Alex Boulton
email: boulton@univ-nancy2.fr

Bringing corpora to the masses: Free and easy tools for interdisciplinary language studies

Keywords: data-driven learning, free tools, practical applications, interdisciplinary, distance learning

Despite huge promise, direct applications of corpus linguistics (CL) have yet to make substantial inroads to the language classroom. The main stumbling block is perhaps psychological: even when they are aware of the possibilities, teachers and learners alike may hesitate to change established practices and challenge traditional roles. It is difficult to alter such mindsets, but it should be possible to make corpus linguistics more user-friendly. In particular, we argue here that even using only free on-line tools with minimal training, learners are quick to see a variety of applications in language learning and other areas. This paper describes a CL option in an English MA course where students are allowed considerable freedom to discover the tools and possibilities for themselves, and pursue them to their own ends. An analysis of 30 resulting research papers highlights the diversity of approaches and topics which can be studied through CL, and discusses the wide range of skills learned in the process.

Introduction

Few fields of human inquiry have been left untouched by the advent of powerful computer tools for the processing of vast quantities of data. In language studies, this has given rise to complete new fields of study, including corpus linguistics (CL). CL in turn has made its influence felt in various areas connected with language teaching and learning, most especially in informing teaching and reference materials as well as considerations of content. However, and despite a plethora of research articles and projects, when it comes to direct applications in the classroom in the form of "data-driven learning" (Johns 1991a) or DDL, the trickle-down effect has yet to turn into the predicted torrent (Leech 1997:2).

To explain this lack of uptake, we cannot ignore a number of obvious potential disadvantages to DDL, whether real or perceived. For example, the changes to current practices might be thought too big to be absorbed in a single step, and perceived as not "serious". DDL definitely constitutes a challenge to teacher and learner roles: the former might resent a loss of power and control, while the latter might not like having to take greater responsibility for their learning. Both might also lament the "fuzzy" nature of language inherent in corpus work, preferring the certainty and comfort (even if illusory) which accompanies a traditional rule-based approach. Some might argue they have insufficient facilities in terms of computers, software and space, or the money to acquire Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université. In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96. them. Even with extensive technological resources, teachers and learners can also feel threatened if they lack sufficient skills to use them. Some training would seem essential for DDL to be of use, but any "learning to learn" can be perceived as a waste of time, serving only to delay the language learning itself.

Many powerful arguments can of course be marshalled to overcome these objections. The widely reported advantages to DDL include promoting autonomy, language awareness and noticing skills in a learner-centred approach using authentic language by applying inductive, hands-on discovery learning. As all of these are central in current research on language teaching and learning, it might be argued that objections to DDL are largely a gut reaction. In a nutshell, learners and teachers simply aren't convinced.

If DDL is to break out of its current research confines and reach a wider audience, it should be seen to offer maximum return on minimum investment. For this to happen, a number of things are necessary:

• The tools should be free. Many working teachers are unaware of the considerable resources available free on the internet; all that is needed to exploit them is a single computer with internet access and a printer. A fully-equipped computer laboratory and expensive software certainly allow greater possibilities, but they are by no means essential to DDL.

• The software should be easy to use. Specialists aside, few people are likely to exploit the full potential of the sophisticated software available; more important is that it should be user-friendly and ready to use with only a few minutes' exploration, even for those with limited ICT skills. Freely-available software has increasingly intuitive interfaces, and is frequently accompanied by concrete examples, guided tours, tutorials, help facilities, etc.

• The techniques should be flexible and transferable. If CL is a methodology rather than a discipline (e.g. Scott & Tribble 2006:4), then it might have applications in a variety of areas above and beyond linguistics and language learning (itself a derivative). The more widely it is used for a variety of purposes, the more likely it is to continue to be used. This paper presents a course in corpus linguistics which attempts to put these criteria into practice. In particular, we discuss the tools that learners find free on the internet, and look at the topics they choose to explore when allowed to use them to pursue their own interests.

An option in corpus linguistics

The course in question is part of an MA in English offered entirely at distance by the Centre de Télé-enseignement Universitaire (CTU) at Nancy Université in France, and the typical student profile is correspondingly quite specific (see also [START_REF] Boulton | Habeant Corpus-They should have the body. Tools learners have the right to use[END_REF]. Most of our learners are mature students with French as a mother tongue, but enrolments come from all over the world. They have relatively advanced language abilities as well as a specialist interest in English, and are comparatively autonomous and used to managing their work alone. Motivation is thus a key factor in such an isolated learning environment, as students try to reconcile their degree with work and family commitments in sometimes very difficult circumstances.

Course considerations

In accordance with the prevailing paradigm in France, language forms the common core of the MA supported by three major options in literature, cultural studies and linguistics, and a number of subsidiary specialisations (e.g. French as a foreign language). Of the three main options, linguistics is the least well-known, and statistically the least popular. It is important then to find a course which appeals to a wide range of interests, does not require too much prior knowledge, and relates to other studies. CL seems to fit this bill on a number of counts. On a practical level, it is unlikely students will have come across it before, thus placing them all on an equal footing despite their varied prior experience of linguistics in general. Apart from a computer and access to the internet, no extra costs are involved in the form of required reading or expensive tools, nor any specialist knowledge of ICT. The focus is on learning about CL by applying it to language areas students are interested in. This highly practical approach aims to downplay the common perception of linguistics as abstract theorising, and fosters a set of skills which are transferable to and from other areas of study, and indeed to non-academic life.

Evaluation is confined to the final research paper submitted at the end of the semester. This system lends itself well to distance learning at MA level, as students can pursue their own interests and work at their own rhythm, with teacher back-up available as required. The paper is a 20-page report (not counting appendices etc.) on a research project they define and conduct themselves on an area of their choice. We deliberately kept the guidelines for the research paper as flexible and open as possible so that learners could develop their own questions in regard to their own interests, which go well beyond language learning; the only restriction being that it should involve English and CL. Indeed, as Bernardini (2001:228) points out, "it would be inherently contradictory to prescribe a methodology when the aim of the approach is to give learners the instruments to develop their own methodologies and make their own discoveries." The mark accordingly is less concerned with the product than with the process-defining the question, choosing the corpus, finding the appropriate tools, using their ingenuity to overcome the problems they will inevitably encounter, and reasoning their way through the whole procedure. A number of past research papers are made available on line as well as a list of all past titles and the most popular sites used. Guidance is also given on the appropriate structure, style and format of a research report in English, linguistics generally requiring rather different styles from research in literature or cultural studies.

