Driad-Lebeau
email: lynda.driad-lebeau@ineris.fr

Peter

LATASTE Bourgeois 4 Bernard

GEOPHYSICAL DETECTION OF UNDERGROUND CAVITIES

ABSTRACT: In this paper, we present a synthesis of the geophysical investigations conducted on experimental sites selected in the Lorraine salt basin and Haute-Normandie marlpits. These investigations were carried out in the frame of a partnership (scientific and industrial) research program on cavities detection and characterization using techniques of high resolution seismic, microgravity and electric resistivity. The objective of the research is three folds: (1) to develop and optimize P and S seismic vibratory sources with an enhanced procedure of generation and optimization of sweeps (2) for each technique, to define the privileged application field and limitations regarding the general context of the considered site (3) to progress in the joint interpretations of the geophysical data. Despite the good quality of the data, the results evidence the complexity in the interpretation of the geophysical response in cavities environment due mainly to the compromise between the resolution and the ratio depth/dimension of the cavities, and the nature of the filling (brine, water, air). In the case of the marlpit mines, the corresponding geophysical anomalies could be correlated to the exact position of the marlpit known according to the archive records. Drilling campaign has confirmed a local collapse of the marlpit identified on the only High Resolution Seismic data. KEYWORDS: Cavities, detection, HRS, microgravity, resistivity, resolution.

RÉSUMÉ: Ce papier présente une synthèse des investigations géophysiques menées sur des sites expérimentaux du bassin salifère lorrain et les marnières de Haute-Normandie. Ces travaux ont été réalisés dans le cadre d'un programme de recherche partenarial axé sur la détection et la caractérisation des cavités à partir des techniques de Sismique Haute résolution (SHR), de la microgravimétrie et de la résistivité électrique. L'objectif du programme est triple : (1) développer et optimiser des sources vibratoires en ondes P et S avec une Procédure de Génération et d'Optimisation de Sweep adapté ; (2) définir pour chaque technique le champ d'application privilégié et les limitations en fonction du contexte général des sites considérés ;

(3) progresser dans les interprétations conjointes des données géophysiques. Malgré la bonne qualité des mesures, des difficultés ont été rencontrées lors des interprétations de la réponse géophysique de l'environnement des cavités avec, pour raison essentielle le compromis entre la résolution de l'imagerie et le rapport profondeur/dimension de la cavité ainsi que la nature du remplissage (saumure, air, eau). Dans le cas des marnières, les anomalies géophysiques ont été carrelées à la position exacte de marnière pilote bien documentée dans les archives. Une campagne de forage a permis de confirmer un éboulement local de la marnière observée sur les données de sismique haute résolution.

MOTS-CLEFS : Cavités, détection, SHR, microgravimétrie, résistivité, résolution

Introduction

The detection and characterisation of underground cavities represent an essential technical and scientific stake in the process and setting up risks prevention plans related to ground movements. Scientific research on geophysical techniques aiming to detect, locate and characterize the underground cavities at depths ranging between ten meters and a few hundred meters is still a matter of debate and remains more than ever of topicality. Indeed, for this range of depth, the majority of these techniques presents a lack of resolution and requires being adapted to investigate site and wanted target, namely delineation and the characterisation of the abandoned underground works and the overburden. Nevertheless, some of these techniques appear promising on a condition that progress is made to optimise and enhance the performance of the acquisition and interpretation tools such as they are implemented today. The use of the geophysical techniques, such as the High Resolution Seismic, the microgravity and the electric resistivity, for the detection of the underground cavities is not a new approach. During the last 20 years, published applications recall the potential of these techniques to detect and locate shallow cavities (depth < 100 m) in various geological and mining contexts (Piwakowski, 1991 ;[START_REF] Lagabrielle | La détection des cavités souterraines[END_REF][START_REF] Lagabrielle | Comparaison de quelques méthodes géophysiques de forage pour détecter les cavités souterraines[END_REF][START_REF] Piwakowski | Imagerie du proche sous sol par sismique haute résolution : Application à la détection des cavités[END_REF][START_REF] Watelet | Tests géophysiques de localisation de carrières souterraines abandonnées: Application au cas du massif de l'Hautil[END_REF][START_REF] Bishop | The detection of cavities using the microgravity technique: case histories from mining and karstic environments[END_REF]Driad and Piwakowski., 2002 ;[START_REF] Grandjean | Détection de carrières souterraines par sismique haute résolution à Annet-sur-Marne (France)[END_REF][START_REF] Baker | Geophysics as tool for delineating cavities -Case Study[END_REF][START_REF] Styles | Detection of Caves by Microgravity, Geophysics, Bahamas, inSinkholes and Subsidence[END_REF]. In the majority of the cases, these publications are matched examples of detected of known cavities, on which the effectiveness of the geophysical technique is tested. However, the problem remains complex taking into account, on the one hand, of heterogeneities and the geological stratifications of the crossed grounds and on the other hand, of the cavities target often under-dimensioned compared to the investigation depth. A shallow and voluminous spherical cavity (depth of the roof of the order of the radius) is a priori detectable with all geophysical techniques. However, under the real site conditions and beyond 20 m of depth, the effectiveness of these the measurements decreases notably because of heterogeneities of the loading medium whose geological noise limits the theoretical depth of investigation. The feedback of these various studies and research made it possible certainly to ensure, a scientific and technical progress in the field of the geophysical detection of the cavities, but also to emphasize the gaps as well as limitations which are always of topicality. The technique based on the high-resolution seismic reflection imaging has shown an interesting potential in detecting underground works in particular cavities. It is indeed the only geophysical method that makes it possible to provide a continuous image of the succession of the grounds and their structural fitting. The success of this technique rests on the performances of the acquisition, the processing and the analysis means available today. The HRS was particularly applied to the sounding of mining underground works (galleries, rooms and pillars...) and to the civil engineering structures (tunnels, foundations, drains.... Husband et al., 1998, Knapp et al., 1986a[START_REF] Miller | Applications of shallow, high resolution seismic reflection to various mining operations[END_REF]. Research and development works were carried out to improve the resolution of the HRS imaging and to increase the capacity to detect cavities which size is restricted compared to the depth of investigation. It was in particular shown that the "type" and the parameters of the seismic source constitute the principal factor of progress in this domain [START_REF] Piwakowski | From high resolution land seismic imaging to very high resolution: state of the art, limits and field examples[END_REF][START_REF] Swoboda | High resolution seismic surveying sweep techniques and the air sound source to detect near surface inhomogeneities[END_REF][START_REF] Portolano | Application of a new seismic source to shallow surface imaging[END_REF]. The scientific questioning related to the problems of detection of the underground cavities are thus located around two aspects: (1) is it possible to improve the seismic resolution to enable it a positive detection of sub-surface structures, in particular of the underground cavities? (2) What is the contribution of each geophysical technique (HRS, microgravity, Eletric resistivity) and what is the relevance of a joint analysis? In this research program, we have attempted to answer the mentioned questions through new HRS developments and experimental measurements in abandoned mines where the cavities are known. After a short note on the HRS developments, the paper focus on the geophysical imaging and the interpretations obtained from the HRS reflection, microgravity and electric resistivity measurements for cavities detection in salt and marlpit mines.

Development of HRS acquisition tools

Vibratory sources

It is commonly known that, to obtain a good resolution of the seismic image it requires the exploitation of waves the shortest possible, i.e. of the high frequencies. Taking into account the fact that the absorption of the medium increases with the frequency, the exploitable frequency is limited. According to universal principles, the optimisation of the frequency requires the use of powerful seismic sources and/or broadbands, able to generate very short impulses. The experience shows that each investigated site requires an individual choice of the type of source and its power. The requirements concerning the choice of the source are sometimes contradictory: (1) a broad band source must emit an impulse as short as possible; (2) a short impulse induces a weak energy which generally implies a weak signal to noise ratio; (3) in theory, the power of the source is limited by the local conditions (site) and by external factors (cost, noise, vibratory harmful effect, safety, possible damage of the site...) and cannot be increased without limits; (4) during the data processing, the stacking operation makes it possible to approach an equivalent power, but because of non repetitivity of the source, the summation is reduced to a low-pass filtering, which limits the exploitable frequency (high frequency). Vibratory source type used successfully in seismic investigations for oil exploration can bring a solution to the mentioned problems. Indeed, the development of mini-vibrators adapted to the specificity of the HRS proved to be necessary taking into account their limited number in Europe. Thus, within the framework of this program1 , three vibratory sources were developed within the laboratory "Electronic Acoustic" of the Central School of Lille (Fig. 1), scientific partner for the HRS investigations.

