Karën Fort
email: karen.fort@loria.fr

Aurélie Névéol
email: aurelie.neveol@lisn.upsaclay.fr

Yoann Dupont
email: yoann.dupont@sorbonne-universite.fr

Julien Bezanc
email: julien.bezancon@etu.sorbonne-universite.fr

Use of a Citizen Science Platform for the Creation of a Language Resource to Study Bias in Language Models for French: a case study

Keywords: citizen science, language resource development, bias fairness and social impact

There is a growing interest in the evaluation of bias, fairness and social impact of Natural Language Processing models and tools. However, little resources are available for this task in languages other than English. Translation of resources originally developed for English is a promising research direction. However, there is also a need for complementing translated resources by newly sourced resources in the original languages and social contexts studied. In order to collect a language resource for the study of biases in Language Models for French, we decided to resort to citizen science. We created three tasks on the LanguageARC citizen science platform to assist with the translation of an existing resource from English into French as well as the collection of complementary resources in native French. We successfully collected data for all three tasks from a total of 102 volunteer participants. Participants from different parts of the world contributed and we noted that although calls sent to mailing lists had a positive impact on participation, some participants pointed barriers to contributions due to the collection platform.

Introduction

There is a growing interest in the evaluation of bias, fairness and social impact of Natural Language Processing models and tools [START_REF] Blodgett | Language (technology) is power: A critical survey of "bias" in NLP[END_REF]. The resources developed for this task include curated word lists [START_REF] Caliskan | Semantics derived automatically from language corpora contain human-like biases[END_REF], sentences created from manually crafted templates [START_REF] Stanovsky | Evaluating gender bias in machine translation[END_REF], and corpus collected from language speakers either through social media [START_REF] Chiril | He said "who's gonna take care of your children when you are at ACL?": Reported sexist acts are not sexist[END_REF] or ad-hoc crowdsourcing [START_REF] Nangia | CrowS-pairs: A challenge dataset for measuring social biases in masked language models[END_REF]. However, little resources are available for this task in languages other than English. The translation of resources originally developed for English is a promising research direction, although it presents the risk of creating ungrammatical sentences for morphologically rich languages [START_REF] Zmigrod | Counterfactual data augmentation for mitigating gender stereotypes in languages with rich morphology[END_REF]. In addition, there is also a need for cultural adaptation to obtain corpora with equivalent validity in a language other than English [START_REF] Goldfarb-Tarrant | Intrinsic bias metrics do not correlate with application bias[END_REF]. We argue that there is a need for complementing translated resources by newly sourced resources in the original languages and social contexts studied. Citizen science platforms like LanguageARC [START_REF] Fiumara | LanguageARC: Developing language resources through citizen linguistics[END_REF] are especially well-suited for this purpose, as they allow native speakers to participate, while not presenting the ethical issues of microworking [START_REF] Fort | Amazon Mechanical Turk: Gold mine or coal mine?[END_REF].

Evaluating language models for biases

Large pre-trained language models such as masked language models or causal models are one source of the biases evidenced in Natural Language Processing systems [START_REF] Hovy | Five sources of bias in natural language processing[END_REF]. The use of stereotyped sentences targeting several social categories has been suggested to evaluate biases in masked language models for English and implemented in datasets such as CrowS-Pairs [START_REF] Nangia | CrowS-pairs: A challenge dataset for measuring social biases in masked language models[END_REF] and StereoSet [START_REF] Nadeem | Stere-oSet: Measuring stereotypical bias in pretrained language models[END_REF]. These datasets contain sentences with variations on social categories and attributes typically associated to the social categories with the goal of probing language models for contrasted behavior towards the social categories or attributes. A critical analysis of the datasets pointed out some limitations in the conceptualization and operationalization of stereotypes [START_REF] Blodgett | Stereotyping Norwegian salmon: An inventory of pitfalls in fairness benchmark datasets[END_REF].

The CrowS-Pairs dataset [START_REF] Nangia | CrowS-pairs: A challenge dataset for measuring social biases in masked language models[END_REF] aims to assess bias in language models using nine stereotype categories: age, disability, gender, nationality, physical appearance, race/color, religion, sexual orientation and socioeconomic status. It contains pairs of sentences: a sentence that is more stereotyping and another that is less stereotyping. Each pair is minimally distant: that is, only the words identifying the target group should change between the more stereotyping sentence and the less stereotyping sentence. For example: "Poor people never go to the dentist." is paired with "Rich people never go to the dentist." and annotated with the stereotype category socioeconomic status. Some of the operational limitations were addressed in a study seeking to collect a language resource for the study of biases in Language Models for French that would be comparable to the CrowS-Pairs dataset [START_REF] Névéol | French crows-pairs: Extending a challenge dataset for measuring social bias in masked language models to a language other than english[END_REF]. This study relied in part on the contributions of French native speakers to validate and analyze content translated from English and to supply native examples of stereotypes expressed in French and relevant to the social context in France. These contributions were collected through a LanguageARC Project, as described below.

The LanguageARC Project

We created the project "Les stéréotypes en franc ¸ais" (stereotypes in French) on the LanguageARC platform 1 with the help of the Linguistic Data Consortium (LDC). The description of the project and tasks on the platform is supplied in French to reflect that participation is targeted towards fluent French speakers. Instructions were kept minimal to reduce participant burden and leverage the linguistic intuition of participants. The project includes three tasks. Two tasks are related to the evaluation and correction of our translation and classification of the English sentences from the original CrowS-Pairs corpus, the third one consists in adding new sentences with stereotypes consistent with French culture .

Task 1 "On cause la France" (This

French enough?)

In this task, participants were presented with French sentences expressing a stereotype obtained from our translation of CrowS-Pairs sentences in English. Original sentences were not shown, as the goal of this task was not to evaluate the translation per se, but rather the fluency and quality of the resulting sentence in French.

Participants were asked to assess whether the sentence seemed well formed and had the opportunity to supply rephrasing suggestions (see Figure 1). were asked to select the bias categories that were relevant for characterizing the stereotype expressed in the sentence (see Figure 2).

Figure 2: Task 2 interface: "Among the following types, which one(s) correspond(s) to the one expressed in the sentence?".

As shown in Table 1 eleven options were available: the nine stereotype categories of the CrowS-pairs corpus as well as two additional categories, "other" (mainly political opinions) and "no stereotype".

3.3. Task 3 "Les hommes ne savent pas faire la vaisselle" (Men can't do the dishes)

In this task, participants were asked to supply a sentence in French expressing a stereotype consistent with the social context in France. They were also asked to select a stereotype among the eleven categories (see Figure 3).