Descriptions of a number of CL courses have been published elsewhere. [START_REF] Maia | Making corpora: A learning process[END_REF], for example, imposed broad topics (e.g. sport or ecology) on her translation students but required a very specific product for evaluation, namely a bilingual glossary. Others (e.g. [START_REF] Dodd | Exploiting a corpus of written German for advanced language learning[END_REF][START_REF] Jackson | Corpus and concordance: Finding out about style[END_REF][START_REF] Bernardini | Systematising serendipity: Proposals for concordancing large corpora with language learners[END_REF][START_REF] Cheng | The language learner as language researcher: Putting corpus linguistics on the timetable[END_REF][START_REF] Chambers | Integrating corpus consultation in language studies[END_REF]) have used the research paper for evaluation purposes in undergraduate degrees; however, most of these are very explicitly language-oriented courses, often with the language topics for investigation provided by the teachers. [START_REF] Jackson | Corpus and concordance: Finding out about style[END_REF] is the exception in allowing his In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96. students considerable choice in their research, although the projects are confined to a CL analysis of literary style. All of these courses thus impose constraints of one type or another on what students can do, none allowing the freedom in the CTU course. In the light of Johns' (1991a:2) oft-repeated comment that "research is too serous to be left to the researchers," we agree with Leech (1997:10) that individual choice is essential in giving the student "the realistic expectation of breaking new ground as a 'researcher', doing something which is a unique and individual contribution." CL would certainly seem to be a fruitful ground for this, as "the traditional divisions between, on the one hand, teaching and research, and, on the other, literary and linguistic analysis, are to an extent arbitrary" (Burgess & Kohn 1995:63).

Introducing corpus linguistics

As students are often wary of linguistics, and CL can be terrifyingly technical on first encounter, it is essential to find a way to make the course as unthreatening as possible. Bowker and Pearson, in their "practical guide to using corpora" (2002:108), attempt this by leaving their "introduction to basic corpus processing tools" (frequency lists, concordances, clusters, collocates, etc.) to chapter 7, half-way through the book. Another way is to provide easy, accessible, and familiar examples wherever possible, and leave much of the work to the learner to discover not just the language, but how to go about tackling it through CL. Learners need differing amounts of guidance, and back-up has to be available on demand, but in general students quickly see the point, and discover much of the rest for themselves in a relatively short time (c.f. also [START_REF] Bernardini | Spoilt for choice': A learner explores general language corpora[END_REF].

The introductory section to the course thus aims to show how applications of CL can be quite concrete in a familiar environment, which also highlights the variety of applications of corpus methodologies. For example, students are generally familiar with amazon.com, but few have explored the "inside this book" and "search inside" functions, some of which offer insights into CL. Looking at a well-known book such as Dan Brown's Da Vinci Code, visitors to the site can read various extracts including the blurb and the first pages, and are introduced to "statistically improbable phrases" with a brief discussion of what that entails (cilice belt, seeded womb, pope interred, corporal mortification, rosewood box, etc.); these are hypertext-linked to the actual occurrences in the text. Other features provide an introduction to important concepts including frequency, search strings, concordances, collocations, co-text, and so on. The statistics provided here can be intriguing, and even the more fanciful ones serve to keep the introduction accessible: the book rates 9.1 on the Fog Index, indicating that 80% of books are harder to read; it has 11.1 words per sentence (82% of books have more); the reader gets 14,105 words to the dollar, and 11,425 words per ounce, meaning that each word costs €0.000056 at today's prices and weighs 0.0025 grams. The first 500-odd words of the novel can easily be copied into VocabProfiler 1 to show other features that texts can reveal, highlighting the importance of lexical frequency within the text. The statistics can also be compared to the language as a whole based on the most frequent words in bands of 1000. Other concepts introduced at this stage include types, tokens, lemmatisation and etymology.

1 URLs to the various sites mentioned can be found in Appendix A.

Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université. In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96. Standard search engines provide another familiar entry point. Although students know and use google.com, for example, few have ever got around to exploring the advanced search features, which can make all the difference to the relevance of hits. The implications for corpus searches are made explicit, including the importance of domain-specific searches, reliability of websites, etc. The limitations of such tools are also explored, showing how they are information-oriented rather than language-oriented. This allows a lead-in to the web as corpus (see e.g. [START_REF] Kehoe | WebCorp: Applying the web to linguistics and linguistics to the web[END_REF], and especially the possibilities of the more linguistic WebCorp. There is an introduction to large public corpora-the British National Corpus (BNC) and the Bank of English (BoE) among others-as well as the common tools associated with them: concordances, collocates and frequency lists, as well as tagging, query language and tips for conducting successful searches. Other "non-linguistic" corpora are mentioned (e.g. Shakespeare, or Old English), as well as the possibilities for students to create and analyse their own corpora. For this, a number of free on-line tools are presented briefly (e.g. Web Frequency Indexer and the demo version of WordSmith Tools), and links are given to numerous other resources on line, including other tutorial pages.

Descriptions of such tools are kept to a minimum as the idea is that students should explore and find out for themselves; in a sense, learners become their own teacher-researchers. This would seem to be particularly appropriate in CL, which, "almost more than any other branch of linguistics, … requires that students have 'hands on' experience of the subject… Only through using corpora can one gain a first-hand sense of their potential" (Leech 1997:7). This works precisely because CL, "by its very nature, has a highly practical and applied orientation" (Hatzidaki 1996:263). We would agree in this with Kirk (2002:156), who also found with experience that his "focus has shifted from teaching corpus linguistics to that of learning to do corpus linguistics." In a way such a discovery approach is really little more than an extension of the data-driven learning philosophy where learners explore language rather than being told about it; there seems little reason why a similar approach cannot be adopted with the tools and methodologies of CL as well as for language learning.

Student research papers

the internet is a "fabulous linguists' playground" (Kilgarriff & Grefenstette 2003:333) and that there is enormous potential for using the web as corpus (see e.g. [START_REF] Kehoe | WebCorp: Applying the web to linguistics and linguistics to the web[END_REF][START_REF] Bergh | Min(d)ing English language data on the web: What can Google tell us[END_REF]). However, almost all of the corpora which were used were collected from the internet for obvious reasons of variety, availability and cost [START_REF] Aston | The learner as corpus designer[END_REF]. As table 1 shows, there were two basic types: 18 papers used one or more large, ready-made reference corpora, while 27 involved at least one home-built corpus of their own.

home-built corpora 0 1 2 3 4 total 0 3 6 1 2 12 1 1 2 4 1 8 2 2 4 3 1 10 reference corpora total 3 9 13 1 4 30 Table 1. Types of corpora used.
The majority of students made use of some kind of large reference corpus, most frequently the BNC (14 papers) or the BoE (12 papers).2 Two used the Brown corpus, three used different specialist corpora (Old English, Switchboard, W3Corpora). In eleven of these cases, two or three of these large ready-made corpora were used for comparison purposes, or simply to collect more data. Only three students relied entirely on such reference corpora, the others compiling ones of their own. Among these, the majority analysed one corpus (9 papers) or two (13 papers); the remaining five used three or four. In eighteen cases both types of corpus were used, either with the reference corpus as a starting point before moving on to the home-built corpus (12 papers), or the other way round with the reference corpus serving to check the results of the purpose-made corpus (6 papers).

The most popular single source type was inevitably online newspapers for reasons clearly outlined in [START_REF] Rademann | Using online electronic newspapers in modern English-language press corpora: Benefits and pitfalls[END_REF]. Not only are the texts easily available, but (depending on the newspaper) they have minimum standards of language and content, are up-to-date, cover a wide range of areas, and are arguably more representative than other registers in that they are written by multiple authors and read by millions (c.f. Burnard 2002:58). This does not mean the students are necessarily interested in the news as such, but as long as they are aware of the limitations of newspapers and do not overgeneralise their findings, newspapers would seem to be a fine source in general (Partington 2001a:63). Twelve students based part or all of their corpora on British or American press found on line, although others were considerably more specialised.

