Götz Bokelmann
email: bokelmann@gm.univ-montp2.fr

Emeline Maufroy

Luisa Buontempo

José Morales

Guilhem Barruol

Testing oceanic subduction and convective removal models for the Gibraltar arc: Seismological constraints from dispersion and anisotropy

We discuss mantle structure and dynamics under the Gibraltar arc. For that region, a large variety of geodynamic models has been proposed, and there was, up to recently, no consensus on which of these models is the best one to describe the geodynamic situation of the region. The key to distinguishing the different models probably lies in the upper mantle, and we propose two specially adapted seismological techniques that help to distinguish each of the two models that are more frequently invoked. These are, on one hand, the subduction-rollback (SR) model, and on the other, continental delamination or convective removal (CDCR). The first technique that we use is based on observations of dispersed P-wavetrains that are expected for certain ray directions. Observations made on the Gibraltar arc suggest the presence of subducted oceanic lithosphere under the Alboran Sea. The second technique is based on shear-wave splitting of SKS phases, and it allows us to look for the radially-oriented flow that must be present in the mantle beneath the Alboran Sea, for a CDCR mechanism. There are however no indications of any radial flow up to this point. Both approaches therefore suggest that subduction-rollback is the best model to explain the complex geodynamic situation under the region.

Introduction

Oceanic lithosphere is recycled into the deep interior of the Earth at subduction zones. While the general principles of this conveyorbelt like motion are understood, this question is more open for continental lithosphere. It is clear though that continental subduction exists, for example from high-pressure phases (e.g. Coleman and Wang, 1995;[START_REF] Yamato | HP-UHP exhumation during slow continental subduction: self-consistent thermodynamically and thermomechanically coupled models with application to the Western Alps[END_REF].

Several models have been proposed to explain how, in which cases, and where continental recycling takes place. [START_REF] Bird | Continental delamination and the Colorado Plateau[END_REF] has proposed that this occurs via lithospheric delamination, while [START_REF] Houseman | Convective instability of a thickened boundary layer and its relevance for the thermal evolution of continental convergent belts[END_REF] have proposed convective removal of a dense overthickened lithospheric root. These two types of models have been invoked to explain various extensional basins around the Earth, and particularly extensional basins along the Alpine convergence zone between Europe and Africa, such as the Pannonian basin and the Alboran Sea basin (Fig. 1).

In this paper, we focus on the latter region, the Alboran Sea between Spain and Morocco, also addressed as 'Gibraltar arc region'. That region has been much studied, and a wealth of information is available about its surface tectonics (e.g. [START_REF] Dallmeyer | Pre-Mesozoic Geology of Iberia[END_REF][START_REF] García-Dueñas | Miocene extensional detachments in the outcropping basement of the Northern Alboran Basin and their tectonic implications[END_REF][START_REF] Jabaloy | The Mecina Extensional System: its relation with the post-Aquitanian piggy-back basins and the paleostresses evolution (Betic Cordilleras, Spain)[END_REF][START_REF] Crespo-Blanc | Extension versus compression during the Miocene tectonic evolution of the Betic chain. Late folding of normal fault systems[END_REF][START_REF] Martínez-Martínez | Mode of extensional tectonics in the southeastern Betics (SE Spain): implications for the tectonic evolution of the peri-Alboran orogenic system[END_REF][START_REF] Michard | How does the Alpine belt end between Spain and Morocco?[END_REF][START_REF] Martínez-Martínez | Active transfer fault zone linking a segmented extensional system (Betics, southern Spain): insight into heterogeneous extension driven by edge delamination[END_REF][START_REF] Balanyá | Structural trend line pattern and strain partitioning around the Gibraltar Arc accretionary wedge: insights as to the mode of orogenic arc building[END_REF]Jolivet et al., 2008, and others), current motion (e.g. [START_REF] Stich | Kinematics of the Iberia-Maghreb plate contact from seismic moment tensors and GPS observations[END_REF], magmatism (e.g. [START_REF] Turner | Magmatism associated with orogenic collapse of the Betic-Alboran Domain[END_REF], and crustal magnetics (see [START_REF] Michard | How does the Alpine belt end between Spain and Morocco?[END_REF]. In addition, there is considerable information from seismicity in the region, focal mechanisms [START_REF] Stich | Moment tensor solutions for small and moderate earthquakes in the Ibero-Maghreb region[END_REF], as well as petrogeochemical data from magmatic rocks [START_REF] Duggen | Magmatic evolution of the Alboran region: the role of subduction in forming the western Mediterranean and causing the Messinian Salinity Crisis[END_REF][START_REF] Duggen | Post-collisional transition from subduction-to intraplate-type magmatism in the westernmost Mediterranean: evidence for continental-edge delamination of subcontinental lithosphere[END_REF], large-scale mantle outcrops (e.g., [START_REF] Vauchez | Seismic properties of an asthenospherized lithospheric mantle: constraints from lattice preferred orientations in peridotite from the Ronda Massif[END_REF] and xenoliths [START_REF] Turner | Magmatism associated with orogenic collapse of the Betic-Alboran Domain[END_REF].

