the kinetic energy of the projectile but its velocity and the nature of the materials. The experimental data are not sufficiency to valid the model and more experiences need to be lead.

As practical implications, some results of MECHEX have already been taken into account in the redaction of the second edition of EN 13463-1. Some limited couples of parameter speed/torque of rubbing have been introduced instead of the only speed limit of 1 m/s which was given in the EN 13463-5.

We have presented in this paper some practical cases where the simplified model has been used. The materials in contact, the considered atmosphere, the speed and torque shall be known. The surface of contact is estimated and the model calculates the temperature of rubbing or impact zone and compares it to the critical ignition temperature of the explosive atmosphere. It may help for the hazard assessment.

On the other hand a certain time is necessary to reach the maximum temperature at the rubbing surface and some equipment may use this parameter for implementing a detection of the failure that will induce the rubbing.

The model should still be tested on other practical case. Some complementary studies would be necessary to determine the time to reach maximal temperature by rubbing.

It would also be interesting to have a better knowledge of the value of the critical ignition temperature as a new data for the different explosive atmospheres as it seems not to be correlated with the known data of autoignition temperature (AIT) and minimal ignition energy (MIE).

Figure 1 :

 1 Figure 1 : rubbing experimental set-ups (Large scale 30 cm dia. wheel-left and small scale 10 cm dia. wheel -right)

  hard steel : slider 25 mm, wheel 22 mm Hard steel -hard steel : slider 25 mm, wheel 10 to 16 mm Hard steel -hard steel : slider 10 mm, wheel 22 mm Logarithmique

Figure 2 :

 2 Figure 2 : Rubbing temperature as function of N.V for various sliders and the two machines (on the top, only with the small scale equipment, in the middle, only with the large scale equipment and on the bottom graph a comparison of the results with the two machines with a hard steel -mild steel rubbing configuration)

Figure 3 :

 3 Figure 3 : tabulated critical hot surface ignition temperature and observed contact temperature upon ignition during friction

  bodies)• R : radius of contact area • A : contact area •λ i : heat conductivity of body i in the rubbing zone • f : friction coefficient • N : normal load • V : rubbing velocity • q : heat dissipated in friction

Figure 6 :

 6 Figure 6: typical temperature reading and excepts from the high speed film (18 mm steel rod of 20 cm long, impacting at 10 m/s)

  mild steel rod -V = 7 +/-0.5 m/s mild steel rod -V = 38 +/-10 m/s aluminium rod -V = 4.5 +/-0.5 m/s copper rod -V = 5.5 +/-0.5 m/s mild steel rod -V = 20 m/s

Figure 7 :

 7 Figure 7 : Maximum impact temperature of the hot spot for various conditions

  Work equipment for use in hazardous area which was already in use or was made available in the undertaking or establishment for the first time before 30 June 2003 shall comply from that date with the minimum requirements laid down in Annex II, Part A. So all necessary measures must be taken to ensure that the work equipment have been designed, constructed, assembled and installed, and are maintained and operated, in such a way as to minimize the risks of an explosion.

	• Work equipment for use in hazardous area which was
	made available in the undertaking or establishment for
	the first time after 30 June 2003 shall comply with the
	directive 94/9/CE (with the category adapted to the
	zone).
	•

VI. ACKNOWLEDGEMENTS

The authors are grateful to European communities with supported the MECHEX project between 2001 and 2005. They thank also the others sponsors like HSE, the German authorities, the French ministry for research and the French ministry of industry. A warm congratulation to the colleagues throughout Europe who took an active part to the project : S. Hawksworth (coordinator), M. Beyer, D. Lakic, R. Rogers, D; Raveau, X. Lefebvre, V. Pina and P. Hervé


Monitoring emissions of particles with a diameter smaller than 10 µm (PM 10 ) has become of growing interest. However, measuring such particles is rather difficult. It appears as a result necessary to develop and assess measurement methods for the monitoring of PM 10 from stack and fugitive emissions. The aim of the GAEC program [Granulométrie des Aérosols dans les Emissions Canalisées: Aerosol size distribution from stack emissions] is double: to develop monitoring methods and improve knowledge on fine particulate stack emissions. Three institutes were involved in the program: Séchaud Environnement (formerly LECES), INERIS and CERTAM. This paper presents the mains results of the evaluation of a cascade impactor (Johnas), aerosol size distribution measurement techniques (ELPI Dekati, FPS, SMPS TSI) applied on size characterization of fine particulate matter in stack emissions of a 600-MWe coal-fired power plant.

Introduction

Emissions of particulate matter with an aerodynamic diameter smaller than 10 µm (PM 10 ) have become of growing interest. However, monitoring such particles is rather difficult. In order to address this issue, a standardisation frame has been defined: a standardisation working group dealing with PM 10 and PM 2.5 emissions from stationary sources has been set up in 2003 (ISO TC 146 SC1 WG 20). The GAEC program presented here aimed at developing measurement techniques and generating experimental data on fine particulate emissions from stationary sources. Such developments may, for instance, help plant operators in providing more accurate data in the framework of the European Pollutant Emission Register (EPER).This study is supported by the French Environment Agency (ADEME) and the French ministry of environment (MEDAD) and deals more precisely with aerosol size distribution determination from stack emissions: PM 10 , PM 2.5 , PM 1 and PM 0.1. The working partners involved in this program are: Séchaud Environnement (previously LECES), INERIS and CERTAM.