It seems then that learners can see the attraction of both large existing corpora and small purpose-built ones (see Aston 1997a for a comparison). In the former case our students used only the dedicated software provided with the corpus (e.g. Sara with the BNC); in creating their own corpora, however, they made use of a variety of different tools available free on the internet. 3 One of the most popular was the Web Frequency Indexer (WFI), used by 15 students. 16 papers made use of WordSmith Tools (WST), although there were occasional alternatives. Tagging was rarely used, perhaps because of the limited availability of free online taggers such as CLAWS or the Brill tagger, which are often restricted to very small samples. Word frequency lists from the BNC proved popular, lemmatised or not, as did VocabProfiler and the Compleat Lexical Tutor in more recent papers. Other software included numerous online dictionaries, pgAdmin, Filemaker, the ubiquitous Microsoft Word and Excel, as well as occasional locally-available tools.

With the exception of general-purpose software (notably MS Word and Excel), what all the above tools have in common is that they are free and easily available on the internet. Although the free versions of some are limited, returning a reduced number of hits or allowing only short texts, they were frequently sufficient for most purposes. A number of students showed surprising ingenuity in finding ways around the limitations, especially with carefully formulated search commands, or cutting the text into small sentence or paragraphlength fragments containing the target language; a handful of others were sufficiently hooked to buy full licences for WordSmith Tools. Another point which bears mentioning is that not all of these tools were mentioned in the course: the students located them and found how to use them on their own.

Research topics

Although CL has permitted many quite revolutionary techniques, in some ways it is "nothing but a methodology" (McEnery & Wilson 2001:1)-at least in the sense that its methods are not limited to linguistics alone. This can be seen in the description of a new journal entitled Corpora: Corpus-based Language Learning, Language Processing and Linguistics. One of its "three main features" is: Interdisciplinarity: the journal will actively seek to promote a cross fertilization of ideas and techniques across a range of areas… and disciplines… in the belief that these areas have something to offer to each other through their common focus on corpus data. 4Hatzidaki (1996:262) argues convincingly for the introduction of CL as a taught course in undergraduate language degrees precisely because of its "strong interdisciplinary character… CL is applicable to a whole gamut of linguistic, and, arguably, non-linguistic disciplines." Partington (2001b) and [START_REF] Adolphs | Introducing electronic text analysis: A practical guide for language and literary studies[END_REF] are among those who point out that the vast majority of social sciences use corpora in one way or another, from psychology to philosophy and beyond. [START_REF] Kettemann | Can the L in TALC stand for literature?[END_REF] ask if the L in TaLC can stand for literature, and this can easily be extended to cultural studies, history, sociology, translation, etc., as well as the pursuit of personal interests. All of this suggests that CL may have applications in far wider areas than just "language" [START_REF] Boulton | When linguistics isn't linguistics: Interdisciplinary approaches to corpus linguistics[END_REF], a factor which it would seem churlish not to exploit, and even key in ensuring continued use of the methodologies beyond the course itself. Indeed, if corpora are to be effective, it seems extremely important to allow students to use them in domains that are of interest to them [START_REF] Maia | Making corpora: A learning process[END_REF], especially if texts are "only useful insofar as the learner is able to authenticate them, i.e. to create a relationship with the texts" (Braun 2005:53; also [START_REF] Widdowson | Context, community, and authentic language[END_REF]. And what better way for Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université. In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96. students to authenticate a corpus than for them to choose the texts based on their own interests?

Given the students' overriding interest in literature and cultural studies, their research projects frequently have a bias towards these and related areas. 5 The tools and procedures needed for the course are quite different from what the students are used to in these other fields, however; even where they have used a "corpus" in the past, the methods involved are likely to be substantially different. The idea is not to oppose the different approaches, but to treat them as complementary (Kettemann & Marko 2004:187). However, the students quickly adapted to the quantitative measures needed, generally supplemented with a sensible qualitative eye. Of the 30 papers looked at here, arguably only 6 started with a purely linguistic question, typically a language point that is difficult to grasp or to translate into the L1. These included phrasal verbs, pairs or groups of close synonyms (e.g. barely / hardly / scarcely; speak / talk), and spelling differences (civilise / civilize; enquire / inquire). A number of others were interested in translations of key items, although only two had a parallel corpus as such. Many others were inspired by their own learning experiences, although again, few made this a major focus of the paper. About two thirds of the students started with a specific question (a bottom-up approach); the other third started with the corpus (a top-down approach).

A brief look at the titles of the research papers submitted shows the wide range of other interests pursued (table 2). Top of the list were cultural issues revolving around politics, current affairs, and social issues such as feminism; other cultural topics related to their studies included history, and especially British and American cultural comparisons, confirming or rejecting stereotypes. Five papers were mainly concerned with literature, although many others had this as a minor element, especially in the form of stylistic comparison. Translation and language learning / teaching formed the core of four more, again relating to other courses offered in the degree. Outside interests can be found in music and popular culture, professional life, ICT, religion, and personal contacts with a particular part of the English-speaking world. Many papers managed to combine several of these interests quite explicitly. Such a broad range of topic areas supports Seidlhofer's (2000:208)

What did they learn?

The course outlined here is, first and foremost, a course in corpus linguistics for language students. As a result, students should end up with more than a passing knowledge of key concepts involved in CL, and be able to use them in the future. These include, in no particular order, an awareness of concordances, KWICs, frequency lists, keywords, distribution plots, collocates lists and tables, mutual information and t-scores, stoplists, picklists, batches, left and right sorting, and so on. Using corpora entails an understanding of the importance of balance and statistical evidence in relation to topic, text type, size, date, language variety and register, number of contributors, text length, representativity, and extrapolation from samples to populations. As with Chambers' (2005:114) course, few if any of our students have an "ambition to become experts in CL, any more than they desire to become lexicographers when using the dictionary;" but it is another potentially useful set of tools for them even without specialist in-depth knowledge (Hatzidaki 1996:256).

In producing their research papers, students knew they would be assessed on the "processes rather than products, on methods rather than outcomes, on resourcefulness, awareness and reflectiveness rather than learnedness" (Bernardini 1997:5). In other words, inconclusive or unexpected results do not automatically entail a low mark. As Maia (1997:5) found in her course evaluation, "although the product of each of the assignments varied in quality, the knowledge gained from the process was considerable." There is inevitably a degree of serendipity involved [START_REF] Bernardini | Systematising serendipity: Proposals for concordancing large corpora with language learners[END_REF], but the students are thus sensitised to the cyclical nature of research, involving continuous feedback and rethinking rather than a strictly linear process. The discovery approach involves defining the question, choosing the corpus, locating appropriate tools and experimenting with their use, using their nous to deal with problems they will inevitably encounter, and reasoning their way through the whole process in a clear and rational way. All of this leaves the students better equipped to use CL techniques in the future.