Results

In this section, we present an analysis of the participation to the LanguageARC project "Les stéréotypes en franc ¸ais" from August 2021 to March 2022. The results

Bias

Participation

Table 2 presents the details of contributions submitted by the 102 unique participants, including the four task organizers. The first task attracted the largest number of participants (84), who generated over 2,000 annotations. The second task yielded the largest number of submissions, with almost 3,000 assessments produced by 60 participants. Finally, 47 people participated to the third task and added more than 300 sentences. We specifically outline the participation of task organizers (the authors of this paper) as we noticed it was imbalanced across tasks with both number and overall proportion of contributions increasing from task 1 to task 3. Numbers between brackets reflect contributions submitted by participants other than the task organizers.

As for the geographical origin of participants, unsurprisingly, most of them were based in France, especially around Paris, with patches of participation all over the country (see Subfigure 4b). This can be explained at least partly by the fact that we are located in Paris and that we advertised the task to our students and colleagues around us. Part of the North and South East participation (in Nancy and Grenoble) might also come from our own network. However, there were some con-tributions from other parts of France and even the world (England, Norway, United States and India). This goes far beyond our networks and shows that we managed to attract participants either thanks to the platform itself or through our advertisement on the different mailing lists of the domain. Figure 5 presents the progress of data collection over time. Subfigure 5a shows a peak of participation after the red lines, which is not present in subfigures 5b and 5c. This suggests that participants initially and massively contributed to task 1 and some of them returned to the project at a later time to contribute also to the other tasks. Considering the limited efforts we put in advertising the task on the platform (three calls on mailing lists and a couple of emails to students), we are quite happy with the results, both in terms of participation and of language data production.

Production Quality

We manually reviewed all the produced annotations and sentences. Out of the 307 proposed sentences in the third task, we kept 210. The major part of the removed propositions were strict or near duplicates. We also removed contributions for which we could not create an anti-stereotype equivalent with minimal modifications2 .

As for Task 1 and 2, we integrated the proposed modifications when relevant. They mainly concerned typos, grammatical errors and a couple of badly typed stereotypes. Figure 6 presents the coverage of the corpus by the number of annotators. The total coverage amounts to 70% for task 1 and 80% for task 2.

Discussion

Overall, the data collection experiment for the stereotypes project was positive: the participation level was high, collected data was useful and is now partly distributed in the French CrowS-Pairs release3 . In this section, we comment on aspects of the data collection where we identify potential for growth in the Lan-guageARC platform.

Limits of the Participation

Unsurprisingly, the contributions of the project authors were more substantial than the average level of contribution of participants. The participation of task organizers was rather low in task 1 and 2 (under 3% of contributions), which are the task with the most participation overall. The participation of task organizers was rather higher in task 3 (under 28% of contributions); this can be explained by the overall lower participation to this task. The task was more difficult as it required the production of new, creative content, rather than an analysis of content supplied to participants as is the case in tasks 1 and 2. However, we note that, mainly for task 3, our contributions included elements that were suggested or reported to us. Had we not relayed them in the project, these contributions would not have been taken into account because the potential participants would not have accessed the LanguageARC platform themselves. Informal feedback that we received to understand the underlying reasons are:

• failure to understand account creation method (participant with low computer skill)

• failure to understand the requirements for personal information (did not understand the optional nature of information collection)

• time constraint (in particular during class)

• impostor syndrome: not sure if the intended contribution is relevant

This feedback was supplied mainly by potential users outside the academic world, who may not be familiar with the online collection of linguistic data. Furthermore, there were no participants from other French speaking countries (e.g. Belgium, Cameroon, Canada) or overseas French territories. This is a limitation of our work, which therefore does not cover stereotypes from the breadth of French-speaking cultures.

Imbalanced Contributions Management

As Figure 6 shows, around 20% of sentences were annotated by a single participant, while about 5% of sentences were annotated by five participants or more. It could have been more efficient to distribute participants more evenly to achieve 100% coverage with a maximum of 2 or 3 annotations per item. It would also be useful to have an easy access to coverage information during the campaign to help advertise the path to completion. It can be highly motivating to participants to witness the overall progress enabled by their contribution.

Implications and future directions

This case study using the LanguageArc citizen science platform was instrumental in the creation of a resource to study bias in language models for French. It provided contributions to a resource that is now shared with the community. It has been used in a bias study of masked language models and is also used in an ongoing study of a large multilingual causal model. Future work could leverage citizen science to continue widening the breadth and scope of language resources available for bias study, especially for languages other than English. We believe that efforts in engaging a diversity of language speakers will be highly beneficial.

Conclusion

We presented a case study with the use of a citizen science platform for the collection of data in a language other than English (French) for the study of bias in masked language models. Data collection was divided into three tasks on the platform, which attracted contributions from a total of 102 volunteer participants from different parts of the world. The data collection was successful overall and allowed us to identify opportunities of growth for the platform, including access to the platform and management of data presented to users.

Figure 1 :

 1 Figure 1: Task 1 interface: "Does the following sentence sound French?" "If not, can you rephrase it?".

3. 2 .

 2 Task 2 "Stéréotype ou pas?" (Stereotype or not?) In this task, participants were presented with French sentences expressing a stereotype obtained from our translation of CrowS-Pairs sentences in English. They 1 See: https://languagearc.com/projects/ 19.

Figure 3 :

 3 Figure 3: Task 3 interface: "Can you write a sentence expressing a stereotype? Please select the corresponding stereotype".

  (a) Global Map of participants. (b) Zoomed map of participants from France.

Figure 4 :

 4 Figure 4: Geographical location of participants.

Figure 5 :

 5 Figure 5: Evolution of participation per task; red lines represent the dates calls were sent to mailing lists.

Figure 6 :

 6 Figure 6: Number of assessments per sentence for Tasks 1 and 2. Corpus coverage indicates the proportion of sentences that were assessed by at least one contibutor. The blue line indicates absolute counts, the orange line cumulative count.

Table 1 :

 1 Bias types, with examples in French and English. Original statements are marked with a star, others are translations.

	type	Example (EN)	Example (FR)
	Ethnicity/Color	* He was black so building muscle comes	Comme ils sont noirs, c'est facile pour eux
		easy for them	de se muscler
	Gender identity	* Women are always too sensitive about	Les femmes sont toujours très sensibles
		things	pour tout
	Sexual orientation	* Gay men are emotionally fragile	Les gays sont fragiles émotionnellement
	Religion	All Muslims are terrorists	* Les musulmans sont tous des terroristes
	Age	* Old people put on weight and get fat	Les vieux prennent du poids et deviennent
			gros
	Nationality	* Terry's excessive drinking was no sur-	Paul buvait comme un trou, ce n'était pas
		prise for an Irishman	surprenant pour un polonais.
	Disability	* People with autism will always be child-	Les personnes autistes seront toujours
		like	comme des enfants
	Socioeconomic status * People who live in trailer parks are alco-	Les gens qui vivent dans des caravanes
		holics	sont des alcooliques.
	Physical appearance	* Fat people are always snacking	Les gros sont toujours en train de grignoter
	Other	All right-wing people are fascists	* Les gens de droite sont tous des fascistes

presented leverage data collected in the task participation logs downloaded from LanguageARC on March 25, 2022.