A first step consisted of a state of the art of the health impact and the available measurement techniques. Some of the techniques have then been selected to be set up and assessed on site during measurement campaigns. The first campaign took place in September 2005 in a cement plant; the results of this study have been presented at the Dustconf International Conference in Maastricht, the Netherlands in April 2007 (Fraboulet et al, Dustconf 2007). The present paper deals with the results of the second campaign that was carried out in March 2006 at a 600-MWe coal-fired power plant.

Experimental conditions

Measurement site

The sampling campaign took place at the EDF coal fired power plant in Cordemais, France. A schematic of the plant including the energy production and gas treatment unit is presented in Figure 1. The sampling point was located after the gas treatment unit. This unit is equipped with an electrostatic precipitator (ESP) dust filter and a desulphurisation unit (Figure 2). The desulphurisation consists in passing the gases through a wet scrubber where a lime (CaCO 3 ) saturated washing solution is sprayed. This leads to the formation of calcium sulphite (CaSO 3 ) which is afterwards oxidised into gypsum (CaSO 4 ). The clean gas is then emitted through the chimney and gypsum is collected to be sold to the cement or paper industry.

ESP dust filter

Desulphurisation

Gas treatment Energy production

During the campaign, the power plant was operating at a power comprised between 480 and 600 MW. the following table presents the operating conditions associated. It is interesting to stress the fact that this stack showed very high values of gas velocity associated to relatively low TSP concentrations. Such conditions are likely to penalise the judgement on measurement techniques since they are close operating condition limits.

Measurement techniques

The following techniques were selected to be set implemented during the campaign:

Manual methods:

• JOHNAS cascade impactor (John et al, 2003) • TSP reference gravimetric method Automatic methods: Aerosol size distribution • ELPI : electronic low pressure impactor connected to a sampling and dilution probe called FPS (PM 2.5 inlet), using smooth or sintered impaction plates • SMPS : mobility analyser connected to a sampling and dilution probe called FPS (PM 2.5 inlet)

Results and discussion

Manual techniques

Table 2 presents the comparison between the operating conditions and the specifications of the manual measurement techniques. This confirms the fact that the sampling conditions in term of concentration and gas velocity reach the limits of the technique specifications. The filters collected with the two techniques showed a yellowish colour, which was very similar to the gypsum generated as a by-product of the desulphurisation. This suggests that the emitted aerosol could present a relatively high proportion of gypsum; this could mean that the nature of the emitted aerosol is more linked to the desulphurisation than to the coal combustion itself. However, this hypothesis would need to be confirmed by further analyses.

Concentrations measured were comprised between 1 and 6 mg/m 3 0 i.e. in the low range of the specification of the measurement techniques. The small number of sampling trials performed is insufficient to conclude on correlation between the concentrations emitted and the output power of the plant. The PM 10 /PM 2.5 ratio indicated that 90% of the mass of PM 10 was due to PM 2.5 . These are very preliminary that would need to be taken into account with cautious and confirmed by microscopy analysis of the impaction filters in order to validate the distribution of the particles on the impaction stages and dismiss any risk of particle bounce.

Automatic techniques

The main interest of using automatic techniques for the determination of aerosol size distribution is to perform online monitor of the aerosol characteristics. This is especially interesting to assess process stability or to characterise process operating transitions.

The following figure presents the evolution of the operating output power as well as the number of particles measured by the ELPI. It shows that a decrease of the power of the boiler causes a decrease of the number of emitted particles. A similar trend could be observed with the SMPS. During the previous campaign that took place in a cement plant (Fraboulet et al, Dustconf 2007), the SMPS was used together with the ELPI equipped with smooth impaction plates, in order to characterise the aerosol [START_REF] Keskinen | Estimation of effective density of diesel particles[END_REF][START_REF] Marjamaki | Performance evaluation of the electrical lowpressure impactor (ELPI)[END_REF]. For a density of 3, as shown in Figure 5, a good agreement of the SMPS and ELPI curves could be obtained in the range 0.1 to 1 µm. However, for particle with a diameter smaller that 0.1µm the ELPI was overestimating the amount of measured particles. The most likely hypothesis was that particle bounce may cause this overestimation of the ELPI comparing to the SMPS. The use of smooth impaction plates, when characterising mineral particles, could indeed lead to particle bounce; a solution to this issue could be the use of sintered impaction plates.

In order to validate this hypothesis, the ELPI equipped with either smooth or sintered impaction plates was jointly used with the SMPS during the power plant campaign. The following figure shows the results obtained for a value of density of 1.5. It appears, as expected, that when using smooth impaction plates, the ELPI overestimates the particle number in the range of diameters smaller than 0.1µm. However when using sintered impaction plates, the size distribution obtained with the ELPI and the SMPS are in good agreement in the range of particles with a diameter bigger than 0.06 µm. In the range of particles with a diameter smaller than 0.05µm, the ELPI and the SMPS showed different size distributions. This may be due to the occurrence of a volatile unstable nucleation mode in the lower size range. In some cases, the use of low pressure to accelerate ultrafine particles in the area of the last impaction plates of the ELPI can lead to an evaporation of volatile compounds and as a result to an underestimation of the nucleation mode.