It is also however a course in corpus linguistics, and students certainly learned a lot of linguistics in the process-perhaps indeed more effectively as the practical approach can make many quite theoretical linguistic concepts that much more "real". Amongst other things we could mention:

• The concept of "word" (lemma, lexical item, etc.), function vs. lexical words, and the separation of lexis and syntax, and associated ideas (affixation, inflection / derivation, compounds, multi-word units, lexical phrases, chunks, collocations, discourse, cohesion / coherence), including the inherently "fuzzy" nature of language, which allows learners to be more confident with approximations. • The importance of frequency, lexical density, type/token ratios etc.; semantics, polysemy, homonymy, part of speech, etymology, borrowings, cognates, polysemy, translation, etc.

Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université.

In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96.

• The importance of text type (genre / register, style, speech vs. writing, etc.) and variation, especially between different varieties of English (geographic, socio-economic, diachronic, etc.); the concept of "error" and standard vs. non-standard varieties.

Clearly, a course such as this is not primarily concerned with language learning per se, but language was by default the mainstay of all projects. Where the specific research question had a language focus, one would hope that the point concerned would be better understood. More importantly as regards language learning, the processes involved are likely to increase noticing and sensitivity to various aspects of language, leading to more efficient learning in the future [START_REF] Allan | Data-driven learning and vocabulary: Investigating the use of concordances with advanced learners of English. Centre for Language and Communication Studies[END_REF]). To use a hackneyed analogy, it would seem better in the long term to provide the hungry with a fishing rod rather than with a fish. As Kettemann and Marko (2004:171) point out, "we do not want to make students aware of something, we want them to be able to become aware on their own." This kind of language awareness is what [START_REF] Mair | Empowering non-native speakers: The hidden surplus value of corpora in continental English departments[END_REF] calls the "hidden surplus value of corpora in continental English departments."

The first port of call for many students was a range of standard reference works, either on paper or on line. These include the dictionary, thesaurus, textbook, usage manual, grammar book, style guide, and so on. Notoriously, such materials are found to be inaccurate descriptions of what actually happens in many languages. Confronting such reference works with corpus evidence, learners might be expected to use them more efficiently or more sensitively in future; even when they are "accurate", they can rarely be complete, and a corpus approach can be seen to add to this (c.f. [START_REF] Dodd | Exploiting a corpus of written German for advanced language learning[END_REF]. A degree of scepticism and critical thinking is also needed in regard to teacher explanations and folk wisdom-not to mention the learner's own ideas-including an ability to think of alternative explanations for apparently simple phenomena [START_REF] Kettemann | Can the L in TALC stand for literature?[END_REF].

The entire process as outlined here involves a great deal of "authenticity" of text, purpose and activity [START_REF] Stevens | Concordancing with language learners: Why? When? What?[END_REF], whether as the main focus or more indirectly, almost as a form of task-based learning. This includes following the course, understanding and using the tools in English, and encountering and manipulating tremendous quantities of English data in their chosen corpora. The lack of an explicit "language learning" aim is therefore not only not a problem, but can be seen as a positive advantage (Kennedy & Miceli 2001); Aston (1997b:210) even claims that learners should "approach the corpus with non-linguistic goals." If, as Johns (1991b:30) has claimed, "effective language learning is itself a form of linguistic research," one wonders if the converse might not also be true in many cases, i.e. that linguistic research can be an effective form of language learning (see also [START_REF] Boulton | Habeant Corpus-They should have the body. Tools learners have the right to use[END_REF]. Following Maia's 1997 article, "making corpora: a learning process", Mauranen (2004:99) claims that "corpus skills constitute a learning task in themselves… Once acquired, they facilitate learning greatly and need not be constantly refreshed."

In such a course, students inevitably discover an awful lot about using ICT, in particular using the internet more efficiently: finding and evaluating sources of texts and tools, advanced functions of search engines and query language, recovering broken links, downloading and copying. Students often found the tools themselves, which means they should be better able to do so again in the future as resources come and go; they also had to find out how to use them by experimentation, as complete description is not possible-or, as we have argued, desirable-in the course itself. We could also mention OCR scanning, transcription, screen shots and image editing, among other things.

The students learned a lot about more advanced functions of software they thought they were already familiar with, in particular MS Word and Excel. This includes the manipulation of layout and formatting, tables and graphs, images and automatic tables of contents, page numbering, spell-checking in different varieties, keyboard shortcuts, find and replace functions, etc. As the final research paper was in English and in a style similar to professional academic articles, there was the obvious opportunity of providing a style guide similar to those frequently found in professional linguistics journals (c.f. Jackson 1997). If these students do continue to do research, they should be better placed to produce an appropriate paper, with considerations of format, structure, style, the importance of evidence, the respective merits and roles of quantitative and qualitative approaches, interpretation and generalisation, background reading and referencing, and so on. Some students took the initiative to contact researchers, publishers and copyright holders in their chosen area; some explored local university and other resources; some acquired experience in doing fieldwork. Most should also be more sensitive to the dangers of preconceived ideas in the formulation of their research.

Finally, the whole problem-solving, hands-on approach using limited tools encourages all sorts of useful behaviour which may serve in future (see also Kettemann & Marko 2004:185ff). The initial idea has to be formulated and framed in such a way that it becomes a valid topic for research; frequently the original research questions cannot be addressed with a simple command, requiring considerable lateral thinking to overcome the problemsespecially given the limits placed on much of the free demonstration software. For example, creative thinking was required to formulate a number of separate but compatible search requests to obtain more than 50 hits with BoE or BNC, how to measure syntactic complexity, alignment of different texts, and so on. Students also needed to be aware of issues of repetition (e.g. with song lyrics), features of spoken vs. written language, non-standard spellings, the presence of other languages or non-Roman scripts. Even if not confronted with these specific problems again, the fact that they have had to solve such problems in the past would seem to put students in better stead to solve new problems as and when they arise in the rest of their studies and beyond.

Conclusion: If…

Corpus linguistics is a hugely promising field with a wide variety of applications, but in language courses as a whole, the uptake so far has been extremely limited. If CL is to play any role here beyond materials development and syllabus design, we have to make sure it is not perceived just as "trendy" research that is irrelevant to the working teacher and the "average" learner (c.f. Braun 2005:47, 48). For language learning as such, we need to show that CL can almost immediately produce results which are "illuminating and very helpful" (Sinclair 2004:288).

Clearly, such an approach increases learner autonomy, but more than that "it is now possible for the learner to be autonomous in ways it was previously impossible" (Uzar Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université.

In N. Kübler (ed.), Selected papers from Teaching andLanguage Corpora 2006. Frankfurt: Peter Lang, p. 69-96. 2003:153). This applies whether the explicit focus is on language or something else, and the crossover for learners like ours is essential if they are to keep using CL in any form. Several are applying it in their other studies, including the long Master's dissertation and subsequent doctoral thesis; others have provided informal feedback that they also use it outside the academic context. The crossover is by no means guaranteed: [START_REF] Chambers | Integrating corpus consultation in language studies[END_REF] found her students in Ireland had great difficulty in making the connection. This might be in part because her students were focusing exclusively on language questions, while a wider range of non-linguistic questions and greater room for learner choice to follow their own interests may maximise the versatility of corpus methodologies.