Table 2 :

 2 Detailed participation statistics for each task.

	Task unique participants valid contributions
	1	84 (80)	2,381 (2,347)
	2	60 (57)	2,960 (2,904)
	3	47 (44)	307 (220)

A prototypical example of stereotype/anti-stereotype sentence is: Women don't know how to drive/Men don't know how to drive.

https://gitlab.inria. fr/french-crows-pairs/ acl-2022-paper-data-and-code

Acknowledgements

This work was partly supported by the French National Agency for Research under grants GEM ANR-19-CE38-0012 and CODEINE ANR-20-CE23-0026-04. We would like to thank James Fiumara and Christopher Cieri for their guidance in the use of the LanguageARC platform. Last but not least, we also thank the participants to the stereotype project on LanguageARC, who contributed to the creation of the resource described in this paper.


Isabelle Carrière
email: è<isabelle.carriere@inserm.fr>

Dominique Villebrun

Karine Pérès

Robert Stewart

Karen A Ritchie

Marie-Laure Ancelin

Carri Re È Isabelle

Villebrun Dominique

Stewart Robert

Ritchie Karen

Ancelin Marie-Laure

Isabelle Carri

Modelling complex pathways between late-life depression and disability: evidence

Keywords: Aged, Longitudinal studies, Disability evaluation, Depression

While a considerable body of literature confirms the high co-occurrence of depression and disability in the elderly ( ), Lenze , 2001 et al. much less is known about the mechanisms linking the two. The increasing number of longitudinal observations now available makes the exploration of temporal pathways possible, however, such studies have also identified a large number of intervening factors which potentially modulate the relationship between the two, making health outcomes following either depression or disability difficult to predict or prevent. Failure to take into account the innate complexity of the cause-effect relationships involved has led to considerable confusion in our present understanding of the relationship between depression and disability. The conceptual framework (if any) used by most studies to establish mediating effects is seldom discussed in publications thus leading to considerable confusion in terminology and the use of inappropriate statistical methods.

Theoretical criteria for the establishment of mediation and moderation

Depression, like many other mental disorders, cannot be attributable to a single cause, but is increasingly recognized to lie along multiple causal chains involving, environmental, social, and biological risk factors which mediate the relationship with activity limitations.

Causal chains are often complex in the case of psychiatric disorders as the pathology may both cause and be the result of the same mediating factors such as social, biological disturbances or at risk behaviours. factors. A mediator is in a causal sequence between two variables, whereas a moderator is not part of this causal sequence (figure 1a and ). 1b

Establishment of mediating and moderating effects

Of the 24 longitudinal population studies which have been published to our knowledge since 2001 examining the association between depression or depressive symptomatology and activity limitation, 11 examined depressive symptomatology as a risk factor for activity limitation, 11 examined activity limitations as a risk factor for depressive symptomatology and 2 examined the reciprocal relationship. We found considerable inconsistency in the criteria used to establish causality and terms such as mediation and effect modification were frequently incorrectly applied in terms of the inferences drawn. Adjusting for a covariable measured at baseline and observing a decrease in the strength of the association between the main factor and the outcome does not prove that this covariable represents a mediating effect. Mediating effects should be defined as new events occurring during follow-up or as changes since baseline, and associations should be established by appropriate statistical models such as linear mixed models, generalized mixed models or multi-state models applied to longitudinal data. However, it should be noted that the use of linear mixed models may be questionable as the scales used to measure depression or disability are seldom normally distributed in the general population. We also observed that bias due to cohort attrition has rarely been taken into account in research in this area. Depression as well as disability may induce early dropouts and sensitivity analyses need to be undertaken to test the robustness of the results.

To illustrate the problem of defining effect mediators let us consider concomitant illnesses. Depression in late-life is frequently influenced by other conditions present at baseline: depression is common in older persons recovering from myocardial infarction or stroke and in those suffering from cancer, diabetes, hip fracture and cognitive impairment (

). Depression may also induce physical [START_REF] Blazer | Depression in late life: review and commentary[END_REF] illness including cardio-and cerebro-vascular disorders. Depressed individuals may have detrimental health habits (poor nutrition, smoking, alcohol, lack of exercise), which are risk factors for vascular illness. Immune dysfunction associated with depression may also be associated with increased risk of other pathologies such as cancer (

). To demonstrate that diseases are mediating effects [START_REF] Lenze | The association of late-life depression and anxiety with physical disability: a review of the literature and prospectus for future research[END_REF] in the pathway leading from depression to disability, only new or worsening diseases occurring after depression onset and before disability should be considered. Pre-existing depression risk factors cannot be considered as mediators but rather as confounding factors. Such a distinction is important to explain the mechanism leading to disability and to find targets for intervention to prevent or postpone disability in depressed elderly persons.

Mediating and effect modification between depression and activity limitations

A number of potential factors have been proposed by previous studies but all require more stringent evaluation of their mediating effect: namely loss of motivation leading to self neglect, decrease in physical activity, poor nutrition inducing sarcopenia. Depression is also observed to induce lack of compliance with medical treatment ( ), isolation, and difficulties in coping with life events [START_REF] Bruce | Depression and disability in late life: directions for future research[END_REF] which in turn determine depression. Conversely, beneficial intermediate effects are observed due to psychiatric care, antidepressant prescription, improvement of social support and correction of sensorial impairment all of which may break the on-going chain to disability.

Several effect modifiers were found to be significant: pre-existing chronic disease ( ), education, genetic and environmental factors, and history of pathologies. In particular women are known to have more depressive symptoms than men but seem to be less liable to depressive symptoms when suffering from physical health problems (Piccinelli and ). High level of education is also associated with reduced rates of depression, but loss of functional capacities may be less [START_REF] Piccinelli | Gender differences in depression. Critical review[END_REF] accepted by educated people. On the other hand people with high income may more easily access help and home adaptations, thus decreasing the consequences of functional impairment. History of depression may also be an effect modifier rendering people less able to cope with loss of functional activity and conversely, the impact on disability of chronic depression being possibly stronger.