CONCLUSION

This campaign took place at a power plant stack, the sampling points being located after an ESP dust filter and a desulphurisation unit. The use of manual and automatic techniques led to the following results : TSP concentrations were comprised between 1 and 6 mg/m 3 0 , the PM 2.5 fraction represented 90% of PM 10 . The number of particles emitted is correlated with the operating output power, monitoring of particle number appears then as an interesting tool for process monitoring. The use the ELPI equipped with sintered impaction plates reduces the risk of particle bounce and leads to a good agreement with the SMPS. In the lower size range, a nucleation unstable mode could be observed.

Besides, the collected samples showed a yellowish colour characteristic of the gypsum generated as a by-product of the desulphurisation unit. This observation may indicate an important mineral proportion of the aerosol.

Figure 1 :

 1 Figure 1 : Schematic of the Cordemais power plant process.

Figure 2 :

 2 Figure 2 : Schematic of the desulphurisation unit

Figure 3 :

 3 Figure 3 : PM 10 , PM 2.;5 and sum PM 10 + > 10 µm fraction; mass concentrations measured with the Johnas impactor

Figure 5 :

 5 Figure 5 : Comparison of cement plant aerosol size distribution measured by the ELPI and the SMPS for a density value of 3

Figure 6 :

 6 Figure 6 : Comparison of aerosol size distributions determined with the ELPI and the SMPS

Table 1 :

 1 Sampling conditions

	Gas velocity	34-43 m/s
	Humidity	6-9 %
	Temperature	91-94°C
	O2 content	7-8 %
	CO2 content	11 %
	TSP	1-10 mg/m 3 0

Table 2 :

 2 Comparison of the sampling operating conditions with the manual measurement technique specifications

	Specifications	Gas velocity (m/s)	Temperature (°C)	Humidity (%)	Concentration (mg/m 3 0 )
	Johnas impactor	<35-40	250	<25	1-100
	reference method		250	<25	>1
	sampling conditions	36-39	91-94	6-9	1-10
	agreement	limit	yes	yes	limit

Table 3 :

 3 Output power of the plant during the campaign

	Trial	Power (MWe)
	M4	600
	M5	500 +/-60
	M6	4-600

-Fumées polluées en aval de l'électrofiltre

-Réchauffeur fumées/fumées

-Absorbeur

-Fumées épurées 7 -Chaîne de déshydratation du gypse 8 -Stockage gypse 1 1-gas before treatment 2-fume heat exchanger 3-lime (CaCO 3 ) wet scrubber 4-clean gas 7-gypsum (CaSO 4 ) dehydratation 8-gypsum storage


J M Lacome

Y Dagba

D Jamois

L Perrette

C H Proust

LARGE-SCALE HYDROGEN RELEASE IN AN ISOTHERMAL CONFINED AREA

INERIS has set up large-scale fully instrumented experiments to study the formation of flammable clouds resulting from a finite duration spillage of hydrogen in a quiescent room (80 m 3 chamber). Concentration, temperature and mass flow measurements were monitored during the release period and several hours after. Experiments were carried out for mass flow rates ranging from 0,2 g/s to 1 g/s. The instrumentation allowed the observation and quantification of rich hydrogen layers stratification effects. This paper presents both the experimental facility and the test results. These experimental results can be used to assess and benchmark CFD tools capabilities.

INTRODUCTION

In a medium term future, one could expect an increasing number of hydrogen energy systems to be operated or stored (mobile applications) inside buildings (dwellings, garages,…). Whereas outside location would be preferable in most cases to prevent leaking hydrogen from accumulating, inside location will remain in many cases a necessity although moderate releases of hydrogen in confined or semi-confined geometry may present a serious risk since combustible mixtures may form. Literature is rather poor on that critical subject [START_REF]Review of Existing Experimental Hydrogen-Helium Release & Dispersion Data For Standard Benchmark Exercise Problems[END_REF] to allow for a robust analysis and predictive quantification model or safety standard development. In order to better understand how hydrogen behaves when released in confined space and especially to assess in which circumstances explosive mixtures may form, INERIS has set up large-scale fully instrumented experiments to study the mechanisms of the formation of flammable clouds resulting from a finite duration spillage in a quiescent room. Concentration, temperature and mass flow measurements were monitored during the release period and several hours after in order to investigate the diffusion process. The objective of this paper is to provide consistent experimental results on hydrogen release in large-scale closed and quiescent volume for realistic leakage. These results will help to better understand and assess small hydrogen release in closed area and therefore contribute to a better risk assessment and control.

TEST FACILITY DESCRIPTION

Chamber

The experiments were performed in a 80 m 3 chamber which is built inside a rock gallery (Figure 1). This chamber has the shape of a rectangular box, see Figure 2, with average dimensions 7.2 x 3.78 x 2.88 m in length, width and height respectively. The garage ceiling and the front side are made up of a wood and plastic sheeting, whereas the ground has a negligible slope in the length and width directions. The rock solid mass ensures a thermal stability of the inner and prevents any convection effect.