If corpus consultation… is to become a common activity for learners across the broad spectrum of language studies (general language learning, literary studies, languages for specific purposes, translation, etc.), it would seem necessary for developments to take place in a broader context… It is perhaps outside the classroom that the next important step in research in this area will take place. (Chambers 2007:13) Like Römer (2006:105), our aim has been to "equip our students with a tool box, containing skills that are transferable from problem to problem across sub-disciplines." Multifunctional tools tend to be kept to hand and used repeatedly, while specialist tools tend to be confined to a dusty drawer and brought out on rare occasions, if they are not forgotten altogether.

Although the tools generally used in CL may have been designed with specific purposes in mind, it is a tribute to their power, simplicity and versatility that they can find a range of other uses. Screwdrivers too can be used for opening tins of paint. In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96.

the internet using FileMaker; this allows analysis of the definition of "word" claimed for the 850-item vocabulary of BE. Comparative statistical analysis is conducted using Larkin and VocabProfiler.

Analysing Sting and the Police lyrics.

Two corpora from different periods are compiled from the internet (10K and 25K words) to analyse maturity of Sting's lyric writing. Focus is on the most frequent nouns using MonoConc (also WST), plus a qualitative interpretation for semantic fields, KWICs and collocates etc.; the results are checked against BNC written / spoken frequency lists.

4. The religious vocabulary in U2's lyrics.

A musical interest in the group instigates background research into their religious inclinations. Lyrics are compiled from the internet in two periods (8K and 12K words), and analysed for religious/spiritual lexical words and collocates using WFI and WST.

5. Fancy, like and love: Expressing liking and feeling. Words don't always have one-to-one translations, and emotions are a particularly difficult area. After looking in dictionaries and comparing translations, these verbs were checked in BoE and BNC, as well as W3Corpora. To Google or Not to Google was used for the target items plus someone or something.

6. Building a lexical map of Nick Cave's lyrics: From The Boys Next Door to The Bad Seeds.

The lyrics were found on the web and compiled into two unbalanced corpora from different periods (5K and 30K words), and compared against BNC spoken corpus (keyword frequencies, collocates etc.). WFI and WST were used but a qualitative comparison was necessary too for syntax and meaning, collocation and idiom, concentrating on emotive lexical words.

7. Should we enquire or inquire about something? Different varieties of English lead to confusion, and dictionaries and etymology are of little help. Usage is compared in relevant subcorpora of BNC and BoE, then in home-built corpora from GB and US newspapers and literature on line (total 25K words). Analysis of key words in each corpus is conducted with WFI and WST: frequencies, collocates and KWICs are used to compare meanings.

8. The different meanings and uses of the word concern and its derivative forms in Tony Blair's and George Bush's speeches. This paper unites interest in politics and the intuition that concern is a "false friend": dictionaries etc. are found unconvincing regarding meaning and use. Two corpora of semi-scripted interviews are compiled from GB and US leaders (120K words each) and tagged with CLAWS. MonoConc and WFI were used to compare different senses and uses in the corpus against frequencies in Brown and BNC, the web as corpus using Virtual Language Centre, KWICs and frequencies etc.

9. The simplified novel, a case study: The Fall of the House of Usher.

The interest here is in literature and reading in a foreign language, as well as vocabulary acquisition. Corpora are compiled from an original text (7K words) and an OCR-scanned simplified reader (2K words). VocabProfiler is used to compare vocabulary (Anglo-Saxon vs. Greek/Roman/French origins), as well as CLAWS and WST for syntax, text structure, omissions and additions; the tagging also helps to check against the simplification guide.

The results are compared against 5 other similar novels from the internet.

10. Civilise or civilize? A comparative study of the verbs ending in -ise and those ending in -ize and their respective nominalizations.

A common problem where dictionaries, usage manuals, etymology and style guides are not much help. On the claim that it may be a US/GB distinction, examples are checked against Brown and BNC, including BNC frequency lists. Two balanced corpora (12K words each) are then compiled from online newspapers from GB (Guardian, Independent, Telegraph, Mirror) and the US (USA Today, Washington Post, New York Times, LA Times) over two weeks. Other software includes SwitchBoard, VocabProfiler and WST.

11. Stylistic features of verbatim reports of speeches in European Parliament debates: The passage from the tapes to the final transcripts.

Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université.

In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96.

Working at the European Parliament with access to in-house and published documents generates an interest in politics and speech/writing differences. The corpora are from unscripted EP speeches by native speakers, comparing transcribed and subsequently edited texts (6K words each). The focus is on spoken features, redundancy, syntactic complexity, connectors, etc. using WFI and WST.

12. Near synonyms for woman in Old English: A corpus analysis.

Interests lie in literature, language evolution, the "real" meaning of words, and gender studies. Synonyms from OE dictionaries are checked against online parsed OE corpora (1.5M words of prose, 75K words of verse); relevant segments are analysed with WFI.

13. A corpus-based approach to the speeches made by Gerry Adams since the Good Friday Agreement.

Contacts in Ireland and subsequent interests in politics and history inspire a corpus analysis of Adams' speeches before (45K words) and after (62K words) the GFA, as well as Paisley's speeches (16K words), all from the internet. The corpora are tagged with CLAWS, frequencies checked with WFI, and compared to each other and to BNC subcorpora and frequency lists; the focus is on aspects of scripted speeches with written/spoken styles. All corpora are checked for religious words and loaded language (WFI, WST) and against WebCorp on British newspapers.

14. Some French words and expressions in English.

Motivated by the crackdown on Anglicisms in France, this looks at the causes of borrowing and the shared history of French and English: frequency, prestige, accommodation, semantic shifts, etc. Definitions and etymology of French borrowings and Anglo-Saxon synonyms are compared in monolingual and bilingual dictionaries, then in KWICs and frequencies in different subcorpora of the BoE. There is also a comparison between US and GB English from editorials of newspapers (total 36K words) using WFI.

15. E-words.

From a combination of previous experience in linguistics (including lexicology) and the IT focus of the course, ICT words are noted in general contexts. New coinages are compared against BoE and BNC frequency lists, and subsequently against an international newspaper corpus (35K words) as well as the web as corpus (Yahoo in different languages). Relevant contexts are downloaded and pasted into WFI.

16. Make or do? That is the question.

A long-standing dissatisfaction with traditional explanations of generalisations or idiosyncrasies is reason to check synonyms and translations for common points in BNC and BoE. A corpus is built from the internet Guardian Friday review and analysed with WST, looking also at translations back into French. Semantic, collocational and structural patterns are compared. 17. Land, country, nation and state: Scotland in focus.

In interest in politics combines with time spent in Scotland and an awareness of Scottish autonomy. Dictionaries and etymology are found unconvincing for the target words, so two corpora are compiled from the Guardian and SNP websites (27K words), the key words checked for frequency, collocates and KWICs with relation to Scotland using WFI, and compared against BoE.

18. Latin and French derivatives in the Guardian Unlimited and the Sun Online.

French borrowings are frequently claimed to be more formal in English. After historical research, corpora were built using pgAdmin to compare the prestigious Guardian and the popular Sun newspapers (12K words each); the corpora were further subdivided into news (more formal) and sport (more popular). Typically French affixes were the key focus.

19. A corpus comparison of the verbs speak and talk.

These verbs posed problems in translation which dictionaries did not answer satisfactorily. BoE was used to provide KWICs of the verbs + preposition, as well as collocates, and translations back to French were compared.