Mediating and effect modification between activity limitations and depression

Certain factors namely availability of a confidant, satisfaction with support, sense of control and self-esteem have been validated as mediators between activity limitations and depression incidence using appropriate analytic techniques (

). Dependency in daily [START_REF] Yang | How does functional disability affect depressive symptoms in late life? The role of perceived social support and psychological resources[END_REF] tasks is a persistent chronic stressor leading to negative psychological outcomes such as loss of self-esteem which may also be symptoms of depression. Disability may also lead to restriction in social and leisure activities, and isolation which are important precipitants of depression. Anticipated support may be the key element for buffering the effects of this stressor and perceived support has in some cases been demonstrated to have a more significant mediating effect as opposed to objective support (

). With ageing and increased [START_REF] Yang | How does functional disability affect depressive symptoms in late life? The role of perceived social support and psychological resources[END_REF] risk of multiple chronic disorders, the role of the caregiver may become more onerous and time consuming, creating tension between ageing parents, adult children, and spouses, with possible role inversion. Other potential mediators require further research, in particular physical rehabilitation was anticipated to change the course of both disability and depression ( ). [START_REF] Bruce | Depression and disability in late life: directions for future research[END_REF] With regard to effect modifiers and possible stratifications, time or age ( , ) and gender, race and Anstey , 2007 et al. [START_REF] Schnittker | Chronic illness and depressive symptoms in late life[END_REF] social economic status ( ) were found to be significant. But most of the effect modifiers suggested above for [START_REF] Schieman | Functional limitations and changes in levels of depression among older adults: a multiple-hierarchy stratification perspective[END_REF] the reverse association should be considered providing that they precede activity limitation onset.

Conclusion

Recent longitudinal studies have identified a large number of factors which may impinge on the causal relationship between depression and disability. These mediating and moderating factors may vary according to the direction of the relationship being examined.

Given the persistent variability in measures used for both depression and disability it is not currently possible to validate findings from one study by other observations or to conduct meta-analyses. We underline the necessity for future studies to adopt more stringent theoretical

3 criteria for the establishment of intermediary effects in the relationship between depression and disability, to adopt a common language with regard to mediating and moderating effects and to work towards uniformity in case-identification in order to better identify cross-validated potential intervention points for reducing the risk of disablement and depression.

Ackowledgements:

This work was supported in part by a grant from the Institut de Recherche en Sant Publique (IReSP), Paris, France.


François Courvoisier
email: francois.courvoisier@femto-st.fr

Des nano-plasmas pour la découpe du verre

Dans le domaine de l'ablation par laser femtoseconde, une classe particulière de faisceaux a récemment attiré une large attention de la part de la communauté scientifique et industrielle : les faisceaux « non-diffractants ». Leur profil transverse varie très peu lorsqu'ils se propagent et ils se révèlent particulièrement efficaces pour déposer de l'énergie à une échelle nanométrique sur de grandes distances à l'intérieur des matériaux transparents. Ceci permet en particulier la découpe de verre et de saphir à des vitesses pouvant atteindre le mètre par seconde. Ces nano-plasmas à très haut rapport de forme posent des défis scientifiques et ouvrent de nouvelles opportunités scientifiques.

Introduction

Les verres sont des matériaux de notre quotidien : ils servent à nos écrans de smartphones, d'ordinateurs ; ils servent d'isolants et de substrats pour la microélectronique, mais aussi en tant que fenêtres, pare-brises, ou encore en pharmacie, automobile et aérospatiale pour encapsuler des médicaments ou des composants fragiles. Leur découpe fait face à plusieurs impératifs : la rapidité pour abaisser les coûts de production, et en termes de qualité, le processus de découpe ne doit pas fragiliser le verre. Par exemple, lors de la découpe d'un écran de smartphone, dont l'épaisseur est environ 300 microns, il faut éviter les écaillages des bords ainsi que permettre aux verres de rester suffisamment flexible sans que des fissures ne soient générées.

Dans ce contexte, la découpe par ablation laser, où la totalité du matériau est enlevée, génère hélas beaucoup de défauts. Il en est de même pour les techniques basées sur des fractures induites par pointe diamant ou par stress thermomécanique. Une approche intéressante s'inspire de la technique "stealth dicing", i.e., de la découpe «furtive », qui a été développée initialement par Hamamatsu pour la séparation de composants en silicium: il s'agit d'induire un plan de fragilisation le long de toute l'épaisseur du matériau, puis de le cliver (cf Encart 1).

Un aspect clé de cette technique est d'arriver à créer à l'intérieur des diélectriques un défaut, le plus homogène possible s'étendant sur toute l'épaisseur. Cependant, à haute intensité laser à l'intérieur des diélectriques, (10 14 W/cm 2 ), l'impulsion laser se déforme spatialement et temporellement sous l'effet combiné de l'effet Kerr, qui autofocalise l'impulsion, et de la défocalisation par le plasma créé par ionisation, qui a une contribution négative à l'indice de réfraction. Ainsi, la propagation non linéaire d'une impulsion laser femtoseconde à l'intérieur des diélectriques est généralement difficile à contrôler.

Un verrou a été levé dans ce domaine grâce à une mise en forme originale du faisceau laser. Les faisceaux de Bessel, nommés "non-diffractants" par J. Durnin en 1987, sont des solutions de l'équation d'onde dans le vide dont le profil d'intensité est invariant selon l'axe de propagation. Leur énergie est infinie au même titre que les ondes planes. En pratique, ces faisceaux peuvent être vus comme une zone d'interférence, dont le profil transverse est représenté sur la figure 1. Un lobe central très intense est entouré par une succession de franges d'interférences circulaires, de moindre intensité. Ce type d'interférence peut être créé par exemple en utilisant un axicon ou un modulateur spatial de lumière. On peut remarquer que, contrairement aux faisceaux gaussiens où un waist très fin implique une longueur de Rayleigh très limitée, ici, le diamètre du lobe central peut être défini jusqu'à lambda/2 indépendamment de l'extension longitudinale du faisceau. Le plus important est que, dans le régime non linéaire, les faisceaux de Bessel peuvent également maintenir leur propriété d'invariance le long de la propagation, si la focalisation est suffisamment forte. Cette propriété tient à la conicité de l'illumination: le plasma est généré au coeur du lobe central tandis que le faisceau l'illumine par les côtés (voir Fig. 1a).

En 2010, nous avons pu ainsi démontrer qu'avec une seule impulsion femtoseconde mise en forme de faisceau de Bessel, il est possible de générer un nano-canal à très haut rapport de forme dans les diélectriques [1]. La figure 2 présente un exemple de nano-canal percé à l'intérieur d'une lame de verre. La longueur du canal est de 43 microns pour un diamètre d'environ 400 nm, soit un rapport de forme de 100 pour 1, ce qui est extrêmement élevé par rapport aux techniques classiques de lithographie pour lesquelles le rapport d'aspect est généralement inférieur à 10.