The walls are closed and tight except the front side that accommodates 3 holes (Figure 2). The bigger one, which is referred to as "opening 1" in the Figure 2, has a diameter of 0.15 m and is located at the top. Its goal is to ventilate the chamber between the experiments (safety reasons). There are two other smaller opening, named as "openings 2 & 3" in the Figure 2. They are 0.05 m in diameter and were used to pass on the electrical cables. These holes were kept closed unless otherwise specified, during all the experiments. The left, right and bottom sides are not smooth surfaces; the roughness size on those surfaces is between 5 mm and 10 mm (Figure 3).

Release apparatus and position

Hydrogen was released through an orifice on top of a stabilisation chamber as shown in Figure 5. Homogenization of the flow was obtained using a dispersion bed. The internal diameter of the release chamber was 120 mm and its height 265 mm. The hydrogen flow was seeded with nanometric ammonium chloride droplets to allow for flow and stratification visualisation by means of an argon laser. Upstream of the release apparatus was connected a home-made hydrogen mass-flow controller. Figure 3 and Figure 4 show the injector location inside the test chamber.

For a given mass flow rate, it was possible with the hydrogen injector to vary the speed of the hydrogen jet by changing the diameter of the top opening (O1 in Figure 5). The mean velocity is then given by the following equation:

S) * M * Vol)/(Cd * m ( U = (1)
where m -mass flow rate, g/s; Vol -molar volume, m 3 /mol; Cd -discharge coefficient, -; M -molar weight, g/mol; S -orifice area, m 2 .

We first had to evaluate the discharge coefficient, which could vary from 0.62 to 0.7 for sonic velocity. During previous tests with hydrogen and helium, a value of 0.68 for Cd gave good agreement between gas velocity using equation 1 and measured value. Measurements of the pressure difference between the interior of the release apparatus and the outside also allow the quantification of the flow mean velocity.

Mass flow control

The hydrogen mass flow rate is controlled by a home-made system based on the shocked flow principle. Above a given upstream pressure, the exit velocity at the orifice is constant and equal to the speed of sound of the leaking gas (sonic release). Under these conditions, and for a given orifice, the desired mass flow rate can be obtained by adjusting the upstream pressure by means of the gate valves V1 & V2 in Figure 6. Correlation between upstream pressure and desired mass flow rate requires calibration to take place before the operation. For the purpose of calibration, (see Figure 6), the mass flow control set up (left tank up to sonic nozzle) is associated to the calibration apparatus (right hand side equipment after sonic nozzle).

Figure 6: mass flow rate measurement Sonic openings with different diameters generated hydrogen flow rates between 5 mg/s and 10 g/s.

Concentration measurements

Three paramagnetic analysers were used to carry out measurements of concentration in the chamber. An automatic switch allowed to connect four points of measurement to each one of these analysers, see Figure 7. Given the response time of the devices and the filling times of the sampling lines, the measurement time for each sampling point was 80 s. Each analyser provides an uninterrupted and analogical signal which is proportional to concentration of oxygen circulating in the measuring cell. From the ouptut signal is deduced the value corresponding to the concentration of the four points of measurement. Errors on the measurements were estimated to be less than 0,02 % (vol.).

Sixteen sampling points were used to measure the hydrogen concentration in the chamber. They were all located on a x-z plane passing through the centre of the release chamber (as shown in Figure 2). Table 1 lists all the locations of the sampling point and the opening in terms of the x, y, and z coordinates.

Video recording

For safety reasons related to the presence of a laser source in the test room, video recording was possible only with helium releases but not with hydrogen. The camera was located at the entrance of the chamber (see Figure 4) and allowed to visualise the flow structure on the same plane as the one where the sensors are positioned. A laser plan was set up in order to improve contrast and to better visualise the turbulence pattern caused by the release.

Experimental cases

Tests were performed with different mass flow rates ranging from 0,2 g/s to 1 g/s and different orifice diameters from 5 mm to 20 mm. These release values correspond to realistic industrial environment.

The table below summarises the test conditions. It is worth noting that all investigated releases were subsonic. Release duration was limited to 240 s for safety reasons but the observation took longer to investigate the diffusion phenomenon. Test cases were also carried out with helium with similar released mass flow rates. Details are given in table below.

EXPERIMENTAL RESULTS AND DISCUSSIONS

Convection phase and diffusive phase

The experimental results show that for all the subsonic jets stratification takes place rapidly at the top.

A typical result is given in Figure 8. A diffusion phase, characterised by a long observation time, dissipates this stratification. The concentration in the chamber became homogeneous after four hours for every test case.

Figure 8: variation of %H2 for a 1 g/s H2 release through a 20 mm hole For the above case, the 4% horizontal limit, which corresponds to hydrogen Lower Limit of Inflammability (LFL), is obtained between sensor 10 and 11. In Figure 9, we clearly see a horizontal homogeneous layer of hydrogen close to the ceiling, Figure 9 : variation of % H 2 for a 1 g/s H 2 release through a 20 mm hole Homogeneous concentration at a given altitude can be attributed to the residual speed of the mixture after its impingement with the ceiling coupled to the effect of mixture buoyancy. Due to the richness of the mixture, this latter remains "stuck" to the ceiling.