A study of two phrases:

To go on strike and to be on strike.

Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université.

In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96.

Current affairs prompts an interest in phrasal uses of strike, which are compared first with French and German equivalents. The key words are analysed in BoE and then a newspaper corpus of Guardian articles covering the event, and the relevant text is checked for various uses, PoS, idioms, argument structure etc.

21. French words in English cuisine.

An enthusiasm for cooking and English recipes prompts interest in borrowings and history. BNC and BoE KWICs allow analysis of accommodation in morphology and structural use as well as semantic coverage against French. A corpus of recipes is compiled from MSN, downloading borrowings for analysis with WFI.

22. Freedom and liberty in George W. Bush's speeches.

Reactions to 9/11 highlight one Germanic / French doublet in English with a lack of comparable difference in French. This leads to work on etymology, but dictionary differences remain unclear. A corpus is built from the official White House site (27K words) and fed into WST; KWICs, WFI and structure provide insight to differences in meaning and use.

23. Corpus study of non-standard past irregular verbs in Appalachian and Piedmont English.

Living in the US, this student's family is critical of the non-standard local variety (especially do). The corpus is largely from a university audiobank supplemented with 3 local informants: interviews, own transcriptions of relevant past forms and focus on the 7 commonest verbs. There is a comparison of (non-)standard usage and variation of form and meaning by register and speaker using tools from the same university.

24. Clone, cloned, cloning: New terms in modern language.

A retired doctor looking at "new" words since Dolly was cloned in 1997, especially as a buzzword outside medicine. WFI and WST were used to compare 4 registers (20K words each) of fiction, newspaper, science and "ethical" texts; collocates by PoS.

25. Technical and non-technical uses of forestry terms.

Working at the Office Nationale des Forêts and reading professional documents, this student compiled two corpora of journal abstracts in fundamental (42K words) and applied (84K words) forestry science over a 4-year period. Lexical word frequencies are compared using WST, as well as technical and sub-technical language (especially compounds), and KWICs for semantic coverage and reference.

26. Is the verb to maintain solely reflective of the maintenance department activity or is it part of the group of synonyms to repair and to fix? Working at LuxAir with access to trade documents, this paper presents a home-built corpus from professional documents comprising 6 manuals (230K words). The corpus is parsed with the Tilburg parse tagger, and WST is used to check frequencies, KWICs and collocates against BNC and BoE.

27. Investigating use of the adverbs barely, hardly and scarcely.

Meaning and usage (syntax) are confusing for these words, and dictionaries and manuals unconvincing. To find out more, frequencies and collocates are gathered from BNC and BoE, as well as KWICs for word order; also WST for a minicorpus.

28. The initiatory book: A case study of The Little Prince. An interest in literature and "initiatory" books as well as translation prompts the comparison of two corpora of the novel in French and English (17K words) from the internet. Ten "interesting" words of f>5 were compared against BoE (frequencies and KWICs); a quantitative and qualitative analysis of translation is provided, especially pronominal replacement; WFI is also used.

29. British and American spelling-meter or metre? That is the question. Different spellings can be intriguing but confusing. After researching historical, cultural and pronunciation differences (homophones etc.) between the two varieties, some of the commonest differences are picked out. These are checked in BoE and BNC subcorpora for frequency and collocates. Two home-built corpora comprise GB and US papers using WFI.

30.

In in phrasal verbs.

Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université.

In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96.

Phrasal verbs often pose problems for French speakers. After checking definitions, the commonest ones are checked in BNC and BoE KWICs and frequencies, and compared against a home-built corpus of fashion magazines (23K words) using WST; other references include Switchboard, ICE and W3Corpora.

Table 2 :

 2 claim that CL can provide "an overarching theme" relevant to many if not all aspects of studies.Alex Boulton. CRAPEL-ATILF/CNRS, Nancy-Université. In N. Kübler (ed.), Selected papers from Teaching and Language Corpora 2006. Frankfurt: Peter Lang, p. 69-96. Popular topic areas.

	topic area	number of papers
	1. politics / current affairs	9
	2. music / popular culture	8
	3. history	6
	4. literature	5
	5. GB / US differences	5
	6. work	4
	7. translation / teaching-learning	4
	8. ICT	3
	9. personal contacts	3

5

A list of the titles plus a brief description of each paper can be found in Appendix B

This section provides an overview of the most popular tools, methods and corpora used by our students, followed by an analysis of the research topics chosen. We look at 30 research papers submitted between 2002 and 2005-not an entirely random selection, as it includes only those which were submitted electronically and got a mark of at least 60%. It also excludes papers which adopted a different approach from that described above: for example, some were more theoretical, while others discussed possible applications of CL in language teaching and learning, and so on. A list of the 30 papers can be found in Appendix B along with a brief summary of each.3.1. Corpora and corpus toolsSurprisingly perhaps, only six papers used the internet itself as a corpus with various tools: WebCorp, Edict VLC, and To Google or Not to Google; Google and Yahoo were also used for searching the web as corpus in various languages. This is perhaps disappointing given that

See e.g.[START_REF] Aston | The British National Corpus as a language learner resource[END_REF] and[START_REF] Bernardini | Systematising serendipity: Proposals for concordancing large corpora with language learners[END_REF] for exploiting large corpora in language learning.

A list of the most popular websites and software can be found in Appendix A.

http://www.eup.ed.ac.uk/journals/content.aspx?pageId=1&journalId=12801 accessed May 2007.

Appendix A. Popular websites

The URLs here were provided by the students themselves; all were operational at the time of the TaLC7 conference in 2006.

Large reference corpora:

• BNC (British National Corpus): http://www.sara.natcorp.ox.ac.uk

Appendix B. Research topics

Below are the full titles and a brief description of each of the 30 projects discussed in this paper, most recent first.

1. From screen-plays to fan fiction: A corpus-based study. Interest in reading and writing science-fiction leads to a comparison of home-built corpora from the internet of Star Trek scripts and amateur "fan fiction" (45K words each). WST is used for various functions to check "techno-babble" overall, then focusing on the 10 most frequent items in each.

Basic English versus Standard English.

Experience teaching English in Germany provokes interest in the core lexicon of English (in terms of frequency, coverage, Germanic origin). Corpora of literary texts in SE and Ogden's BE (25K words each) are compiled from


Etienne Huot

Isabelle Herlin

Nicolas Mercier

Gennady Korotaev

Evgeny Plotnikov

Motion estimation from satellite image sequences: validation

Introduction

The issue of surface velocity estimation from satellite images has been extensively studied in the literature [START_REF] Horn | Determining optical flow[END_REF][START_REF] Nagel | Displacement vectors derived from second-order intensity variations in image sequences[END_REF][START_REF] Cohen | Optical flow and phase portrait methods for environmental satellite image sequences[END_REF][START_REF] Mémin | Optical flow estimation and object-based segmentation with robust techniques[END_REF][START_REF] Béréziat | A generalized optical flow constraint and its physical interpretation[END_REF][START_REF] Corpetti | Dense estimation of fluid flows[END_REF][START_REF] Isambert | A multiscale vector spline method for estimating the fluids motion on satellite images[END_REF]. Data Assimilation (DA) techniques have been applied in the last five years and gain importance in the scientific community [START_REF] Herlin | Estimation of a motion field on satellite images from a simplified ocean circulation model[END_REF][START_REF] Papadakis | Image assimilation for motion estimation of atmospheric layers with shallow-water model[END_REF][START_REF] Béréziat | Solving ill-posed Image Processing problems using Data Assimilation[END_REF][START_REF] Titaud | Assimilation of image sequences in numerical models[END_REF]. The key points of the DA approach are: availability of heuristics on the dynamics of a satellite sequence, knowledge on links between velocity and image data.