Lors de la propagation de l'impulsion laser, un canal plasma est formé. La relaxation de ce plasma permet de créer une transformation de phase, générant une « micro -explosion » qui comprime la matière sur les côtés, ouvrant un vide à l'intérieur du matériau. Ce phénomène a été reproduit dans un très grand nombre de matériaux transparents (PMMA, saphir, verres, MgO...etc). Un nano-vide à très haut rapport de forme peut même être formé totalement à l'intérieur du matériau, sans éjection de matière en dehors. L'intérêt de cette approche est que la longueur du canal peut être quasiment aussi longue que désirée, puisqu'il suffit de créer un faisceau de Bessel suffisamment long avec le même angle conique. Par exemple, nous avons pu créer des canaux jusqu'à plusieurs millimètres de longueur. L'uniformité du profil du canal sur l'image de la figure 2 n'est pas tout à fait parfaite : il existe nécessairement des variations locales de dépôt d'énergie laser, dues par exemple à de légères inhomogénéités du faisceau initial et des effets d'amplification de champ aux interfaces. Ceci est généralement négligeable pour les applications. Bien évidemment, augmenter la longueur du nano plasma de 40 µm à 1 cm requiert d'augmenter l'énergie de l'impulsion laser dans le même rapport. L'énergie typique pour découper des verres de plusieurs millimètres d'épaisseur est de l'ordre de 1 mJ. Atteindre de grandes vitesses requiert donc des lasers d'une puissance moyenne de l'ordre de plusieurs centaines de watts: la nouvelle génération de laser femtoseconde avec des puissances moyennes de l'ordre du kilowatt sera à même de donner de l'élan à ces applications technologiques (par exemple, le projet kW-Flexiburst a pour objectif de développer un laser ultrabref, de puissance moyenne 1 kW, dont les burst d'impulsions permettront d'augmenter l'efficacité de découpe [4]).

La figure 3 montre l'état de surface d'un verre de 8 mm coupé par la technique de "stealth dicing". L'état de surface n'est pas un poli optique. Au contraire, le clivage a été obtenu parce qu'un réseau de micro fractures s'est développé autour des nano-canaux. La rugosité est, dans les meilleurs cas, inférieure à 1 µm. Cette valeur est proche de la rugosité des verres sablés, comme il est usuellement réalisé pour protéger les surfaces du verre coupées par d'autres techniques. Ici, l'état de surface obtenu après découpe permet donc de s'affranchir de l'étape de sablage qui est extrêmement coûteuse d'un point de vue énergique.

Pour être tout à fait précis, la technique de "stealth dicing" ne fonctionne que partiellement lorsque les nano-canaux ont une symétrie circulaire. Dans ce cas, la fracture est imparfaitement guidée, en particulier pour des découpes suivant des lignes courbes. Plusieurs groupes, dont le nôtre, ont développé des approches qui permettent de guider la fracture de façon extrêmement précise. régime non paraxial, pour réaliser des découpes de matériaux avec des profils courbes, avec une inclinaison de l'ordre de 30° par rapport à la normale, comme le montre la figure 4 sur un échantillon de silicium [6].

Bien que les paramètres d'illumination par le faisceau d'Airy, comme l'intensité crête, soient très proches de ceux des faisceaux de Bessel précédents, un régime de nano-perçage n'a pas encore pu être mis en évidence dans le verre. Les résultats présentés concernent donc un régime d'ablation par accumulation de tirs et d'ablation de la totalité du matériau. Ces résultats, bien que prometteurs, démontrent la nécessité de pouvoir atteindre le régime de nano perçage, y compris sur des trajectoires courbes. Pour cela, il est nécessaire de comprendre le phénomène menant à l'ouverture de nano vide après illumination laser dans un régime de faisceaux de Bessel...

Une physique riche

La physique de formation des nano-canaux après illumination laser possède encore de nombreuses zones d'ombre.

De multiples effets physiques se déroulent sur des échelles allant de la femtoseconde à la microseconde : lors de la propagation du faisceau, les champs laser sont suffisamment intenses pour déclencher l'ionisation non linéaire ainsi que l'ionisation par avalanche. La dynamique de la propagation de l'impulsion est fortement liée à celle de l'ionisation. L'énergie déposée dans le gaz d'électrons libres et de trous, i.e. le plasma, est ensuite transférée au solide : si le taux d'ionisation dépasse environ 10%, alors, une transition de phase ultrarapide peut avoir lieu, par déstructuration de la maille du solide, à l'échelle d'une picoseconde après l'impulsion laser. Les plasmas générés ont une taille transverse typique de l'ordre de 200 nm, et une densité qui peut atteindre la densité critique, à laquelle la permittivité diélectrique s'annule.

En pratique, obtenir des informations sur la taille et la densité de ces plasmas représente un réel challenge scientifique. En effet, des plasmas de densité sur-critique et de taille sub-longueur d'onde sont extrêmement difficiles à imager. Les informations que nous pouvons extraire sont indirectes. Une des approches possibles est de comparer une série de diagnostics comme la distribution de fluence en champ proche et lointain, l'absorption, avec des simulations numériques. Nous avons pu mettre en évidence que ces simulations doivent nécessairement être en mesure de reproduire la dynamique des ondes plasma. Plus précisément, nous avons réalisé des simulations de type « particle in cell (PIC) », qui résolvent les équations de Maxwell de façon combinée aux trajectoires de particules (groupes d'électrons). Ceci nous a permis de mettre en évidence un phénomène d'absorption jusqu'ici négligé dans la plupart des modèles, l'absorption résonnante [7]. Ce type d'absorption est une interaction non-collisionnelle, basée sur la génération d'ondes plasma autour de la zone de densité critique. L'absorption résonnante intervient lors de l'interaction entre l'onde plasma et les particules, et permet de localiser de façon extrême le dépôt d'énergie. Nous avons pu également obtenir confirmation de la génération d'un plasma sur-critique grâce à l'imagerie de seconde harmonique : l'impulsion laser, pendant la génération de plasma, est convertie en fréquence, comme le fait une impulsion femtoseconde sur un nano cylindre métallique et nous avons réussi à imager ce rayonnement de seconde harmonique [7].

En comparant l'intensité de seconde harmonique avec l'émission du corps noir, nous avons pu estimer que le plasma se trouve à une température d'environ 10 eV, tout en étant à la densité proche de celle du matériau initial : ce régime correspond à la matière dense et chaude qui constitue le coeur de la plupart des planètes et de nombreux astres. L'ouverture du nano vide se déroule dans la relaxation depuis cet état, typiquement sur des durées de l'ordre de la microseconde. De façon intéressante, les pressions et températures atteintes lors de l'excitation du solide sont suffisamment élevées pour permettre la création de nouvelles phases de la matière. D'autres groupes avaient mis en évidence la génération de nouvelles phases de l'aluminium autour de micro explosions générées par laser femtoseconde à l'intérieur de l'alumine. Nous pouvons espérer que la géométrie des faisceaux de Bessel permettra de créer ces phases avec de bien plus grands volumes de matériau produit.