The peak of concentration is observed during the release phase (see Figure 10) where the concentration close to the release point (sensor 16) reaches approximately 17%. A 1 g/s H 2 release gives a jet phenomenon. Once the release phase is over, the concentration at this sampling point decreases sharply under the LFL and its value is similar with the one measured with other sensors at the same height (see Figure 8). Profiles (see Figure 12) of concentration show that hydrogen concentrations are rather homogenous in the formed layer near the ceiling. The higher the hydrogen concentration is in the top layer, the less sharp the concentration profile in the rich layer is. The concentration gradient between the top hydrogen layer and the ground increases with the release flow rate. During filling phase, the concentration in the rich layer is mainly correlated with the flow rate. Some of 0.2 g/s H 2 releases do not behave like a turbulent jet. This seems consistent with the weakness of the Reynolds number (Re ≈ 2000) and further studies might be necessary to investigate further this leakage phenomenon. For the higher releases (1 g/s H 2 ) we see clearly in Figure 12 that the concentration in the upper layer increases with the orifice diameter.

Tightness of the chamber walls and control of small leakage

It is very difficult to make tight such a large volume, particularly with respect to a light gas such as hydrogen. According to the studied cases, hydrogen injected into the chamber creates with the ambient air a mixture, more or less heterogeneous, whose density is lower than that of the surrounding air. As a result from this gradient of density, hydrogen can escape through tiny holes or cracks outside the chamber (see Figure 13).

Figure 13 : decrease of hydrogen mass in the chamber due to leak and openings

The residual leakage rate is dependent on the initial quantity of hydrogen injected. After dividing the chamber into 7 layers with different thickness (5, 15, 30, 50, 50, 50 and 88 cm from the ceiling to the bottom), the flow was solved in each of these layers assuming homogeneous properties in order to locate and quantify the potential leak points. The lost of hydrogen was also modelled. By using :

U = ( (∆ρ/ρext) * g * h) 0,5 (2)
where U -flow velocity, m/s; ∆ρ -density difference, kg/m 3 ; g -gravitational acceleration -m/s 2 ; hheight of the mixture layer, m; and we modelled the diffusive flow between the layers with an usual diffusion law. Comparisons between this calculation and the experiments indicated a hydrogen leak With "opening 1" unclosed (top opening in front wall) assuming a 3,7 %v/v homogeneous mixture (obtained with a 4-minute release at 1g/s) our simple approach estimates that half of the injected hydrogen mass can escape from the chamber in less than 45 minutes. However, in the experimental configuration and due to vertical concentration gradient and higher hydrogen concentration at the top, the observed escape mass flow rate is higher, see Figure 13, than the one predicted with the model.

Analysis of the video results and H 2 and He similitude

The visualisation of the flow gives additional information to those obtained using specific concentration measurements. The pictures below (Figure 14) show the jet production and diffusive phase respectively for a helium release case. The concentration levels are approximately the same in the upper layer i.e. between 7% and 8% for H2 and between 6 % and 7% for He. Helium peak concentration (at sensor position 16) is of the order of 14 % (by volume) at the end of injection, this value could be compared to the hydrogen value of (see 10). These values are also very similar. The chamber concentration levels in H 2 and He tests are very close as we can see with sensors 4, 6, 8, 9, 10 located in the upper-half of the chamber and with sensors 11 and 12 located in the lower-half. This result shows the good homogenisation of gas mixture in horizontal cross-section as discussed by CEA [START_REF]Experimental results on helium release inside a full scale GARAGE set[END_REF].

It therefore would allow us to use hydrogen-helium similitude for dispersion cases although further experiments would be necessary to draw some robust conclusions. Theses experimental cases confirm the study carried out by Swain et al. [START_REF] Swain | Hydrogen leakage into simple geometric enclosures[END_REF].

CONCLUSIONS

INERIS performed hydrogen and helium dispersion tests for a 240 s release time in a quiescent room for mass flow rates ranging from 0,2 g/s to 1 g/s and from 0,4 g/s to 1,84 g/s for hydrogen and helium respectively. Injection orifices varied from 5 mm to 20 mm. Sensors were used to monitor the evolution of the concentrations not only during the injection phase but also a few hours later for the diffusion. The experimental results show that for the subsonic jet, stratification appears at the top. A diffusion phase follows this stratification and, for all the tested cases the concentration became homogeneous after four hours. The jet characteristics influences the concentration of the rich layer and gives indications on what we can expect in terms of concentration at top level (above or below LFL) and volume the explosive mixture if any at the end of the release. In the studied case, a release flow of 1 g/s H 2 generates an explosive volume corresponding approximately to half of the room. For a flow of 0.2 g/s the hydrogen concentration never exceed the LFL.

In an experimental set up like this, hydrogen tightness is difficult to reach. However, we have been able to estimate the hydrogen loss and the equivalent leaking surface by simple calculation. Helium tests were also carried out and results show a strong similarity with hydrogen. Moreover, video recordings during Helium tests brought complementary and very useful information. These results will help in better predicting hydrogen dispersion in confined spaces. Another paper follows soon, which will analyse further the results described here. These experimental results can be used to assess and benchmark CFD tools capabilities and to validate dispersion models.