VibroPA VibroPP VibroS

Figure 1. P and S wave vibratory sources allowing acoustic (VibroPA) and mechanical (VibroPP) coupling

Procedure of Generation and Optimisation of Sweeps (PGOS)

The selection of the input signal is governed by the dependencies existing between the emitted vibratory signal and the parameters of the seismic records. Indeed, the input signal is fixed according to the quality of the seismic image obtained during the test phase and evaluated according to well-defined criteria's. The adopted solution was to develop ( 1 It should be noted that this software of the figure 2 is operational and applicable to the sources developed within the framework of this research program, but also on any P and S vibrator.

Geophysical investigation

Experimental sites

The selected sites belong to two different geological and mining contexts. (1) Salt mine exploited by dissolution between 1971 and 1993 in the Lorraine basin (North-east France).

2

The wells split into three tracks were drilled into the base of the first salt seam and connected together by hydro-fracturing. The salt cavities that resulted from the workings are located at depths of 110 to 180 m. In addition, the variety of their geometrical forms, the various geological setting and the nature of the infilling (brine, air) have given a particular interest to the selected site. Indeed, the cavities progress naturally through the marls until they reach a stiff and resistant layer called "Beaumont Dolomite"; as soon as this banc breaks, than the global collapse of the cavity occurs. Note that in December 2004, less than one year before the geophysical measurements, a major collapse (expected) has affected the site along the track3 at the cavities 50 and 51 (Figure 3). (2) The Marl-pit mines exploited during the last two centuries in the Haute-Normandie regions (North-West France) to extract "soft chalk" used to compensate for the natural acidity if the silt ground. The risk of collapse related to the voids left by the marl-pits affect, in a considerable way the entire region, which appears among the French areas most, affected. The major resulted cavities are situated at depth of 20 to 45 m covered by altered chalk, shale and silt (alluvium) near the surface. For the geophysical investigations, an experimental site (Eturqueraye, department of Eure) overlays a known marpit for which the exact position (on the surface and in depth) has been not revealed before the data analysis was completed.

2 Water injection and salt extraction wells

Detection of salt cavities

HRS analysis: The acquisition design and parameters were defined with respect to the general context of the investigated site and the targets (see Kosecki et al, 2008b for more details). The first step consisted in testing the new acquisition system composed by the vibratory sources and the related software. Reference vibratory sources in P and S waves have used for comparison on the test profile crossing the cavities 37, 38 and 39 along the track3 (Fig. 3). shows masking effects of the deep reflectors and signal perturbation of the salt roof interface (Fig. 5). The described seismic behaviour is observed mainly at the positions of the known salt cavities suggesting a positive detection. In the case of the cavity 37 whose sonar image is not available, it would be of low dimension, located in salt and embanked mud saturated with brine, it might be "acoustically" hidden, the detection is, thus, not possible. Note that majority of cavities are filled of brine, the masking effect is not highlighted accurately as it would be seen if the cavities were empty; the experience in data processing made it possible to come out with interpretable sections (Kosecki et al, 2008b).

Microgravity analysis: the measurements were carried out with Scintrex CG-5 allowing a resolution of 1 µGal and a standard deviation lower than 5 µGal (Styles, 2005). The design of measurements was optimised to detect cavities at depth greater than 100 m profiles on tracks 1, 2 and 3. In addition, a 3D-grid was applied above the cavity 41 (track1) believed to be instable and thus likely to induce variability in the gravity anomalies. The residual gravimetric anomalies, representative of local heterogeneities, were obtained after classical processing consisting in the different corrections (tide, drift, regional geology…). The residual map of the figure 6 highlights (1) a dominant negative residual anomaly in the collapsed zone (tracks 3) expressing the gravitational field induced by the crater (deficit of mass) following the collapse; (2) a negative anomaly at the Eastern border of the investigated zone that would act of a geological fault not indexed but identified on the electric resistivity map (see further); (3) a correlation (large wavelength) between a slight negative corridor of the residual anomalies and presence of cavities 36 to 45 along the main track. It should be noted that the correlation between presence of cavities and negative anomalies is in fact not striking. In addition, it is not easy to "individualise" the gravimetric anomalies in association with the isolated cavities; (4) just as on track 3, cavity 31 belonging to the track 1 is translated by a hardly negative anomaly with a light side shift. Electric resistivity analysis: the method known as "rectangle" was used to attempt the detection the salt cavities. It consists in injecting the current on a pair of fixed electrodes, relatively distant one from the other (bipole injection), and maping the distribution of the electric field which results from it inside a rectangle centered between these two electrodes. After transforming the measurements into apparent resistivities, an empirical topographic correction was applied to suppress the strong correlation observed between the measured apparent resistivity and the altitude given by a differential GPS. A map of apparent resistivity corrected for the topographic effect was thus obtained ((Fig. 7). Globally, we we identify two types of anomalies: (1) geological structures such as a "graben" and faults oriented according to the direction of regional fracturing. It is interesting to note that the major fault identified on the western side of the graben is prolonged to the collapsed zone. According to the owner of the mine, it is probable that this fault contributed to the evolution of cavities 50-51 towards surface until its collapse. In addition, the secondary fault detected in the East of the track 2, near cavity 45, is known from the owner, it probably had an impact on the evolution of this cavity which up to now under observation;

(2) anthropic structures including a large conducting zone which includes two parallel tracks of cavities and a resistive zone associated to the collapsed zone.

Joint geophysical analysis: the isolated cavity on the track 1 has been selected for the joint measurements. The cavity of 35 m X 10 m is located at the base of dolomite to 110 m of depth and would be filled with water unsaturated with salt. If one considers individually the geophysical images, none the techniques could detect the cavity in a convincing way. In addition to the nature of the infilling, we believe that the ratio between the dimension and the depth of the cavity is not optimal for geophysical detection.

Detection of marlpit

The geophysical surveys were conducted on selected experimental site (Eturqueraye) where the marlpit mine location was known. The information on the exact position of the mine was purposely "hidden" during the measurements and the data analysis.

HRS analysis: two perpendicular profiles have been designed and deployed on the only basis of depth range of 20-45 m the marlpit. Taking into account the unfavourable geological context (strong absorption due to the local geology) the data processing was confirmed very complex, the exploitable frequency is indeed particularly weak (Figure 8). However, it was possible to identify on the two profiles, an anomalous zone which corresponds to a relatively significant attenuation of the amplitudes in the first 20 meters of the basement. Although this zone is very attenuated, it seems that the waves crossed the disturbed medium. The HRS images do not evidence the presence of cavity, the observed anomaly could be related to collapsed zone in the first 20 m of the cover. Finally, the correlation to the marpit real position and the drilling data confirmed the seismic interpretation. Indeed, the marlpit was found collapsed at the drilling point on the first 23 m (depth of the marlpit).

The local conditions of the selected site did not enable the use of the vibratory source therefore the latter has been tested on another site located in Goderville (Haute-Normandie) where the marpit is well known and was subjected to HRS reflection measurements in 2002 [START_REF] Driad | Détection des marnières par Sismique Haute Résolution[END_REF]. The first HRS investigations were carried out using impulsive source providing an uncompleted section due to the inaccessibility of an inhabited zone (private houses). Thus the same profile as in 2002 has been measured with a non-destructive vibratory source allowing the access in private garden. The comparative results shown in figure 9, evidence a better seismic image with the vibratory source confirming undeniably the advantage in using a non-destructive source.

RS investigations were carried out using impulsive source providing an uncompleted section due to the inaccessibility of an inhabited zone (private houses). Thus the same profile as in 2002 has been measured with a non-destructive vibratory source allowing the access in private garden. The comparative results shown in figure 9, evidence a better seismic image with the vibratory source confirming undeniably the advantage in using a non-destructive source.