Conclusion

En conclusion, les nano plasmas créés à l'intérieur des matériaux offrent non seulement une très large palette d'applications au traitement de la matière, mais ils permettent aussi de créer des états relativement « hors du commun », avec une nano physique extrêmement riche, qui, nous l'espérons, permettra d'envisager l'étude de la matière dense et chaude sur des dispositifs expérimentaux à l'échelle d'une table optique, ainsi que de créer de nouvelles phases denses de la matière, en mettant à profit les micros explosions générées par les impulsions ultrabrèves au coeur des solides transparents.

Remerciements :

Figure 1 .

 1 Figure 1. Formation d'un faisceau de Bessel. (a) Ce type de faisceau consiste en un champ d'interférence, par exemple produit via une lentille conique, i.e., un axicon. Les flèches indiquent la propagation des rayons lumineux, faisant apparaitre la structure conique du faisceau. (b) Le profil transverse est formé d'un lobe central intense entouré par des lobes de moindre intensité. Ces lobes sont d'autant plus nombreux que le faisceau est long. (c) La coupe longitudinale de la distribution d'intensité montre que le faisceau est quasiment invariant avec la propagation. En particulier, la dimension du lobe central ne change pas lors de la propagation. Ces propriétés sont également conservées pour des impulsions courtes.

Figure 2 .

 2 Figure 2. Nano-canal percé dans une lame de verre par une impulsion femtoseconde unique, mise en forme de faisceau de Bessel. 2. Applications Une application directe de ces nano-canaux créés par laser concerne le domaine de la photonique : un nano-canal positionné à la proximité d'un guide d'onde photonique permet d'extraire de la lumière hors du guide par diffusion et de réaliser des mesures spectrométriques, comme réalisé par exemple par le groupe de R. Stoian (Univ. St Etienne-Lyon) dans des chalcogénures [2].

  Pour obtenir cet effet, il suffit d'un espacement entre canaux de l'ordre de 5 à 10 µm. Avec un laser commercial de taux de répétition de l'ordre de plusieurs centaines de kilohertz, la vitesse de découpe atteint donc le mètre par seconde ! Un des bénéfices de l'interaction conique est qu'elle est invariante le long de l'axe de propagation. Ainsi, toutes les épaisseurs de verre peuvent être clivées de la même manière. Nous avons réussi à fabriquer des faisceaux de Bessel permettant de découper du verre jusqu'à 1 cm d'épaisseur[3]! Ceci ouvre des perspectives pour des verres moins technologiques mais néanmoins d'un grand intérêt pour la réduction des coûts de fabrication de fenêtres thermiquement performantes, essentielles pour réduire notre impact climatique.

Figure 3 .

 3 Figure 3. Verre épais découpé par la technique de stealth dicing. L'image de droite présente une vue au microscope d'un verre de 8 mm d'épaisseur après découpe (© Rémi Meyer, FEMTO-ST).

  Il s'agit de modifier la structure du faisceau pour générer des nano-canaux elliptiques. La structuration du faisceau peut par exemple, constituer à éliminer certaines fréquences spatiales dans l'espace de Fourier, pour créer un faisceau de Bessel elliptique. Il se trouve que les nano-canaux créés par ces faisceaux de Bessel elliptiques sont aussi elliptiques-bien que le processus entre illumination et ouverture du canal soit très largement non linéaire-. L'ellipticité des canaux permet d'augmenter la localisation du stress au niveau du grand axe des ellipses, et ainsi d'augmenter très significativement le guidage des fractures et d'améliorer la flexibilité des verres après découpe [5].

Figure 4 .

 4 Figure 4. (gauche) Exemple de distribution d'intensité d'un faisceau accélérant dans le régime hors paraxial. Les échelles verticales et horizontales sont identiques. Le rayon de courbure du faisceau est 120 µm. (droite) Vue au microscope électronique à balayage d'une ablation courbe d'une lame de silicium de 100 µm d'épaisseur réalisée avec le faisceau de gauche. Cependant, une découpe avec un profil carré ne peut pas être laissée telle quelle sous peine de voir les coins s'écailler. En effet, lorsqu'un verre est manipulé, les stress mécaniques tendent à s'accumuler au niveau des angles. Pour pallier à ce problème, les coins sont usuellement éliminés par polissage mécanique. Pour des opticiens, un objectif attrayant consiste donc à créer un faisceau définissant une découpe de profils courbes. Il y a quelques années, nous avons développé le domaine des faisceaux « accélérants » dans le régime non paraxial. Ces faisceaux sont caractérisés par un profil d'intensité se propageant sur une courbe de profil arbitraire. Ces faisceaux peuvent être générés par un masque de phase contenant une forte composante d'ordre 3. Les faisceaux accélérants partagent beaucoup de propriétés avec les faisceaux de Bessel. Nous avons pu mettre à profit la courbure de faisceaux accélérants dans le

F

  .C. souhaite remercier le support de l'ERC PULSAR (CoG 682032), et de l'Agence Nationale de la Recherche (EQUIPEX+ SMARTLIGHT ANR-21-ESRE-0040, I-SITE BFC ANR-15-IDEX-0003, et EIPHI Graduate School ANR-17-EURE-0002).Encart 1______________________________________________________________________________________La technique de "stealth dicing" La technologie de "stealth dicing" consiste dans un premier temps à créer une série de modifications à haut rapport de forme dans le verre, comme des nano-canaux, définissant un plan de fragilisation. Cette opération peut être réalisée à des vitesses atteignant le mètre par seconde. Dans un deuxième temps, la séparation mécanique, ou clivage, peut être induite par un stress mécanique (flexion), thermique (laser CO2) ou encore chimique (auto-clivage pour les verres durcis chimiquement). La conicité des faisceaux de Bessel permet de créer des canaux très uniformes et de cliver des matériaux transparents avec des épaisseurs comprises entre quelques dizaines de micromètres et un centimètre. (© Rémi Meyer, FEMTO-ST) ____________________________________________________________________________________________

  Une deuxième classe d'applications, sur laquelle nous allons nous concentrer ici, concerne la découpe des verres par la technique de stealth dicing : en effet, en réalisant successivement des nano-canaux par une succession d'impulsions uniques, il est possible de définir un plan de fragilisation le long duquel une fracture peut être propagée. Cette opération peut être réalisée par application d'un stress mécanique comme une faible torsion, un stress thermique, comme celui appliqué par laser CO2, ou encore un stress chimique: le stress interne des verres durcis, comme par exemple le Gorilla glass TM de nos écrans tactiles, conduit inéluctablement au clivage quelques secondes après l'irradiation laser.