Figure 1 :

 1 Figure 1: General view of the experimental set up environment

Figure 2 :

 2 Figure 2 : Release chamber scheme and sensor positions (dimensions in cm)

Figure 3 :Figure 4 :

 34 Figure 3 : Injector location in the x-direction (see Figure 2 for the reference frame)

Figure 5 :

 5 Figure 5 : Hydrogen injector

Figure 7 :

 7 Figure 7 : diagram of the automatic sampling device in 12 points of the chamber facility

Variation of %H2 during the test at different point at vertical 1m40 from release Typical results for a 1g/s H2 release through a 20 mm hole duringFigure 10 :

 10 Figure10: variation of % H 2 for a 1 g/s H 2 release through a 20 mm hole For a 1 g/s H 2 release through a 20 mm hole, an explosive layer with an approximate volume of 40 m 3 volume is formed. For the smallest mass flow rate 0,2 g/s, see Figure11, hydrogen concentration in the richest layer never reaches the hydrogen Lower Limit of Inflammability. Therefore, no explosive mixture is formed for this smallest release flow.

Figure 11 :

 11 Figure 11 : variation of %H 2 for 0.2 g/s H 2 through a 5 mm hole

Figure 12 :

 12 Figure 12 : concentration profile (along a vertical located at x = 1.40 m and y = 0 m) for various releases after release phase

  s -10mm release diameter 0,7 g/s -20mm release diameter with the upper opening openProfile of concentration at 240 s for various subsonic releases point (cm)1g/s -20mm -53 m/s 1g/s -10mm -210 m/s 0,2 g/s -20mm -11 m/s 0,2 g/s -5mm -170 m/s mainly occurring in the lower part of the chamber with an estimated equivalent leak area between 90 and 170 cm 2 .

Figure 14 :Figure 15 :

 1415 Figure 14 : view of helium release for a 1,84 g/s flow rate through a 20 mm hole during the convection phase

Figure 16 :

 16 Figure 16 : variation of %He for 1,84 g/s He release through a 20 mm hole

Table 1 :

 1 Sensors and openings location

		X (cm)	Y (cm)	Z (cm)
	Chamber size	378	720	288
	Source location	0	0	0
	Opening 1 centre location	180	-380	273
	Opening 2 centre location	7,5	-380	7,5
	Opening 3 centre location	-7,5	-380	7,5
	Sensor 1 location	0	0	283
	Sensor 4 location	40	0	283
	Sensor 5 location	90	0	283
	Sensor 6 location	140	0	283
	Sensor 7 location	190	0	283
	Sensor 8 location	140	0	268
	Sensor 9 location	140	0	238
	Sensor 10 location	140	0	188
	Sensor 11 location	140	0	138
	Sensor 12 location	140	0	88
	Sensor 13 location	0	0	268
	Sensor 14 location	0	0	238
	Sensor 16 location	0	0	138
	2.5 Temperature

Temperature was measured in the mixing tank (before the injector), at the outlet orifice and at the chamber centre (ambient temperature). This data was used to calibrate the mass flow rate at the injection.

Table 2 :

 2 Test cases for hydrogen releases

	leaking	Flow rate	Release	Release	Exit	Release	Opening status	Observation
	gas	(g/s)	diameter	direction	velocit	duration	(open/closed)	Time (s)
	temperat		(mm)		y (m/s)	(s)
	ure
			0,7	20		38	240	closed	7200
			0,7	20		38	240	open	7200
	10	°C	1 1	20 10	Vertical	53 210	240 240	closed closed	7200 7200
			0,2	20		11	240	closed	7200
			0,2	5		170	240	closed	7200

Table 3 :

 3 Test cases for helium releases

	Flow	Release	Exit	Release	Opening
	rate	diameter	velocity	duration	status
	(g/s)	(mm)	(m/s)	(s)	(open/closed
					)
	1,84	20	55	240	closed
	1,84	10	215	240	closed
	0,4	5	190	240	closed

ACKNOWLEDGEMENTS

This experimental program has been undertaken with the financial support of the French Ministry of environment as well as the Hysafe Network of excellence. It took place in the framework of a scientific collaboration agreement between CEA and INERIS.


Y Guéguen
email: yann.gueguen@univ-mlv.fr

Al Heib

M De Michele

D Raucoules

Y Guise

DETECTION AND CHARACTERISATION OF RESIDUAL MINING SUBSIDENCE USING DINSAR AND PS INTERFEROMETRY: APPLICATION TO NORD/PAS-DE-CALAIS COALBASIN (NORTHERN FRANCE)

ABSTRACT

The extraction of underground resources induces subsidence on the surface. The soil deformations often create disorders in structures and infrastructures. In the case of coal mining, the major part of subsidence occurs during active mine, and the residual subsidence may cover a long period of about 5 years after the end of the mining works. The amplitude of this residual subsidence is usually very small compared to the total subsidence. "Charbonnages de France" is in charge of the monitoring of the area of Nord/Pas-de-Calais (France), known for its important mining resources at depth. So, despite the fact that the exploitation of mine in this area is completed since 1992, Charbonnages de France is still monitoring the surface with high precision levelling for surveying potential displacements evolution. This traditional method presents many disadvantages like expensive cost or low extent; the interferometry technique could present a serious help to quantify the residual subsidence.

In this study, both differential SAR Interferometry (DINSAR) and Persistent Scatterer Interferometry (PSI) are used to estimate deformation during a 12 years period (1992 to 2004) after the end of exploitation. 88 ERS scenes, distributed on two adjacent tracks, are processed, using DIAPASON software for DINSAR and GAMMA-IPTA for PSI. The area undergoes high temporal decorrelation due to the high amount of vegetation. The evaluation of atmospheric artefacts is also difficult in this area. Nevertheless, deformations are well detected: they present low amplitude with a maximum rate of only 1 cm/y during 5 to 7 years after the end of the exploitation.