Microgravity analysis Microgravity analysis: the measurements were carried out following a grid of dimension 200 x 400 m with 10 m spacing in order to detect the marlpit situated at depth range of 20-40 m. The obtained map of residual gravity anomalies shows two negative anomalies of -25 µgal, located at the east and the west of the investigated zone (Fig. 10). These two anomalies are separated by a central positive anomaly, which reaches +22 µgal. The negative anomaly, located in the western part, is correlated with the presence of the marlpit. The drilling data made it possible to associate the negative anomaly to decompressed zone corresponding to the local collapse of the marlpit. This interpretation would not have been validated without the drilling information. The negative anomaly located more in the east does not correspond to a known cavity; it is associated to a locally decompressed zone identified by drilling. Finally, the positive anomaly located at the centre, would correspond to stronger density than the embanked rocks; related probably to a more significant concentration of flint in this zone. Indeed, the flint has a density of approximately 2600 kg/m3 whereas the density of soft chalk is of the order 2000 kg/m3. The accumulation of flint might also be connected to the exploitation of the marlpit, which could be rejected at this place by the owners. Electric resistivity: three profiles A, B, C were implemented, of which the profile C is located out of the influence zone of the marl-pit (reference profile). The analysis of the resistivity tomography for the three profiles is obtained from pole-pole and pole-dipole measurements; the latter device is generally the more used since it provides a good resolution. It should be noted that the expected anomaly related to the presence of the marl-pit should be very resistant; the contrast of resistivity is theoretically less significant when the cavity is stowed. The tomography model obtained after from data inversion of the three profiles is shown in the figure 11. It arises a resistant anomaly located directly above the marlpit with a slight side shift on profile A. It would have been simple to interpret this anomaly as being the effect of the marlpit if this same anomaly were not observed on the other profiles, in particular the reference profile C. The various approaches of modelling unfortunately did not succeed in bringing more precise details. Moreover, the differences observed between the profiles carried out in the marlpit zone and outside this zone are not sufficiently contrasted to distinguish without ambiguity the resistive anomaly (i.e. marlpit) in a relatively conductive medium. This observation probably reflects the limit of the technique in term of resolution in the context of marlpit mines all the more reason that the expected anomaly would represent a collapsed zone (i.e decompressed ground). Joint geophysical analysis: since the electric measurements were not conclusive, the joint analysis was limited to the superposition of the seismic and microgravity anomalies. The results evidenced a positive correlation of both seismic and residual gravity anomalies corresponding to the marlpit position (Fig. 12). This encouraging result can be regarded as a validation of the interpretation, but does not draw aside a control by drilling in order to associate the geophysical anomaly the target i.e. marl-pit.

Discussion and conclusions

This partnership research program on geophysical detection of underground cavities offered the opportunity of developing new acquisition system for High Resolution Seismic (HRS) investigations, and of testing joint measurements of HRS, microgravity and the electric resistivity in the context of salt mines of the Lorraine (North-east France) and the marlpits of Haute-Normandie (North-West France).

The experimental site of salt mine, where the cavities were known, was the object of many seismic tests to validate the newly developed acquisition system (vibratory sources in wave P and S, PGOS). The results obtained from exhaustive and comparative analyses revealed the interest of the developments carried out and confirmed the contribution of the adopted technical solutions (electronic, mechanical, algorithms...) and methodology for obtaining a better resolution of HRS imaging. The tests source showed that the VibroPP vibrator can have a potential similar to that of an explosive source which gets objectively the best resolution, but must be used (VibroPP) in an optimal way in order to exploit all the advantages, of which the non-destructive character. It is in particular a question of ensuring an acceptable and repetitive coupling and of carrying out preliminary tests in order to select the source appropriate to the site conditions and target. Although the vibratory sources at the stage "prototype" were developed for research, it was shown that the detection of salt cavities at depth greater than 100m is possible by analysing the masking effects of deeper reflectors. Taking into account the nature of the infilling (brine), the interpretation of the HRS sections was not simple. Indeed, the masking effect is less marked than if the cavities were empty (air). Note that the lack of resolution to great depth did not make it possible to precise the position of cavities. In addition, the HRS highlighted anomalies related to the collapsed zone (not shown in this paper) and geological faults observed on resistivity and microgravity images.

The microgravimetric detection of the salt cavities turned out less effective taking into account the depth of investigation (110-180 m) compared to the dimensions of the cavities. Moreover, the fact the cavities are mainly filled with brine, the density contrast is smaller than if the cavities were empty (air); thus, the lack of mass is not sufficient for an accurate detection. The electrical measurement made it possible to obtain a complete and interesting image of the global structure in particular geological faults, but the precise and individual detection of the cavities was not possible. Indeed, the limitation is due essentially to the compromise between dimensions of the cavities and the depth of investigation, which is not optimal.

In the context of the marlpits , the geophysical measurements were carried out on a marl-pit whose exact position was revealed only after the data analyses. The HRS and microgravimetric imaging highlighted both individually and jointly anomalies associated with the presence of the marlpit. The observations were validated by drilling which identified decompressed zone up to 24 m depth corresponding to a partial collapse of the marlpit. In addition, complementary HRS measurements on the Goderville marlpit using the new acquisition system enabled an accurate detection since the non-destructive character of the vibratory source made it possible to access inhabited zone and implement a longer profile. The detection attempt of the marlpit by resistivity tomography was not conclusive; this technique indeed came up against its limits of resolution in this geological and mining context.

Finally, through this research program we could evidence the limitations and the potential of the investigated geophysical techniques but also the necessity to progress in new developments and increase the performance in acquisition, processing and analysis. In comparison to the past investigations, it has been shown that the research work carried out on the HRS acquisition system contributed strongly to the detection underground cavities in the condition of the selected sites. Further more, other geological and mining context should be considered to come out with a complete methodology applied to the geophysical detection of underground cavities.

  ) a Procedure of Generation and Optimisation of Sweep (PGOS), an algorithm allowing an optimal selection of the input signal (or a series of signals) in comparison with the objectives of the measurements; (2) a program package baptized "SABAL" shown in the figure2, a complete solution integrating the PGOS, the piloting system of the vibrator and a parametric analysis, in particular the design of the measurements. The theoretical basis and the procedures relating to the design and the development of the PGOS are inspired by work of[START_REF] Brouwer | The quantification and monitoring of data quality and the design of optimum sweeps[END_REF][START_REF] Brouwer | Shallow High-Resolution Reflection Seismics[END_REF]

Figure 2 .

 2 Figure 2. Principle (top) and the front face of the program SABAL including the PGOS and the parametric analysis

Figure 3 .

 3 Figure 3. Experimental site selected in an old salt mine (North-East France). Sonar imaging of the salt cavities, situation of the collapsed zone on the track 3(left) and the geological setting (right)

Figure 4 .

 4 Figure 4. Marlpit mines in the Haute-Normandie (North-West France). (a) Example of collapsed marlpit in private garden, (b) vertical section of typical marlpitThe results of the comparative analysis of the seismic profiles obtained from 6 sources (impulsive and vibratory sources) revealed the performance of each source and made it possible to identify the factors limiting the resolution of the seismic image. The combination of the two operations (tests source, comparison section SHR) showed how the characteristics of a source influence the quality of a seismic image(Kosecki et al, 2008a). Furthermore, three HRS reflection profiles have been carried out along each track. The longest profile overlaying the major cavities was done using the VibroPA source along the track 3 up to the position of the cavity 42. The resulted time section shows masking effects of the deep reflectors and signal perturbation of the salt roof interface (Fig.5). The described seismic behaviour is observed mainly at the positions of the known salt cavities suggesting a positive detection. In the case of the cavity 37 whose sonar image is not available, it would be of low dimension, located in salt and embanked mud saturated with brine, it might be "acoustically" hidden, the detection is, thus, not possible. Note that majority of cavities are filled of brine, the masking effect is not highlighted accurately as it would be seen if the cavities were empty; the experience in data processing made it possible to come out with interpretable sections(Kosecki et al, 2008b).

Figure 5 .FaultFigure 6 .

 56 Figure 5. Interpreted High Resolution Seismic reflection time section obtained for the long profile of the track 3.

Figure 7 .

 7 Figure 7. Apparent resistivity map obtained for the salt mine

Figure 8 .Figure 9 .Figure 10 .

 8910 Figure 8. Interpreted HRS reflection time section obtained for one profile crossing the marpit. The position of the marlpit and the seismic profiles and anomalies (top) seismic section for depth ranges of 10-20 m and 40-60

Figure 11 .

 11 Figure 11. Tomography of the electric resistivity obtained for panels A, B and C (reference profile) using acquisition device pole-dipole.

Figure 12 .

 12 Figure 12. Correlation of seismic (in brown) and microgravity (in blue) anomalies above the marlpit. The western isolated microgravity anomaly corresponds to a decompressed zone identified by drilling.