This paper proposes an analysis and a validation of the DA approach for motion estimation from ocean satellite images. Two Image Models were proposed in [START_REF] Korotaev | Retrieving ocean surface current by 4-D variational assimilation of sea surface temperature images[END_REF][START_REF] Huot | Assimilation of SST Satellite Images for Estimation of Ocean Circulation Velocity[END_REF][START_REF] Huot | Estimating apparent motion on satellite acquisitions with a physical dynamic model[END_REF]. They express heuristics on the dynamic of the motion field. The comparison of the estimation using the two models allows us to analyze the impact of these heuristics. The main issue of the paper is then to validate the estimation approach by evaluating the quality of the result, compared to real data.

The motion estimation is performed with NOAA/AVHRR Sea Surface Temperature (SST) data acquired over the Black Sea. The analysis is conducted by comparing the stationary and the shallow-water heuristics. The validation is obtained by quantifying the discrepancy of the water layer thickness, estimated with the shallow-water image model, and the one computed from altimetry data. The altimetry measures, used in this study, come from the Envisat and GFO sensors.

The paper is organized as follows. Section 2 summarizes the principles of variational data assimilation. The definition of the Stationary Image Model (SIM) and Shallow Water Image Model (SWIM) is given in Section 3. Section 4 describes the application of DA to perform motion estimation. Section 5 describes the SST images (5.1), displays and analyzes the estimated motion result (5.2), describes the altimetry data (5.3), and validates the approach (5.4).

Variational Data Assimilation

Mathematical setting

Let X being the state vector depending on the spatial coordinate x (x = (x, y) for image data) and time t. X is defined on A = Ω × [0, τ ], Ω being the bounded 1 spatial domain and [0, τ ] the temporal domain.

We assume X is evolving in time according to:

∂X ∂t (x, t) + ▼(X)(x,t) = 0 (1)
▼, named the evolution model, is supposed differentiable.

Observations Y(x, t), for instance satellite image acquisitions, are available at location x and date t and linked to the state vector through an observation equation:

Y(x, t) = ❍(X)(x,t) + E O (x, t) (2)
In this paper, we assume that one component of X is directly comparable to Y.

Consequently, ❍ reduces to a projection operator. The observation error E O simultaneously represents the imperfection of the observation operator ❍ and the measurement errors.

We consider having some knowledge on the initial condition of the state vector at t = 0:

X(x, 0) = X b (x) + E b (x) (3)
X b is named background value of the initial condition and E b the background error. E b and E O are assumed to be Gaussian and fully characterized by their covariance matrices B and R.

Variational formulation

In order to solve the system (1), (2), (3) with respect to X having a maximal a posteriori probability given the observations, a functional E(X) is defined and minimized:

E(X) = A [Y(x, t) -❍(X)(x,t)] T R -1 (x, t)[Y(x, t) -❍(X)(x,t)])dxdt + Ω [X(x, 0) -X b (x)] T B -1 (x)[X(x, 0) -X b (x)]dx + Reg (4)
In this formulation, we consider no correlation of the errors between two spacetime positions. Reg is a regularization term used to obtain a convex function and allow the minimization process to converge to a global minimum. The minimization of E(X) is carried out with an iterative method based on the one described in [START_REF] Béréziat | Solving ill-posed Image Processing problems using Data Assimilation[END_REF] and summarized in the following.

At each iteration k, the analysis X k a is obtained from the background X k b by computing the increment δX at t = 0.

X k a (x, 0) = X k b (x, 0) + δX(x) (5)
1.

Initialization (a) k = 0 (b) Compute X 0 b (x, t) from the initial condition X b (x) of the state vector at t = 0 in (3): X 0 b (x, 0) = X b (x) (6)
∂X 0 b ∂t (x, t) + ▼(X 0 b )(x, t) = 0, for t = 0 to τ (7)
(c) Initialize the analysis X 0 a (x, t):

X 0 a (x, t) = X 0 b (x, t) ∀t ∈ [0, τ ] (8)
2. Repeat (a) Compute the adjoint variable λ from t = τ to t = 0:

λ(x, τ ) = 0 (9) - ∂λ ∂t (t)+ ∂▼ ∂X * λ(t) = ❍ T R -1 [Y(t)-❍X k a ], for t = τ to 0 (10) (b)
Update the value of the background variable:

X k+1 b = X k a (11)
(c) Compute the incremental variable δX at t = 0:

δX(x) = B(x)λ(x, 0) (12)
(d) Update the value of the analysis variable:

X k+1 a (x, 0) = X k+1 b (x, 0) + δX(x) (13)
(e) Compute X k+1 a (x, t) from the initial condition:

∂X k+1 a ∂t (x, t) + ▼(X k+1 a )(x, t) = 0, for t = 0 to τ (14) (f) k = k + 1 Until ||δX|| 2 ≤ ε 3. Final result is X k a .
Equation [START_REF] Béréziat | Solving ill-posed Image Processing problems using Data Assimilation[END_REF] makes use of the adjoint model ∂▼ ∂X *

. In our study, the discrete adjoint model is automatically obtained by the Tapenade software1 .

Image models

The two Image Models used in the paper are based on the assumption that a pixel value is a passive tracer transported by the surface velocity field. The state vector X includes the motion vector W and a tracer q that can be directly compared to the image observations. The evolution of q is given by the advection-diffusion equation:

∂q ∂t + W • ∇q = ν q ∆q (15)
with ν q standing for the diffusion coefficient.

The Stationary Image Model (SIM) is based on the restrictive assumption that, at each position, the velocity is constant over time. The underlying hypothesis is that the surface velocity field evolves much slower than the temperature field. This heuristic is acceptable for a large range of marine processes. If a vortex, whose spatial scale is more than 10 -50km, is transported with a velocity less than 0.1 to 0.5m/s, then the temporal scale of that phenomenon will be more than one day. It means that the surface velocity field can be considered as stationary during one day. Defining X = (u, v, q) T , with u and v the two components of the 2D motion vector W, SIM is defined as: However, the stationary hypothesis makes this image model only applicable on a short temporal window.

The shallow-water equations, derived from the Navier-Stokes equations, link the 2D velocity (u, v) of the layer to its thickness h and take into account the gravity and Coriolis forces. The state vector X is (u, v, h, q) T and the Shallow Water Image Model (SWIM) is defined as: with B = gh + 1 2 (u 2 + v 2 ), g the reduced gravity, f the Coriolis parameter (depending on the latitude), ξ the vorticity (ξ = ∂v ∂x -∂u ∂y ).