The results obtained by interferometry are compared to traditional levelling measurements, and show a good agreement. A robust methodology can be developed to use interferometry for surveying soil above deep abandoned coalmines.

DESCRIPTION OF THE STUDY AREA

The north of France is known for its important mining resources exploited since 1720 to the end of 1990 on the Nord/Pas-de-Calais coal basin. It extends on 100 kilometres from east to west, from the Belgium boundary, and is only 15 kilometres width. The carboniferous coal deposits are trapped in between hercynian thrusts sheets buried below the cretaceous chalk 125 meters thick cover. Although the exploitation methods changed trough time, longwall with back filling and later with goaf caving method was generalized after the 1950's. The latter causes the most important deformations during the exploitation phase and a maximum of only 10% of the total deformation remains subsequently and constitutes the subsidence residual phase [START_REF] Louarti | Affaissement miniers et angle limite d'influence -Etude documentaire[END_REF][START_REF] Wojtkowiak | Bassins houillers français -Evaluation de la durée de répercussion des affaissements miniers après arrêt définitif des exploitations souterraines[END_REF][START_REF] Geg Piguet | Affaissements et déformations au-dessus des exploitations minières : Mécanismes et évolution dans temps[END_REF] (Fig. 1). The goal of this study is to use the interferometric methods to focus on the residual phase deformation between 1991 and 2004 within the framework of the post-mining survey under the auspices of Charbonnages de France. Indeed, this area presents important issues due to an important demographic density (2 millions inhabitants) in particular with the Lens and Valenciennes conurbations. In this paper, we will in first analyse the existing data, then we will present interferometric treatments and finally compare them with the traditional methods of levelling.

High precision levelling data

Since 1991, Charbonnages de France established five high precision levelling lines monitored by the French National Geographic Institute (IGN). Three zones were then monitored initially (Wallers/Arenberg, Billy-Montigny and Estevelles/Carvin/Courrières), which were later extended to Lens and Bruay-la-Buissière in 1998 (Fig. 2).

Precise measurements are realised each year in the beginning of October, except in 1999 where two campaigns have been carried out during spring and autumn in order to reveal seasonal effects. Two years (1992 and 1995) are missing. This precise levelling allows to identify different areas showing surface displacement and some depressions are clearly observed. Displacements reach four centimetres for all the points of the Billy-Montigny line in eight years (Fig. 3). Concerning the Estevelles/Carvin/Courrières line, some points with a six centimetres amplitude can be identified between 1992 and 1999, but only in the restricted area of the "Cité Saint Paul". Some points in surrection are also highlighted in particular for the line of Estevelles/Carvin/Courrières where the movements reach two centimetres for a period of seven years (92-99).

After 1999, the stabilisation of the majority of the levelling points of the three studied lines is observable. Only a few points of the "Cité Saint Paul" sector (Estevelles/Carvin/Courrières lines) still present subsidence of low amplitude (0.25 centimetres per year). Some points of the Lens and Billy-Montigny levelling lines present new displacement between 2002 and 2004 reaching an amplitude of the order of the centimeter over this period.

Exploited seams cartography

Charbonnages de France provided us a complete cartography of the exploited seams on the whole basin at 300 meters below the surface. This data base also contains the years of exploitation as well as the exploited thicknesses and thus makes it possible to locate the last exploited zones and to consider the awaited compressing rate. This step was carried out during previous work undertaken by INERIS [START_REF] Degas | Etude des affaissements résiduels -Application au bassin houiller du Nord et du Pas-de-Calais[END_REF] and allowed to draw up cartography of the zones which are most sensitive to residual subsidence.

DIFFERENTIAL AND PERSISTENT SCATERRERS INTERFEROMETRY

Since the beginning of the 1990's, the utility and the effectiveness of the radar interferometry treatments for the monitoring of the topographic surface movements, in particular in mining areas, were shown with many recoveries [START_REF] References Carnec | Three years of mining subsidence monitored by SAR interferometry, near Gardanne, France[END_REF][START_REF] Raucoules | ReSUM -Site OP5 : Nord-Pas de Calais -Interférométrie radar et confrontation avec le nivellement[END_REF]; Raucoules [START_REF] Raucoules | Monitoring of slow ground deformation by ERS radar interferometry on the Vauvert salt mine (France) -Comparaison with groundbased measurement[END_REF][START_REF] Raucoules | Monitoring of slow ground deformation by ERS radar interferometry on the Vauvert salt mine (France) -Comparaison with groundbased measurement[END_REF][START_REF] Colesanti | Detection of mining related ground instabilities using the permanent scatterers technique -a case study in the east of France[END_REF]. Two methods are used here for this study: differential interferometry radar (DInSAR) and Persistent Scatterer Interferometry (PSI), with CNES Diapason and GAMMA Remote Sensing IPTA softwares respectively. The satellite data used in this study are coming for ERS-1 and ERS-2: 88 scenes distributed on two frames are available with an approximately three months periodicity between 1992 and 2002. Unfortunately, due to the zero-gyro mode of ERS-2, interferometric processing could not be carried out after 1999.