The research work was carried out within the Ph.D. thesis of Arkadiusz Kosecki with the close collaboration of the Central School of Lille (North France)


Gloria Senfaute
email: gloria.senfaute@ineris.fr

Wassermann Jérôme
email: wassermann_jerome@yahoo.fr

Homand Françoise
email: francoise.homand@ensg.inpl-nancy.fr

INDUCED MICRO-SEISMICITY AND MECHANICAL RESPONSE DURING THE EXPERIMENTAL FLOODING OF AN IRON ORE MINE

Keywords: micro-sismicité, ennoyage, exploitation minière, effondrement, niveau d'eau

ABSTRACT: In order to understand the impact of flooding on mining collapses and its relationship with the micro-seismic activity, a controlled experiment of cavity flooding was performed in an underground iron mine. 134 micro-seismic events (magnitude < 0) were recorded during the experimental flooding. The micro-seismic activity showed a correlation with the water injection evolution in the cavity suggesting that micro-seismic activity was triggered by the stress modification induced by water introduced in the cavity. Two main processes are suggested: a) micro-seismic events (class 1) forming a group of perfect multiplet families and a frequency spectrum higher than 200 Hz; these events were triggered by the reactivation of pre-existing fractures; b) micro-seismic events (class 2) not forming a group of multiplets families with a frequency spectrum lower than 200 Hz; these events would be the result of complex interactions between the increase in elastic stress and the cumulative effects of increased pore pressure. The largest seismic events were triggered only during the water rise in the cavity suggesting that the water rise phase is a particularly sensitive trigger for the largest mechanical instabilities. KEYWORDS: micro-seismicity, flooding, minning, collapses, water level RÉSUMÉ: Une expérimentation d'ennoyage contrôlé a été mise en oeuvre dans une mine de fer souterraine afin d'étudier l'impact de l'ennoyage sur les instabilités minières et les corrélations avec l'activité micro-sismique. 134 évènements micro-sismiques (magnitude < 0) ont été enregistrés lors de l'ennoyage du site expérimental. L'analyse de l'activité micro-sismique montre une corrélation significative avec l'évolution du niveau d'eau dans la cavité et suggère que cette activité a été induite par les changements de contrainte générés par l'ennoyage de la cavité. Deux processus principaux sont proposés : a) évènements micro-sismiques (classe 1) regroupés en famille de multiplet avec des fréquences supérieures à 200 Hz; ces événements ont été induits par la réactivation des fractures préexistantes; b) événements micro-sismiques (classe 2) qui ne sont pas regroupés en famille de multiplet avec des fréquences inférieures à 200 Hz; ces événements semblent être déclenchés par les interactions complexes entre l'augmentation des contraintes élastiques et l'effet cumulé de l'augmentation de la pression des pores. Les événements microsismiques les plus importants ont été induits uniquement pendant la montée de l'eau dans la cavité suggérant que cette phase transitoire de montée de l'eau serait la plus sensible aux instabilités mécaniques.

Introduction

A triggered seismicity occurs when the change in the stress state by an external factor is sufficient to cause failure. This can occur either due to an increase in the stress induced by field driving the fault or due to a decrease of the shear strength of the fault. [START_REF] Fletcher | Earthquakes related to Hydraulic Mining and Natural Seismic Activity in Western New York State[END_REF] show that an increase in pore pressure of 1 to 5 MPa may be enough to trigger seismic events of magnitude -1 to 1. Simpson and Negamatullaev (1981) found that a change in water level only a few meters (of the order of 0.01 MPa) is enough to trigger seismicity. The seismic activity in this particular case is associated with the filling of a number of large water reservoirs. Two different stress modifications have been suggested as the dominant mechanisms responsible for seismic activity during the filling of large reservoirs: 1) the direct effect of loading, through increased elastic shear stress; 2) the effect of increased pore pressure, through decreased effective normal stress (Simpson 1988).

During the mining activities in the underground iron mines in Lorraine (East of France) over a period of 100 years, the ground water table was lowered due to the mine drainage. At the end of the mining, the drainage pumps were stopped and the excavation began to be flooded. Some mining collapses occurred during this transient stage of flooding. In order to understand the possible impact of flooding on mine collapses, several experiments were performed in the framework of GISOS (Research Group on the Impact and Safety of Underground Workings) research program. An experimental site was selected in the iron mines of the Lorraine basin (Tressange mine) and a controlled experiment was carried out between March 2002 and July 2003 for simulating the flooding of an abandoned mine. The Tressange site was isolated with watertight barriers and the cavity was filled with water for a period of 17 months. The aim was to study the impact of flooding on the mechanisms involved in mine instabilities and the relationship with micro-seismic activity. This paper presents the controlled field experiment simulating a mining cavity flooding, the microseismic activity triggered during the cavity flooding and its correlation with the water level and the associated mechanical instabilities.

Geological setting and description of experimental site

The experimental site is located in the Tressange mine which belongs to the iron ore basin of France. This iron basin is almost 100 km long and 40 km wide, from Luxembourg to Nancy (France). The iron ore formation has 60 m thickness and in the deepest iron ore layers is 250 m deep. The mining method was room and pillar under Tressange town. In this area three ironbearing layers were mined: the brown-iron layer (the deepest layer at about 225 m), the grey-iron layer, and the yellow-iron layer. These three mined iron layers are separated by inter-seam rocks (clay, limestone) and by superimposed pillars of 18 m width and 28 m length. The flooded level of the field experiment was in the brown-iron layer where galleries with around two pillars (G and F) were entirely flooded. The areas around ten other pillars were partially flooded (figure 1a). The cavity was isolated with ten tight concrete barriers built in the galleries and delimiting the flooded area.

The local micro-seismic network

A micro-seismic network composed of three 3D stations was installed in the inter-seam rocks between the flooded brown-iron and grey-iron layers. Each seismic station contains geophones with a frequency-band between 40 Hz and 1.5 kHz cemented at a depth of 5 m in vertical boreholes drilled from the galleries at the bottom of the grey layer (figure 1b). The geometry of the seismic network allowed monitoring of the flooded pillars and the inter-seam rocks. Date acquisition was carried out using a computer at a sampling rate of 10 kHz and a resolution of 16 bits. The sensors were connected to the acquisition system and the complete network was tested using hammer blows. • seismic events triggered during the flooding site. 134 micro-seismic events were recorded from which the seismic signature is completely different from those associated with blasting or earthquakes. Indeed, these events are of short period (a few hundredth seconds) with a large frequency band, between 50 Hz and 1.5 kHz that corresponds the highest frequency of the geophones (figure 2c). In the following, the analysis of these 134 micro-seismic events recorded during the experiment of cavity flooding is presented.

Micro-seismic activity and water level

The correlation between 134 micro-seismic events recorded during the experimental flooding and the water level could be divided in to mainly four phases (figure 3):

-Phase 1: before flooding (~15 days of monitoring). Only one seismic event was recorded.

-Phase 2: during flooding (~15 months). 45 micro-seismic events were recorded with three particular periods: period 1 : after beginning of the water injection, the micro-seismic activity started when the water level in the cavity reached 2.5 m high (the cavity height is 4.5 m), the micro-seismic activity increases continuously with flooding. Period 2 : the cavity is flooded but the water level is not yet completely stable and the water level is still raising progressively before completely stabilising; in this period, the micro-seismic activity is lower than in period 1. Period 3: the cavity is completely flooded, the water level is constant and kept at its maximum level for about 10 months, the micro-seismic activity continues to increase (see figure 3). -Phase 3: during drainage (~40 days). The micro-seismic activity is characterised by a strong increase and 88 micro-seismic events were recorded. -Phase 4: When the cavity is completely drained (~30 days of monitoring). No micro-seismic event was recorded during this period. The small number of seismic stations and the unfortunate malfunctioning of the horizontal sensors make it impossible to locate the seismic events. Thus, we propose a spatial classification based on the signals only recorded by the vertical sensors for which the arrival times are also taken into account. Two classes of seismic events were identified :

• Class 1: seismic events recorded only by one station. 107 events recorded only by the station GL and 2 events recorded only by the station GB. • Class 2: 25 seismic events recorded by all the sensors (GL, GB, GE). The signal characteristics of these two classes are also completely different. Approximately 95% of the events recorded only by one sensor (class 1) show a frequency spectrum higher than 200 Hz and around 99% of the events recorded by all stations (class 2) are associated with a frequency spectrum lower than 200 Hz (see figure 4). The signature was evaluated using the doublet technique or multiplet selection applied to all the seismic events. The aim was to identify seismic event families based on the assumption that each family belongs to the same failure mechanism. We define multiplets as a set, and a doublet as a pair with a similar appearance. The objective is to characterize the degree of similarity of a pair of events using the modulus of the coherency spectrum, which is the smoothed cross-spectrum normalized by the smoothed autospectra of each windowed seismogram [START_REF] Got | Deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of Kilauea[END_REF]. The smoothing function of the spectral densities is given by Fourier Transform. The multiplet selection showed that the events of class 1 (signals recorded only by the station GL) clearly form a group of two multiplet families: family 1 belongs to the seismic events recorded only during the cavity flooding (Figure 5a) and family 2 belongs to 86 events recorded during the cavity draining which 81 events form a group of perfect multiplet (figure 5b). Micro-seismic events of class 2 do not form a group in a multiplet family.