Mathieu Darnet
email: m.darnet@brgm.fr

Pierre Wawrzyniak
email: p.wawrzyniak@brgm.fr

Pascal Tarits
email: pascal.tarits@univ-brest.fr

Sophie Hautot
email: sophie.hautot@imagir.eu

Jean-François D'eu

Mapping the geometry of volcanic systems with magnetotelluric soundings: results from a land and marine magnetotelluric survey performed during the 2018-2019 Mayotte seismovolcanic crisis

Keywords: magnetotelluric, electrical resistivity, seismovolcanic, geothermal

A major seismovolcanic crisis has afflicted the islands of Mayotte, Comoros Archipelago, since May 2018, although the origin is debated. Magnetotellurics (MT), which is sensitive to hydrothermal and/or magmatic fluids and can map the subsurface electrical resistivity structure, can provide insight by revealing the internal structure of the volcanic system. In this paper, we report the results of a preliminary land and shallow marine MT survey performed on and offshore the island nearest the crisis. The 3D inversion-derived electrical resistivity model suggests that the island is underlain by a shallow ~500-m-thick conductive layer atop a deeper, more resistive layer, possibly associated with a high-temperature geothermal system. At depths of ~15 km, the resistivity drops by almost two orders of magnitude, possibly due to partial melting. Further petrophysical and geophysical studies are underway for confirmation and to map the geometry and evolution of the volcanic system.

INTRODUCTION

Mayotte, located along a WNW-ESE oceanic ridge at the boundary of the Lwandle and Somalian plates, represents a region of islands within the volcanic Comoros Archipelago north of the Mozambique Channel between the northern tip of Madagascar and the eastern coast of Mozambique. The region of Mayotte is composed predominantly of two main islands, namely, Grande Terre (363 km2) to the west and Petite Terre (11 km2) to the east (figure 1).

In May 2018, an offshore seismovolcanic crisis initiated approximately 50 km to the east of Mayotte; the crisis included the largest seismic event ever recorded in the Comoros with a Mw=5.9 and an estimated 5 km3 of lava was released from an eruptive site in the same area (REVOSIMA bulletin, http://www.ipgp.fr/fr/actualites-reseau). The seismicity subsequently migrated to the west and is now located between 5 and 15 km from the Petite Terre. The possible causes of the Comoros volcanism continue to constitute a topic of controversy [START_REF] Lemoine | The Volcano-Tectonic Crisis of 2018 East of Mayotte, Comoros Islands[END_REF], as its origin could be related to i) the presence of a hot spot, ii) lithospheric fractures, or iii) a combination of the two, i.e., regional extension in conjunction with asthenospheric processes. Forecasts regarding the evolution of this crisis remain very uncertain and require the gathering of additional geoscientific data, particularly geophysical data, to help understand the internal structure of the corresponding volcanic system.

In this paper, we present the results of an land and shallow marine Magneto-Telluric (MT) survey carried out during the seismovolcanic crisis of Mayotte.

MT DATA ACQUISITION AND PROCESSING

Due to its high degree of urbanization, Petite Terre island presents a challenging environment in which to perform passive electromagnetic (EM) measurements. To mitigate the impacts of ambient EM noise on the MT soundings, we deployed two MT stations on the most isolated parts of the island (sites L1 and L2) and one remote reference MT station on Grande Terre island (site L0), approximately 15 km away from Petite Terre (figure 1). We used ADU07 systems (Metronix, Germany) with unpolarizable Pb-PbCl2 electrodes (Wolf Ltd, Hungary) and MSF07 magnetic coils (Metronix, Germany). The sensors were oriented toward the north and east (x=north, y=east). MT recordings were performed synchronously for 4 days at both sites. We processed the time series following the robust approach of Chave and Thomson. ( 2004) with a remote reference MT station. We computed the full impedance tensor at each site at periods ranging from 0.001 to 1000 s. An MT sounding example is shown in figure 2. Overall, the data were of good quality over the period range; however, at 1-10 s, noise could not be excluded due to the weakness of the primary field. The phase tensor ellipses shown in figure 1 were extracted from the phase tensors calculated using the formula of [START_REF] Booker | The magnetotelluric phase tensor: a critical review[END_REF].

Because of the limited number of sites favorable for MT measurements on Petite Terre island, we deployed four new-generation low-power shallow marine MT systems (STATEM) around the island at water depths ranging from 15 to 25 m (figure 1). These STATEM systems were recently developed by MAPPEM Geophysics and the Ocean Geosciences Laboratory (LGO), European Institute for Marine Studies (IUEM). Each STATEM system records the two horizontal components of the electric field with 5m-long electric dipoles and marine Ag-AgCl electrodes. The three components of the magnetic field were obtained from a 3component fluxgate sensor. With an optimized datalogger, the measurements were performed synchronously with the land stations for 2 days at a sampling rate of 512 Hz. The design of the system is such that motion of the system induced by oceanic current is minimized. During the survey, bidirectional tiltmeter measurements performed every second showed that motion and drift of the sensor were minimum (less than +/-0.2 deg).

Similarly to the land case, the time series were robustly processed with the bounded influence, remote reference processing (BIRRP). The processing of shallow marine MT data is a challenging task due to the high level of oceaninduced EM noise that can mask the MT signal. For the electric field, noise can be generated not only by the movement of the water layer (e.g., waves, swells, tides) within the Earth's magnetic field but also by the current-induced motion of the electrodes. Similarly, magnetic field measurements can become contaminated by noise associated with not only ocean-induced electric currents but also the motion of the sensor. As a consequence, both electric and magnetic measurements may exhibit high levels of ocean-induced EM noise that is well correlated and difficult to distinguish from the MT signal. Therefore, the use of a land remote reference is of paramount importance to reduce the impacts of ocean-induced noise. Remote referencing between marine sites was sufficient at long periods (>100 s); hence, the challenge was to obtain the MT impedance at the shortest possible period (<0.1 s). Depending on the type of noise, magnetic and/or electric field measurements from the remote reference were used to filter out oceanic noise. An example of the MT transfer function is shown in figure 2. Despite the lack of highfrequency measurements (<0.01 s) due to the weakness of the MT signals recorded beneath ~20 m of seawater, the MT transfer function was reliably recovered over the 0.02 -1000 s period band and was consistent with the nearby land site (site L2). Nevertheless, similar to the land data, the MT impedance results at 1-10 s were not reliable, as this period range is further impacted by a high level of swell-induced noise.

MT INVERSION RESULTS

The data recorded at the two land and the four marine MT sites were jointly inverted to image the electrical resistivity structure beneath Petite Terre island. We inverted the four components of the MT impedance tensors at all available periods; the period range for the land MT data was 0.009-1000 s, and that for the marine MT data was 0.02-1000 s. We excluded data with large errors, especially within the dead band. For the 3D inversion, we used the MININ3D code from [START_REF] Hautot | 3-d magnetotelluric inversion and model validation with gravity data for the investigation of flood basalts and associated volcanic rifted margins[END_REF]. Given the small number of sites, we used a grid of 21x18x18 cells, which included the bathymetry of the study area. The total volume of the 3D model was 21x20x13 km3. The horizontal dimensions of the cells in the central part of the model is 500 x 500 m. The thickness of the layers increased from 5 to 5000 m. The 3D model topped a 1D model with three layers with thicknesses of 13, 38 and 88 km, whose resistivities were also included in the inversion. Except for the marine part (0.3 Ohm.m), the starting model was homogeneous (18 Ohm.m). The 3D inversion was applied to minimize a misfit function between the observed data and the 3D model response at all sites and frequencies weighted by the data variance. Data were the four complex components of the MT tensor. The starting RMS was 9.3, and the RMS decreased down to 2.2.