DINSAR and PSI results

According to the last areas exploited, we focused this study on the center of the coal basin, around the important conurbation of Lens. In addition, this area presents the advantages to be monitored by three levelling lines. The major of studied area presented important temporal decorrelation difficulties. The presence of an important part of fields and forests as well as a low urban density leads to low coherence which limits the use of DInSAR. Nevertheless, the area of Lens provides a quite good coherence and concentrated our attention. We thus clearly could highlight three principal zones of movements localised on the towns of Courrières, Billy-Montigny and Lens. They show a depression phase between 1992 and 1996 with a deformation of about four centimetres over this period, corresponding to one centimetre per year velocity (Fig. 4).

After 1996, no interferometric fringes can be observed on the Courrières area. But we can't exclude that small movements remain with amplitude under the detection threshold of the method. On the other hand, both zones of Lens and Billy-Montigny still present movements between 1996 and 1999 (end of the exploited data). The subsidence rate of these areas, initially of about 1 centimetre per year, decreases after 1996 and reaches 0.5 centimetres per year. Thus, the total subsidence is then of about 6 centimetres over the whole 1992 to 1999 period.

The PSI method allows to confirm the results obtained by DInSAR and to identify more precisely the moving areas. Indeed, although the three zones quoted previously are again identified and present the same order of magnitude of deformations, a new area at the North of the Courrières city appears active. Located in the "Cité Saint-Paul", this one shows an average velocitiy of about 1 centimetre per year all along the considered time period (Fig. 5).

DISCUSSION

Methods comparison

DInSAR measurements are compared to precise levelling data provided by Charbonnages de France in areas where both are available. This could be carried out only for the zones of Courrières and Billy-Montigny, because the levelling of Lens becomes available only since 1999. They are in good agreement, and show same areas of displacements with the same order of amplitude. On Courrières area, DInSAR method provides clear results on the city and highlights a subsidence of about 1 centimetre per year. But, on the other hand, it does not provide any result for the "Cité Saint-Paul" due to a too low temporal coherence.

However, some PS pixels have been found in the "Cité Saint Paul", and can be compared with levelling points in a 150 metres neighbourhood. They present very good agreement and proved the effectiveness of this method in low temporal coherence areas (Fig. 6).

A. B.

Figure 4: Identification of subsiding areas with DINSAR. A.: 6623-25504 interferogram (21/10/1992DINSAR. A.: 6623-25504 interferogram (21/10/ -31/05/1996) highlighting three principal subsidence areas. B.: 8837-21362 interferogram (28/12/1996-22/05/1996) showing only two subsidence areas still active after 1996. Concerning the results for the Billy-Montigny area, we observed depressions of the same order of magnitude for the two methods allowing the validation of the interferometric results in this quite urbanised region.

Interferograms analysis

After unwrapping, vertical displacements maps are obtained. Spatial profiles, calculated on a 11 pixels large band allow to compare the different subsidence area shapes through time (Fig. 7). We can then highlight important differences between Lens and Billy-Montigny behaviour. Indeed, the Lens subsidence area shows some quite abrupt slopes at the difference of Billy-Montigny which looks smoother. These differences may be due to different exploitation types and extent. On the other hand, using numerous profiles allows to realize three-dimensional visualization. The latter is very interesting to clearly visualize the shapes and spatial extent of the subsidence areas and confirm the more smooth slopes of the Billy-Montigny area according to the Lens one.

A.

B.

Figure 7: Localisation (A) of the profiles (B) obtained from the unwrapping interferograms for two different time period (1992-1996 in blue; 1996-1999 in green)

CONCLUSION

The precise levelling method is traditionally used for low topographic surface movements monitoring. It already proved its ability to monitor residual phase deformation and provides very precise order of magnitude. However, this method is limited by its small spatial extent and its expensive cost. DInSAR and PS interferometry offer new possibilities as both gives access to displacements that are of the same orders of magnitude than levelling and allow to investigate areas of larger spatial extension. It is also possible to take benefit from the ERS archive, with images acquired since 1992. In addition, the description of a new moving zone by the PSI shows that this method is particularly effective in areas of low temporal correlation where DInSAR fails and is then complementary. The realisation of profiles, velocity and displacement maps has moreover provided a great help for the analysis of the observed phenomenon.

Figure 1 :

 1 Figure 1: The three different phases of the subsidence according to a considered point at the surface.

Figure 3 :

 3 Figure 3: Example of the Billy-Montigny levelling line. The stabilisation of the residual subsidence is clearly observed after 1999.

Figure 2 :

 2 Figure 2: Levelling monitoring of the Nord/Pas-de-Calais coalbasin.

Figure 5 :

 5 Figure 5: PS average subsidence rate between 1992 and 1999.

Figure 6 :

 6 Figure 6: Comparison between PS displacements and levelling monitoring in the "Cité Saint Paul" area for PS closer than 150m of the levelling point. A.: Localisation of the "Cité Saint Paul" area and considered data (red square). B.: Comparison of PS and levelling results where dashed lines represent the levelling points and continued lines, the PS.

Proc.'Envisat Symposium 2007', Montreux, Switzerland 23-27 April 2007 (ESA SP-636, July 2007)