Seismic energy released and mechanical measurements

The estimation of seismic energy released by each seismic event was determined using the following equation:

(

1) t V E N i i z Δ = ∑ 2
Where E z is the estimated seismic energy recorded by each vertical sensor, V i is the amplitude in m/s for each pulse duration (Δt ) and N the total point numbers. The calculation was performed for each vertical geophone recording a signal. The energy of the seismic events was set as the average energy calculated for each sensor. For the evaluation of the size of seismic events recorded during the experimental flooding, we propose an approximate comparison with the events induced by underground mining; the magnitude is calculated using a classical and empirical correlation between seismic energy and local magnitude [START_REF] Gibowics | Magnitude and energy of subterranean shocks in Upper-Silesia[END_REF]:

M L = a log (E) + b (2)
where E is the energy; a and b are the coefficients fitting the local magnitude (M L ) determined from the national or international seismic network. Figure 6 shows the correlation between seismic energy and magnitude proposed by several authors for evaluating the local magnitude of mining induced seismicity [START_REF] Senfaute | Microseismic monitoring of underground coal mining at the Provence Coal Mines[END_REF], Tobyas and Mittag 1992[START_REF] Gibowics | Magnitude and energy of subterranean shocks in Upper-Silesia[END_REF]. The extrapolation of these correlations towards smaller events suggests that the magnitude range of seismic events recorded during the cavity flooding would be in range of -2 to -3 (figure 6). This range could be compared with the magnitude (about -2.5) of micro-seismic events induced by flooding that occurred at the underground Ikuno mine in Japan [START_REF] Ogasawara | Micoseismicity Induced by Heavy Rainfall Around Flooded Vertical Ore Veins[END_REF]. In addition to the geophysical monitoring, mechanical behavior of the cavity was by measuring the gallery convergence during the cavity flooding. Displacement sensors (LVDT type with precision of 0.02 mm) were installed between the floor and the roof of the cavity for measuring the evolution of the vertical displacements. Figure 7 shows the seismic energy released by each seismic events, the correlation between the cumulated seismic energy of each class of events, the displacements of the gallery wall and the water level measured using electromagnetic sensors. The evolution of seismic energy released during the cavity flooding shows that the largest seismic events were recorded during the period when the water level was increasing in the cavity. The largest vertical displacements of the gallery convergence were also recorded during the same period. When the cavity was completely filled and the water level remained constant the released seismic energy was smaller than that of the last period. During drainage of the cavity, a significant number of seismic events of class 1 (recorded only by sensor GL) were recorded with a seismic energy similar to that of the events triggered during the period of constant water level.

Discussion

A significant but small seismic activity (magnitude < 0) was recorded while a controlled flooding experiment was performed in an underground iron ore mine cavity. The seismic activity increased progressively with the water level in the cavity and shows a clear correlation between water level and the triggered seismic activity. The seismic activity would be triggered by the stress change induced by water. Two processes might be responsible for the micro-seismicity triggered during the flooding:

-Seismic events of class 1 recorded only by one sensor (sensor GL). This seismic activity is interpreted to be associated with the reactivation of pre-existing cracks in the roof. Field observations have confirmed that the sensor GL was effectively located in an area with significant natural fractures. This class of events has been regrouped in perfect multiplet families: the family 1 which the events were recorded during the cavity flooding and the family 2 which events were recorded during the drainage phase. However these two multiplet families have also strong similarities with a relatively mono-spectral frequency spectrum and a main frequency peak at 200 Hz (see figure 5). The multiplet families indicate that this class of events belongs to the same failure mechanism and suggests that the reactivation of pre-existing fractures triggers the seismic events of class 1. These seismic events were most likely triggered by the fractures reactivated during the water injection in the cavity and during the rapid decrease of the water level. However the seismic activity disappeared when the cavity became completely empty. These results show that seismic events are induced by an elastic response to the cavity loading mainly affecting the sectors with pre-existing fractures. Simpson et al (1988) and [START_REF] Bell | Strength changes due to reservoir-induced pore pressure and stress and application to Lake Oroville[END_REF] concluded from the investigations concerning induced seismicity related to reservoirs that the dominant mechanism responsible for the triggered seismicity is the direct effect of loading through an increased elastic shear stress.

-The interpretation of the failure mechanism is more complex for seismic events of class 2. This population does not form a group of doublets or multiplets suggesting that they have not been associated with any particular pre-existing fracture. These seismic events were mainly triggered during the cavity flooding. No event was recorded prior to the site flooding and only one event was triggered during the cavity emptying. Following the research of Bell and Null (1978), Simpson (1986) on induced seismicity related to reservoirs, we can suggest that these events result from complex interactions between the elastic stress increase, the cumulative effects of an increased pore pressure, diffusion mechanisms and the initial stress state. An explanation of failure mechanisms is proposed by Grgic et al 2006 who applied numerical modelling to the experimental site and concluded that the liquid pressure at the walls, floors and roofs, generate transient tensile mean effective stresses at the roof and floor. Tensile stresses, localized on the cavity (roofs and walls) induced tensile failures which would be responsible for triggering micro-seismic activity.

The largest seismic events were triggered during the water rise in the cavity. These events were triggered before the water level reached the roof of the cavity and remained constant (figure 7b). These results suggest that the water rise phase is a particularly sensitive trigger for the largest seismic events. Displacement sensors used to measure the gallery roof-floor closure showed that the displacement increases only during the water rise in the cavity. These results confirm that the first water filling phase, before the water level reaches a steady-state is the most sensitive that may result in mechanical instability of the cavity. Similar results have been observed by [START_REF] Simpson | Two types of reservoirs-induced seismicity[END_REF] who studied two reservoirs sites: Monticello in USA and Manic 3 at Quebec. At both sites, the seismicity reached a peak near the time of first stage filling, and gradually decayed as the water level remained constant. The largest seismic events occurred before the water level had reached its maximum level.

Conclusions

A local seismic network installed in an abandoned iron ore mine cavity recorded significant seismic activity during a flooding experiment. The results show clear correlation between the water level and the seismic triggered activity suggesting that seismic activity has been triggered by the stress modification induced by the water filling the cavity. Two kinds of seismic activity have been recorded: a) seismic events regrouped in multiplet families which were triggered by an elastic response to the cavity loading affecting directly the sectors with pre-existing fractures; b) seismic events which are not regrouped in multiplets and probably result from complex interactions between an elastic stress increase, the cumulative effects of increased pore pressure and the initial stress field.

The largest seismic events were triggered during the water rise in the cavity and before the water level reached the roof of the cavity. When the cavity was completely filled, the water level was kept constant, and during the draining phase, the energy of seismic events was smaller than that of the events triggered during the water rise. These results suggest that the water rise phase corresponds to a sensitive stage regarding the mechanical stability of mines and cavities subjected to flooding.

Figure 1 :

 1 Figure 1 : Experimental site: a) Vertical section of three iron layers mined: yellow-iron layer, grey-iron layer and the flooded brown-iron layer. b) Local micro-seismic network installed in the inter seam rocks between the flooded browniron layer and the grey-iron layer (sensors GB, GE, GL)

Figure 2 :

 2 Figure 2: Example of seismic events recorded during the cavity flooding : a) seismic event associated with mining blasting; b) seismic event associated with natural earthquake; c) local seismic event induced by the cavity flooding. The example shows signals recorded by the vertical sensor GL.

Figure 3 :

 3 Figure 3: Correlation between micro-seismic events recorded during the experimental flooding and the water level. The experiment could be divided in to mainly four phases: I. before flooding; II. during flooding; III. during drainage; IV. the cavity was completely empty.

Figure 4 :

 4 Figure 4: Classification of seismic events in class 1 and class 2 following the frequency spectrum

Figure 5 :

 5 Figure 5: Examples of signals regrouped as a multiplet belong to micro-seismic events of class 1

Figure 6 :

 6 Figure 6 : Correlation between seismic energy and magnitude used by several authors for evaluating the local magnitude of mining induced seismicity and extrapolation towards smaller events recorded during the cavity flooding.

  Figure 7 : a) Seismic energy released by each seismic event; b) Cumulated seismic energy of class 1 and class 2 events c) the evolution of the water level into the cavity and the vertical displacements of the gallery .