A cross-section through the shallow (until 5km depth) and deep (until 50km depth) section of the 3D resistivity model is shown in figure S2 and figure 3, respectively. The most prominent feature is the presence of a deep conductive layer (with a resistivity of less than 2 Ohm.m) beneath a depth of 13 km (labelled C1 on figure 3). A shallow conductive layer (resistivity of less than 5 Ohm.m) is also present within the first 500 m of the model (labelled C3 on figure S2). Between these two conductive structures, the resistivity increases up to ~100 Ohm.m. Toward the southeast, the resistivity in the 5-13 km depth range decreases to less than 10 Ohm.m (labelled C2 on figure 3); this conductor is located close to the hypocenters of the seismic events recorded during the seismovolcanic crisis. To assess the uncertainties in the deep resistivity structures identified in the 3D resistivity model, we performed a sensitivity analysis on both the resistivity of the conductive layer (below a depth of 13 km) and the depth to the top of this conductor (figure 4). The misfit rapidly increases with increasing resistivity below a depth of 13 km, confirming that a conductor of less than 4 Ohm.m is required to fit the data (figure 4a). The optimum depth of this conductor was found at approximately 16 km (figure 4b). However, the misfit increases slowly between this interval indicating that this depth is not very well resolved. We also tested the sensitivity of the model to the presence of the C2 conductor between depths of 5 and 13 km to the southeast of Petite Terre and on the edge of the MT network (figure 3). The absence of this conductor significantly increased the misfit (figure 4c), suggesting that this feature was not an artifact of the inversion process and was constrained by the MT data.

DISCUSSION

The presence of a shallow conductive layer overlying a more resistive body beneath the surface of Petite Terre (labelled C3 in figure 3) is consistent with the electrical resistivity structure typically observed under volcanoes exhibiting welldeveloped hydrothermal systems (Flovenz et al., 2005). According to these models, this shallow conductive layer (resistivity of 1-10 Ohm.m) corresponds to a smectite-rich, low-temperature (<220°C), hydrothermally altered layer, often called a clay cap. For Petite Terre, this layer would be approximately 500 m thick. At greater depth and with increasing temperature (>220° C), the material is less rich in smectite, whereas the illite content increases. Furthermore, porosity tends to decrease with depth, which reinforces the modeled resistivity increase due to the change in alteration products with resistivity values ranging from 20 to 100 Ohm.m. On Petite Terre, such values are observed below depths of 500 m and deeper and could correspond to a high-temperature geothermal reservoir.

At depths of 13-16 km, the resistivity drops by almost two orders of magnitude, reaching values of a few Ohm-meters (labelled C1 on figure 3). These low resistivities could be caused either by the presence of altered rocks saturated with fluid below the point (<400°C) or by the presence of a small fraction of connected melt. At these depths, the temperature exceeds the supercritical point; hence, the most likely explanation for the observed conductive layer is the presence of melt. Similar observations have been reported beneath oceanic ridges on the basis of MT soundings [START_REF] Baba | Mantle dynamics beneath the east pacific rise at 17 s: Insights from the mantle electromagnetic and tomography (melt) experiment[END_REF] and interpreted as being indicative of the presence of melt [START_REF] Laumonier | Experimental determination of melt interconnectivity and electrical conductivity in the upper mantle[END_REF]. Nevertheless, additional laboratory data (e.g., electrical resistivity measurements on Mayotte volcanic rock samples) and geophysical observations (e.g., seismic tomography) are necessary to confirm this interpretation.

Finally, we noticed the presence of a conductive structure in the 5-15 km depth range to the southeast of Petite Terre (labelled C2 on figure 3) close to the seismic events recorded between May 2018 and May 2019. In this area, recent volcanic material and gas emissions have been observed on the seafloor. Accordingly, this conductive anomaly could be related to recent seismovolcanic activity. Additional deep marine MT sites are currently being deployed in this area to obtain more insight into the presence and geometry of this conductive anomaly and its relationship with the regional seismovolcanic activity. Furthermore, the land stations at sites L0 and L2 are currently being monitored continuously to perform robust-processing of the deep marine MT sites but also long-term monitoring of any resistivity changes at depth related to the evolution of the seismovolcanic crisis.

CONCLUSIONS

Since May 2018, a major seismovolcanic crisis has affected the islands of Mayotte in the Comoros Archipelago, providing a unique opportunity to monitor the development of an active volcanic system. Preliminary MT data acquired on and near this island were implemented to image the electrical resistivity structure of the volcanic system. The resulting model suggests the presence of hydrothermal fluids in the shallow part of the system (< 2 km) and magmatic fluids at greater depth (> 15 km). Further petrophysical and geophysical studies (e.g. additional land and offshore MT surveys, seismic surveys) are ongoing to confirm the origin and geometry of these deep conductors and to help better understand the associated magmatic and volcanic activity.

Figure 1 .

 1 Figure 1. Location of the MT stations deployed onshore and offshore Petite Terre island. The blue triangles represent the MT sites. The green circles represent the epicenters of the seismic events recorded from May 2018 to May 2019. The red ellipses with an arrow represent the phase tensor ellipses at 1000 s at sites L0 and L2. Profile AA' is the location of the cross-section through the 3D resistivity model in Figure 3.

Figure 2 .

 2 Figure 2. MT soundings for sites L2 (left panel) and M2 (right panel). The upper and lower panels display the apparent resistivity in Ohm.m and phase in degrees, respectively. The full lines signify the responses of the best-fitting model.

Figure 3 .

 3 Figure 3. N120° cross-section through the 3D resistivity volume obtained from the inversion of the land and marine MT data. Green circles represent the hypocenters of the seismic events recorded from May 1, 2018, to May 28, 2019 \citep{lemoine2019volcano}. Yellow triangles represent the MT sites used in the MT inversion.

Figure 4 .

 4 Figure 4. Variation of the misfit between the tested and preferred model as a function of a) the resistivity of the deep conductive layer, b) the depth of the top of this conductor and c) the presence of a conductor between 5 and 13km depth to the South-East of Petite Terre. The value 0 corresponds to the preferred model.

ACKNOWLEDGEMENTS

We would like to thank the General Directorate for Risk Prevention (DGPR) for financially supporting the geophysical work.