Application of Data Assimilation

Data Assimilation is applied to perform motion estimation. The sequence of SST images T (x, t) is assimilated in the two models SIM or SWIM, using the incremental method described in Section 2.2. As said in Section 2.1, the pixel value T (x, t) is directly comparable to the component q(x, t) of the state vector. The observation operator ❍ reduces to a projection operator, ❍(X(x,t)) = q(x, t). The regularization term is based on the L 2 -norm of the motion gradient (to obtain a smooth vector field) and on the motion divergency (incompressibilty assumption). Its impact is analyzed in [START_REF] Huot | Estimating apparent motion on satellite acquisitions with a physical dynamic model[END_REF]. As we consider perfect models, the value of X(t) is obtained from the initial conditions X(0) by integrating in time. Hence, the cost function (4) only depends on the initial conditions and is rewritten as:

E(X(0)) = A (T -q) T R -1 (x, t)(T -q)dxdt + Ω X(0) -X b T B -1 (x) X(0) -X b dx Ω α(|∇u| 2 + |∇v| 2 )dx + Ω β|div v| 2 dx (16)
The choice of the covariance matrix R is crucial for the quality of results. As the satellite images are provided with meta-data information (see section 5.1), the quality of the acquisitions is approximately known. R -1 (x, t) is then given a small value when the acquisition is noisy at (x, t) (because of cloud occlusion for instance). The choice of the initial background conditions has also a strong impact on the quality of the result. It has been discussed in [START_REF] Huot | Estimating apparent motion on satellite acquisitions with a physical dynamic model[END_REF] that the best results are obtained with the first observation as background for q, null value for W and a constant value h m for h, with h m being the thickness value at rest state. As the background value of q is reliable, B q is given a small value.

Results

Image data

A huge amount of images are acquired over the ocean by space remote sensors. Those obtained by optical instruments, such as Sea Surface Temperature (SST) data, display a high space-time coherence. The images, used in the paper, are acquired on-board NOAA-AVHRR satellites. Their spatial resolution is 1.1 km 2 at nadir and the temporal revisit is at best one day. However, several acquisitions over the same area are usually acquired on the same day by different satellites. Some of these data are contaminated by clouds or corrupted by noise.

Analysis

In this paper, motion estimation is tested on a sequence of four images, displayed on Figure 2. The cyan areas, on the third and fourth frames, correspond to missing data.

The two Image Models are used to estimate the surface velocity on these data. Figure 3 compares the motion fields estimated with SIM and SWIM, at t = 0. The results obtained with SWIM visualize a cyclonic vortex on the western part of the Black Sea. SWIM, due to its physical assumptions on the dynamic, permits a more realistic motion estimation and characterizes structures occuring on the sea surface. In comparison, the potential of SIM highly depends on the size of the temporal window compared to the dynamics involved during that period. That makes SIM no more relevant for data such as those displayed in Figure 2. In conclusion, the DA approach for motion estimation permits to retrieve the major currents of the Black Sea basin. Moreover, the high resolution of NOAA/AVHRR images allows to better evaluate the size of some well known mesoscale structures [START_REF] Korotaev | Retrieving ocean surface current by 4-D variational assimilation of sea surface temperature images[END_REF].

Altimetry data

Satellite altimeters provide an accurate measure of the Sea Level Anomaly (SLA) that corresponds to the sea surface deviation from its rest state (see the black curve on Figure 4).

The altimeters are nadir-pointing instruments providing an along-track acquisition. The coverage of Envisat1 over the Black Sea is for instance displayed on Figure 5. In this paper, we use altimetry measures provided by Envisat2 with a 35 days cycle and by GFO3 with a 17 days cycle.

Validation

The outputs of SWIM are W, the surface velocity, and h the thickness of the surface layer. The thickness anomaly, denoted h SW IM , is estimated from h as its deviation from the value at rest. On another hand, the altimeters are 1dimensional instruments measuring the Sea Level Anomaly, denoted h alt , along their tracks. We then compare h SW IM and h alt at the same positions. The physical formula linking these two quantities is:

ρ × h alt = ∆ρ × h SW IM ( 17
)
with ρ being the density of the upper layer, ∆ρ the difference of density between the upper and the lower layer, h alt the sea level anomaly measured by the satellite, h SW IM the thickness anomaly (h -h m ) of the shallow-water model. The number of altimetry measures available on the same space-time period than the SST data is rather small. However, we apply the conversion given in (17) and perform a quantitative comparison of h alt and h SW IM along a track. Figure 7 displays these curves for the two tracks displayed on Figure 6: on the left with Envisat and on the right with GFO. Black crosses locate the altimeter measures. The shapes and values of h alt and h SW IM curves are very similar. There is no error in the slope directions. The extrema are well localized. It is almost perfect in the case of Envisat. As the velocity field is strongly related to the shape of the thickness image, these promising results on thickness estimation validate the estimation of the motion. Figure 8 illustrates the link between velocity and thickness: a bump correspond to an anticyclonic velocity field and a bowl to a cyclonic one.

Conclusion

In this paper, we propose an analysis and validation of the data assimilation approach for motion estimation from satellite image sequences. We compared two dynamic assumptions, i.e. we assimilated the same data in two image models, SIM and SWIM, and analyzed motion results. Moreover, we used altimetry data to quantify the quality of the estimation. The comparison between the surface anomaly estimated by SWIM and measured by altimeters validates our approach.

Figure 1

 1 displays a SST image acquired over the Black Sea in October 2005, with the cyan color corresponding to clouds or noise.

Figure 1 :

 1 Figure 1: Cyan area corresponds to clouds or noise.

Figure 2 :

 2 Figure 2: SST data acquired from October 23th to October 24th, 2005.

Figure 3 :

 3 Figure 3: Motion estimation. Up: SIM; down: SWIM.

Figure 4 :

 4 Figure 4: Sea Level Anomaly.

Figure 5 :

 5 Figure 5: 35 days cycle of Envisat1.

Figure 6 :

 6 Figure 6: Two altimeter tracks displayed over the average of h SW IM .

Figure 6

 6 Figure 6 displays the value of h SW IM , averaged in time. The two straight lines represent altimeter tracks. The green line comes from Envisat and the pink one from GFO.The number of altimetry measures available on the same space-time period than the SST data is rather small. However, we apply the conversion given in (17) and perform a quantitative comparison of h alt and h SW IM along a track. Figure7displays these curves for the two tracks displayed on Figure6: on the left with Envisat and on the right with GFO. Black crosses locate the altimeter measures. The shapes and values of h alt and h SW IM curves are very similar. There is no error in the slope directions. The extrema are well localized. It is almost perfect in the case of Envisat. As the velocity field is strongly related to the shape of the thickness image, these promising results on thickness estimation validate the estimation of the motion. Figure8illustrates the link between velocity and thickness: a bump correspond to an anticyclonic velocity field and a bowl to a cyclonic one.

Figure 7 :

 7 Figure 7: Sea Level Anomaly, given by the altimeters, compared with the estimation with SWIM.

Figure 8 :

 8 Figure 8: From left to right. 1. 3D water layer thickness. 2. Its 2D projection. The magenta line figures the track of an altimeter. 3. SLA along this track. 4. Velocity field.

http://www-sop.inria.fr/tropics/

http://envisat.esa.int

http://ilrs.gsfc.nasa.gov/satellite_missions/list_of_satellites/gfo1_general.html