Acknowledgements

This research was supported by the French Ministry in charge of Industry and the Lorraine region. It was carried out in the framework of a research programme of GISOS (Research Group on the Impact and Safety of Underground Workings, www.gisos.org). The authors gratefully thank them for their collaboration and financial support. They also thank Rory Moltimore, professor at the University of Brighton, Göran Bäckblom of Conrox AB and Mehdi Ghoreychi, director of the Soil and sub-soil department of INERIS for the critical review of the paper.


Cauvin Maxime
email: maxime.cauvin@mines.inpl-nancy.fr

Salmon Romuald
email: romuald.salmon@ineris.fr

Verdel Thierry
email: thierry.verdel@mines.inpl-nancy.fr

DEALING WITH UNCERTAINTIES IN THE CONTEXT OF POST MINING HAZARD EVALUATION

Keywords: Uncertainties, Risk analysis, Probabilities Incertitudes, Analyse de risque, Probabilités

Risk analyses related to a past mining activity are generally performed in a strong context in uncertainties. A PhD Thesis has been undertaken in 2004 in order to draw up solutions to take into account these uncertainties in the practice. The possibility of elaborating a more quantified evaluation of risk has also been discussed, and in particular the contribution that probabilistic methods may brought to an analysis. This paper summarizes the main results of the Thesis. It particularly highlights the benefits that certain tools that have been developed may have regarding the process of expertise.

Introduction

Closing down mining operations has not eliminated all the risks likely to affect surface land within the geographical limits of old underground workings. Damages (surface instabilities, surface flooding, gas emission, severe environmental impacts, etc.) may indeed develop, sometimes as soon as work has ceased but also much later.

In such a context where public safety and infrastructures are at stake, risk analysis procedures are generally undertaken in order to study, post and manage the underground issues. In a post mining context, and more widely in the large field of rock mechanics, those risk analyses strongly lay on the geotechnical expert and on his experience. This characteristic is not without raising some difficulties. As he studies natural objects that are often non homogeneous, fractured, discontinuous, and so on, and because science on the topic is unachieved, the expert has to work in a context of important doubt and uncertainty.

It appears today that methods and tools that had been developed by the geotechnical expert, suffer from not bringing answers that would really allow seizing this context of uncertainty. Only few indirect methods, as the use of safety margins or of partial safety factors, allow to integrate the doubt existing of the result of any risk analysis but they remain highly insufficient. The methods and tools developed by the geotechnical expert also suffer from not being able to lead to a real quantification of the hazards related to underground issues and of their probability of occurrence.

However, a probabilistic expression of risk would perfectly fit the current statutory context, in which demands of quantification and better posting of risk issues are numerous.

In such a context, a PhD Thesis began in 2004 at the LAEGO laboratory in collaboration with the INERIS institute and the Lorraine regional Council [START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF]. This paper will present several important results that have been obtained within the framework of this Thesis.

The notion of uncertainty in the context of the elaboration of a risk analysis

Before trying to deal with the uncertainty that must be faced in any risk analyses, or even to reduce it, the notion of uncertainty itself has first to be clearly introduced. This notion is defined in the framework of the Thesis as the vagueness existing about the result of a risk analysis and which is due to the vagueness about all the parameters intervening in the process of that analysis. The parameters being mentioned are identified to be the "global resources" of the study, the "expert" in charge of the analysis, the "models" used to represent the different parts of the system he studies and the "data" provided to the analysis. Those parameters are of very different natures. However, they own the particularity to be closely linked to each others. The definition of "uncertainty" that is proposed might appear very general. Such a choice is however perfectly justified. The real aim of the work is indeed to allow operational solutions to be given to concrete problems. There is also a clear will in that definition to go further than a linguistic and restricted debate. The given definition thus integrates the different concepts of vagueness, variability, randomness, imprecision, indecision, subjectivity, lack of knowledge, etc. that all can have important consequences on the result of an analysis.

Using the proposed definition, the notion of uncertainty has been divided in four classes (figure 1). The so-called established typology of uncertainty has been built in order to reach three objectives: 1) to post the various sources of uncertainty that may be faced by the expert in an analysis, 2) to adapt to the process of realisation of a risk analysis, and more especially to the strong links existing between the parameters such as previously described, and 3) to allow an operational integrationand whereas possible a reduction -of uncertainties into the study. [START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF] "Resources uncertainties" deal with the knowledge about both the general scientific context of the study and its local particularities. More especially, it concerns the existence of information about the processes being investigated and the objects being studied. "Expertise uncertainty" concerns all the choices, actions or decisions that can be made by the expert while realizing the study. It mainly relies on his particular experience as an individual, on his subjectivity and on the way he represents and interprets the information he has gathered. "Model uncertainty" is basically induced by the use of tools to represent the reality. Finally, "data uncertainty" represents both the natural variability existing on data, the lack of knowledge about their exact values and the difficulty to clearly evaluate them. It is indeed particularly linked with the notion of value of a parameter. Concrete illustrations of those different types of uncertainty may be found in the Thesis.

Once the uncertainties he can face in his daily work are classified using the drawn-up typology, the expert must try to integrate them into his analysis. A "toolbox" has therefore been provided in the Thesis in which he will find operational solutions to help him to take into account the different categories of uncertainty (table 1). Most of the methods detailed in that toolbox have been gathered from a literature survey and their advantages and drawbacks are discussed in the Thesis.

Index of existence

* take quantitatively into account the doubt about the existence of a source of danger * method elaborated in the framework of the Thesis [START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF] Some specific solutions have also been developed in the framework of the Thesis when lacks in the existing methods were identified. The index of existence in particular appears as a powerful tool that makes it possible to deal with the problem of uncertainty related to the existence of the underground workings (figure 2). It allows to indicate how confident the expert is about the fact that a source of danger really exists at a given place. This tool may perfectly complete risk analyses as they are currently performed as it would allow a better communication of the results with the Authorities or the population [START_REF] Cauvin | Représentation cartographique de la connaissance d'expert dans le cadre des Plans de Prévention des Risques Miniers[END_REF]. When a concrete integration of uncertainty into the analysis (via a quantification for example) is not possible, the confrontation can be an excellent way of taking it into account. A comparison between the results induced by such or such choice, between the use of such or such tool, can thus make it possible to illustrate the influence that certain uncertainties can have in the analyses.

However, in practice, the real choice of the methods used to take into account uncertainty in an analysis strongly relies on the characterization of the whole uncertainties that are present into the analysis. The close links between the various categories of the typology actually induce these important methodological choices. An efficient method to reduce a certain type of uncertainty will therefore not be very interesting with respect to the final result of the analysis if the other types are not also taken into account. As a consequence of that remark, a qualitative treatment of uncertainty will appear particularly adapted to studies where uncertainties in the resources play an important rule.

The use of probabilities in risk analyses

Probabilities appear as excellent tools to manage some categories of the established typology of uncertainties. Data uncertainty, for example, can be considered and incorporated into the studies in describing parameters as probabilistic distribution functions.

In the framework of the Thesis, an original and innovative methodology was developed in order to face uncertainties related to the choice and the use of models to evaluate values of different parameters. More precisely this method allows to integrate (1) the existence of several models to represent a single feature, (2) the fact that each model combines the input data in a certain manner and that it only allows to approach reality in an approximate way, (3) the vagueness of the parameters that calibrate models, and (4) the vagueness of input data that are integrated in models. This methodology thus appears as an efficient way to integrate some expert (1), model (2 and 3) and data (4) uncertainties. It basically consists in performing Monte Carlo simulations in which data as well as models are considered as random values. The practical use of that methodology makes it possible to identify the most important sources of uncertainty present in the study and thus to help the geotechnical expert to reduce them (figure 3).

Figure 3. Methodology for a probabilistic integration of different sources of uncertainties into a risk analysis [START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF] The use of probabilities in risk analyses must however be performed with a lot of vigilance. Probabilistic results may indeed be interpreted according to two different approaches. The frequentist approach considers probabilities as a limit of frequency. Its practical implication for the expert is that it makes it possible to indicate how frequent (over space or over time) a given phenomenon may be. On the contrary, the epistemic approach considers probabilities as a tool to express the lack of knowledge about a particular value. In that case, the scattering of the probability distribution function illustrates the part of the doubt existing on the parameter of concern.

In practice, each distribution function that allows characterizing the uncertainty existing on a parameter of a risk analysis generally integrates these two approaches. The scattering of the dimensions of pillars in a large mining panel can, for example, be explained partly by natural -here spatial -variability (the mining extraction process induced the fact that all pillars do not have exactly the same shape) and partly by lack of knowledge (data are not sufficient enough to evaluate precisely the modes of the probabilistic distribution of the dimensions of pillars).

Assumptions on the nature of the uncertainty of every parameter of the study are therefore prevailing towards the concrete meaning of the probabilistic result of an analysis. If spatial variability is preponderant in the problem, the interpretation of the result as a "spatial frequency" will appear to be possible. The likelihood that a hazard occurs at a given place could then be quantified. Figure 4 presents for example a map illustrating the frequency of sinkholes that may be expected at a given place above an old gypsum mine. This map has been obtained in combining Monte Carlo simulations to geometrical and mechanical models. The probabilistic values may be interpreted as: "we can expect that between n min and n max sinkholes will occur about every 100 crossroads in an homogeneously coloured area".

Figure 4. Probabilistic hazard map for sinkhole occurrences above an old gypsum mine [START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF] Even though the obtained results allow to improve the posting of a hazard in a spatial point of view, the integration of time in the analysis still appears to be difficult. The evaluation of the probability that an event occurs at a given location during a given period of time, that is the definition of "hazard", is therefore not yet possible.

Only few works exist in the scientific literature about the integration of time in the natural risk analyses. They can basically be divided in two groups. On the one hand, some works use database in order to forecast what may occur in the future from what occurred in the past (van Besien and Rockaway (1988), [START_REF] Carter | Some observations on the time dependency of collapse of surface crown pillars[END_REF]). On the other hand, some works focus on integrating empirically the time dimension in the mechanical models of failure, through for example the reduction of the strength properties of a rock body or of its geometrical dimensions [START_REF] Kemeny | The time-dependent reduction of sliding cohesion due to rock bridges along discontinuities: a fracture mechanics approach[END_REF], [START_REF] Van Der Merwe | Predicting coal life pillar in South Africa[END_REF]). Even if the general philosophy of those methods is different, they both remain closely linked to the quality of the database that allows either to build the temporal model or to calibrate the mechanical one.

The integration of the time component into the analyses is nevertheless very important as it would allow a better management of risk. A methodology to evaluate the temporal occurrence of surface collapse events in large undermined areas is proposed in the Thesis and is illustrated and discussed for the mechanism of sinkholes (figure 5). The methodology basically consists in (1) performing a literature survey in order to identify the most important parameters to take into account in the problem and ( 2) sketching a mathematical model to represent the main trend of the evolution. Back-analyses using collapse events inventories may then be realized in order to calibrate the model and to adapt it to specific undermined sites. Such a work has to be seen as a first step towards the integration of time in post mining hazard analyses. It constitutes a way to reduce "resources" uncertainties due to the lack of knowledge of the expert about the real evolution of rock failure over time. This work however raises a problem of "model" uncertainties as model parameters appear difficult to be calibrated. It thus highlights the necessity of systematically developing database of events that occurred in the past. That would allow, through back-analyses for example, to improve the global knowledge about the mechanisms at stake.

Main conclusions

The work undertaken in the Thesis and that has been illustrated on several cases of application has arisen the fact that probabilities can constitute a powerful tool allowing to take into account uncertainties that may be encountered in a technical study. Probabilities may thus provide a real help to an expert while his following of to the process of realization of a risk analysis. They can accompany him throughout his work in insisting on particularly problematic points and, if necessary, in bringing solutions for the concrete integration of those difficulties. Moreover their adaptation to the currents methods of analysis seems possible and relatively easy to be put into practice.

Besides being a powerful expertise-aid tool, the integration of uncertainties and the use of probabilities in the studies also constitute a solution for a better risk management. The geotechnical expert has indeed the vocation of transmitting the results of his risk assessment to decision makers or to the population being concerned. The communication between the various actors involved in risk management therefore appears as a key for a good analysis. Results have to be understandable and exploitable by people whose technical and scientific culture is sometimes very different. While taking into account uncertainties in his study, the geotechnical expert may then highlight his own limits and the lack of resources he has, but he may also insist on the efforts he undertakes to reduce them. That "popularisation" of the expertise process and of the difficulties that an expert may face then constitutes a first step towards a real dialogue between the various partners of risk management. The decision following the risk assessment will thus be in that case really concerted.

An interesting prospective of the Thesis would be then to make a study in which various actors (expert, the authorities, population concerned, insurance companies, etc) would be integrated from the beginning of the work and would have presented their real expectations so that a concerted answer is obtained.

Figure 1 .

 1 Figure1. Typology of uncertainty[START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF]

Figure 2 .

 2 Figure 2. The index of existence as an excellent complement of hazard maps

Figure 5 .

 5 Figure 5. Adjustment of a general model for assessing the evolution of sinkhole occurrences over time to in site data

Table 1 .

 1 Several methods to deal with uncertainty, depending on the type of uncertainty

	Type of uncertainty		Strategy and remarks	References and examples
			* deal with the problem of assessing the exact	Dejean (1981)
		Use of safety margins	value or localisation of a parameter	Favre et al. (1998)
	Data	Use of probability functions	* qualitative method * deal with the problem of assessing the exact value or localisation of a parameter * quantitative method	Tritsch (2004) Kim and Gao (1995) Duncan (1999) Favre (2004)
		Comparison between model outputs and reality	* deal with problems related with vagueness and reliability of models	Azzouz et al. (1983)
			* deal globally with problems related with
		Use of safety margins	vagueness of models and data	Favre et al. (1998)
	Model		* qualitative method
			* deal with problems related with vagueness
		Integration of various models into the analysis	and reliability of models * method elaborated in the framework of the	Cauvin (2007)
			Thesis
		Comparison between outputs of various models	* allow a posting of uncertainty	Husein Malkawi et al. (2000)
	Expertise	Use of a decision-aid approach Use of countervaluation	* allow to homogenise the expertise process when a single expert realises the analysis * limit the subjectivity that may be induced	Merad (2003) GEODERIS (2003)
		Use of technical methodological guides	* help expert in providing a wider overview of methods	INERIS (2004)
			* take into account the doubt about the
		'as if' strategy	existence of a source of danger
			* implicit safety margins
	Resources


Isabelle Contrucci
email: isabelle.contrucci@ineris.fr

Emmanuelle Klein
email: emmanuelle.klein@ineris.fr

Pascal Bigarre
email: pascal.bigarre@ineris.fr.

Armand Lizeur

Anthony Lomax

Klein Emmanuelle

Bigarre Pascal

Lizeur Armand

Lomax Anthony
email: anthony@alomax.net.

Tastet Jessica

CAMI-AFT: A SCIENTIFIC FIELD EXPERIMENT TO CALIBRATE REAL-TIME MONITORING SYSTEMS DEDICATED TO THE MANAGEMENT OF POST-MINING RISKS

Keywords: Microseismic monitoring, Post-mining risks Surveillance microsismique, Risques après-mine

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

INERIS has deployed more than 30 microseismic real-time monitoring networks covering the most hazardous areas of the Lorraine iron-basin region of Eastern France (Figure 1-a). In this region decades of iron ore mining have left extensive underground cavities beneath or in the vicinity of urban areas. To ensure public safety, INERIS has developed, tested and validated a microseismic monitoring technique as a volumetric method for detecting underground microseismic activity that may occur before a collapse [START_REF] Bennani | Surveillance des risques d'effondrement dans l'après-mine, besoins, méthodes : apports de la microsismique[END_REF][START_REF] Couffin | Permanent real time microseismic monitoring of abandoned mines for public safety, Field Measurements in Geomechanics[END_REF][START_REF] Senfaute | Qualification of the microseismic monitoring technique applied to the risk of collapse or iron mines[END_REF]. This monitoring method was deployed after the major collapse in 1996 at Auboué located in the Lorraine iron-basin region. Since then, no major collapse has occurred in any of the monitored areas. Though most of the monitored sites show little microseismic activity, small microseismic events are recorded and form a background, underground noise level. Usually recorded by single 3D probes, the associated seismograms show very low amplitude (~10 -6 mm/s), corresponding roughly to negative magnitudes for sources at the mine level. The small size of the events and recording on single probes causes difficulties for reliable automatic analysis, especially source location, a fundamental parameter for any appropriate analysis and understanding of stability analysis (Driad et al., 2005).

In this context a specific experiment, named CAMI-AFT, was initiated in 2005 for six instrumented zones. The experiment consisted of sequences of small blasts in underground mine pillars accurately controlled in terms of location, orientation and energy of the explosive source. This unique database of underground events was then used to calibrate procedures to estimate the 3D hypocenter location and the source energy. This paper describes the design of the field experiment and the main characteristics of the induced events, as well as the method used to process and analyse the recorded data. Preliminary scientific results are then presented and discussed.

Experiment design

The field experiment was performed between October 2005 and May 2006. A total of 70 blasts were shot, with explosive mass ranging from 0.5 to 12 kg (see Table 1 for details, Figure 1-a), leading to more than 1200 high quality seismograms.

Small isolated dynamite blasts were shot in mine pillars located at a depth ranging from 50 m down to 250 m depending on the site. Each site is monitored by microseismic stations (Figure 1-b), which are composed by a 1D surface probe, a 1D probe located at ~15 m depth, and a 3D probe located at ~ 50 m depth (Figure 2). To produce a relevant microseismic database, location, size, orientation and energy of the explosive sources had to satisfy mainly three requirements, which are : source locations must ensure the best possible coverage in terms of offset and azimuth and dip angles of the incident microseismic signals, taking into account constraints related to accessibility and safety.

the size and energy of the explosive sources were chosen, based on empirically predicted amplitudes, in order to optimise signal-to-noise ratio to get usable waveforms while avoiding saturation.

all stations of a same site had to be triggered. Specific local source patterns (for example: successive blasts of varying energy or orientation in a same pillar; blasts in upper and lower excavated layers in case of superposed layers etc.) were also designed to study source effects and test capabilities of numerical routines to discriminate and quantify them.

Site Number of blasts

Mass of explosives

Number of seismograms

Data analysis

The data analysis of the 3D seismograms strategy is the following : systematic analysis of all seismograms per site and microseismic station in terms of amplitude and energy is undertaken in order to assess data quality; polarization analysis to check consistency between measured incident angles and expected values. These values are of the utmost importance, given their role in source location when dealing with a very limited number of P or S-wave arrival times;

velocity model determination by minimizing errors on estimated P and S wave travel times using a global inversion; automatic location procedure, based on numerical inversion of P and S wave arrival times, incident angles and an appropriate velocity model, is tested for performance assessment.

empirical determination of a source energy, depending from the source-to-sensor distance and sensor energy. This work has already been presented in a previous paper [START_REF] Tastet | Large-scale field experiment to calibrate microseismic source parameters applied to real-time monitoring of postmining instabilities[END_REF]; for further details about the method, please refer to this paper.

Wave polarisation analysis

The polarisation analysis consists in the calculation of incident P-wave angles (azimuth and incident angles) giving the direction of propagation at the 3D sensor (Figure 4-a). The computation was performed assuming a homogeneous medium with straight ray paths between blast source and oriented 3D microseismic probes. This information, when obtained through careful polarization analysis of 3D seismograms, represents a unique means of constraining an ill-conditioned 3D localization problem due to the lack of P or S arrival times or other a priori information.

The polarization formulation currently used [START_REF] Dodo Amadou | Seismic and geotechnical investigations following a rockburst in a complex French mining district[END_REF][START_REF] Abdul-Wahed | Source location estimation using single station three-component seismic data[END_REF] was improved by introducing automatically calculated quality and uncertainty factors to assess 1D 1D surface 3D reliability of the numerical analysis, such as rectilinearity and polarization index functions [START_REF] Tarantola | Generalized Nonlinear Inverse Problems Solved using the Least Squares Criterion[END_REF][START_REF] Lomax | A reanalysis of the hypocentral location and related observations for the great 1906 California earthquake[END_REF].

For most of the stations, calculated and measured azimuth and incidence angles show a very good agreement. The average azimuth error is found to be less than ± 5° with no bias, except for subvertical ray paths for which sensitivity to azimuth errors is numerically expected. The average error on the measured incidence angles is found to have a bias of 7° deeper inclination, most likely due to the straight ray paths assumption. A multi-layered velocity model could probably correct this bias, at least partially.

The good quality of these results can be attributed to (1) all 3D microseismic probes are fully grouted 50 to 70 m deep avoiding most of the perturbations introduced on primary front waves by free surface effects or tube waves from boreholes;

(2) geological overburden, although clearly stratified sub-horizontally, remains globally homogeneous when considering seismic wave length and short distances of propagation.

Note that for a few 3D microseismic stations, calculated and measured azimuth angles showed an important bias most likely corresponding to an error of measurement during the grouting of the buried probe (Figure 4-b). In that case, the apparent bias may be introduced as a definitive correction factor.

Microseismic event First 3D sensor move

Optimisation of velocity models

A specific routine was developed in order to invert measured arrival times, through an inclined multi-layered velocity model, from well-known hypocenter coordinates to well-known station coordinates [START_REF] Contrucci | Le monitoring microsismique pour évaluer l'intégrité d'un stockage de CO 2[END_REF]. Monte-Carlo simulations were then performed, resulting in distribution of solutions per velocity class, for each layer (Figure 5).

For a numerical, multi-layered, inclined velocity model in such a sedimentary geological context is to improve the accuracy of hypocenter determination, requires comprehensive knowledge of the lithology. In the case of the CAMI-AFT field experiment, detailed geological description was largely available in many mining data files.

Conclusions

This experimentation contributes to quantifying the sensitivity of the monitoring systems as well as to improving and validating the processing tools. The results benefit from the referenced database composed of the 1200 "induced-blast" seismograms. The CAMI-AFT data processing allows to :

(1) validate the polarization analysis tool ;

(2) calculate P-waves velocity models of each site ; (3) validate the 3D localization approach ; (4) construct an empirical source energy law from the sensor energy for each experimental site (see [START_REF] Tastet | Large-scale field experiment to calibrate microseismic source parameters applied to real-time monitoring of postmining instabilities[END_REF] for details).

The final objective of this experiment is to improve the quality and reliability of the operational real-time monitoring of the French National Monitoring Centre for Ground and Underground Risks (CENARIS) of INERIS. The developed tools will improve and generalize the operational procedures for the characterisation of post-mining instabilities events. Furthermore, the developments will improve decision-making aid for crisis management.

  Figure 1: a) general location map of the CAMI-AFT experiment sites. b) mining map of the Tressange site. Grey spots represent the microseismic stations. Dark crosses represent the blasts.

Figure 2 :

 2 Figure 2: Seismograms of a blast recorded by a microseismic station of the Tressange network (amplitude in mm/s versus time in s). The 3D probe is the deepest one; the 1D surface probe is the shallowest one.

Figure 4

 4 Figure 4: a) stereographic representation of azimuth and incidence angles for one 3D probe. b) Plotting azimuth errors versus incidence angles for 4 microseismic stations, Gerbault station show a systematic bias of ~ 50° and Jardin station a systematic biais of ~-200°.

Figure 6 :

 6 Figure 6 : localization using an homogenous velocity model, example of the Tressange network.

Figure 7 :

 7 Figure 7: Localisation using a multi-layered velocity model, example of the Tressange network.

Table 1 :

 1

Basic parameters of CAMI-AFT experiment layout.

Table 3

 3

shows the best average values obtained for the Tressange site. Velocities are found to be consistent and in good agreement with other sites located in the same geological context. Although

Acknowledgments

Special thanks to the French Ministry of Economy, Finance and Industry for the financial support. We fully acknowledged E. Rovinalti (MECATRAMINE), A. Marnet (DRIRE Lorraine) and J-P. Josien (GEODERIS) for authorizations and their helpful contribution to the field experiment. We also thank M. Bennani and L. Driad-Lebeau (INERIS) for their helpful expertise in the field of data processing. We thank the ANR (Agence Nationale de la Recherche) who funded a part of this work.

the velocity model inversion has not been run on all sites, intermediate results obtained show that, low contrast is found between the different geological strata, except for the altered surface layer, which is poorly constraint numerically.

Source localization

Most of the microseismic events recorded by the Lorraine monitoring systems are usually detected by only one single 3D station. This configuration, one 3D station and only P-wave arrivals, is unfortunately inefficient to locate microseismic event, except if the microseismic event is supposed to come from the mine works, which is a strong hypothesis. To circumvent this problem, a new algorithm was developed, which integrate polarization parameters (incident and dip angle), in addition to the P and S waves arrival times. Tests were performed to check the robustness and the capacities of this algorithm to locate a microseismic event with a few numbers of stations. Multilayered velocity models were, as well, integrated into the location procedures (Figures 6 and7).

The available microseismic data for the blasts, where the locations are known, were used to test the precision of the new localization algorithm. An absolute localization was performed on the Tressange site, using a numerical homogenous velocity field and P-wave arrival times and the incident and dip angles, recorded by 4 stations at the same time. The results showed an estimated hypocentral error in XYZ is ~ 70 m (Figure 6). Using a multi-layered velocity model the location error is improved of about 20 % (Figure 7). Indeed, a multi-layered velocity model takes into account the real ray path, reducing localization errors.