Nevertheless, this wealth of information has apparently not lead to a consensus on the type of model that best explains the geodynamic situation and evolution in the region. In the contrary, there is an ongoing debate as to that effect [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Gutscher | Reply to comment "Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Platt | Comment to "Evidence for active subduction beneath Gibraltar[END_REF]. For resolving structure and dynamics of a region, much hope is generally attributed to seismic tomography, and that type of constraint has been extensively used in recent years (e.g., [START_REF] Blanco | The P-wave velocity structure of the mantle below the Iberian Peninsula: evidence for subducted lithosphere below southern Spain[END_REF]Calvert et al., 2000a;[START_REF] Wortel | Subduction and slab detachment in the Mediterranean-Carpathian region[END_REF]. However, ray coverage in the area is not favourable, and as explained in more detail below, resolution is thus limited. In addition, seismic tomography cannot distinguish easily between oceanic and continental lithosphere, since both appear in tomographic images with similar velocity. In fact, average velocities of the two types of lithosphere are nearly identical.

Our approach is instead based on directly testing the geodynamic models, using adapted seismological techniques. We extract essential and required features from each model such as associated structure and mantle flow, and we present two techniques that allow to critically test each of the two models, and to distinguish them. The key to understanding the geodynamic situation of the region probably lies in the upper mantle, and we thus focus our attention on that depth region, and on the seismic waves that propagate through it. We use constraints based on observations of P-wave dispersion that are indicative of anomalous (layered) structure along the propagation path, and also seismic anisotropy, since it is capable of constraining the geometry of mantle flow, which is also rather indicative of the kind of geodynamic environment.

Test case of lithospheric recycling: the Alboran Sea region

The geodynamic situation of the Alboran Sea region is typically approached by considering one of the two large-scale tectonic processes that occurred in the Mediterranean area, that is, a) the Cenozoic opening of the western Mediterranean, thus addressing the role of the extension and formation of oceanic lithosphere, and b) the collision of Europe and Africa, thus addressing the role of the continental lithosphere during the collision. The many geodynamic models that have been proposed for the region thus typically fall into one of two categories, either 'oceanic models' or 'continental models'. The ocean models suggest subduction of oceanic lithosphere that is either ongoing or has ceased its activity. Different dip directions of the subduction are considered in these models [START_REF] Torres-Roldan | An early Miocene arc-tholeitic magmatic dike event from the Alboran Sea: evidence for precollision subduction and back-arc crustal extension in the westernmost Mediterranean[END_REF][START_REF] Sanz De Galdeano | Geological evolution of the Betic Cordillera in the western Mediterranean, Miocene to Present[END_REF][START_REF] Royden | Evolution of retreating subduction boundaries formed during continental collision[END_REF][START_REF] Docherty | Evidence for the eastward migration of the Alboran Sea based on regional subsidence analysis: a case for basin formation by delamination of the subcrustal lithosphere?[END_REF][START_REF] Lonergan | Origin of the Betic-Rif mountain belt[END_REF][START_REF] Morales | Active continental subduction beneath the Betic Cordillera and the Alborán Sea[END_REF][START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF], but most models assume an eastward-dipping subduction. On the other hand, 'continental models' propose past or present subduction, delamination, or convective removal of overthickened continental lithosphere [START_REF] Platt | Extensional collapse of thickened continental lithosphere: a working hypothesis for the Alboran Sea and Gibraltar Arc[END_REF]Seber et al., 1996a;[START_REF] Morales | Active continental subduction beneath the Betic Cordillera and the Alborán Sea[END_REF]Calvert et al., 2000a). These two types of models are illustrated in Fig. 2.

Geological indications of a westward migration of the orogenic arc suggested the presence of an 'Alboran block' [START_REF] Andrieux | Sur un modèle explicatif de l'arc de Gibraltar[END_REF]) that escaped to the West during the North-South convergence. An Calvert et al., 2000a): The two types of geodynamic models that we test in this paper, a) the retreating subduction model of [START_REF] Lonergan | Origin of the Betic-Rif mountain belt[END_REF] and [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF], and b) the convective removal model of [START_REF] Platt | Extensional collapse of thickened continental lithosphere: a working hypothesis for the Alboran Sea and Gibraltar Arc[END_REF]. For explanations see the text. eastward-dipping subduction zone rolling back toward the West (Fig. 2a) would naturally explain this, and also the extension that has subsequently been identified, and incorporated into the model [START_REF] Royden | Evolution of retreating subduction boundaries formed during continental collision[END_REF]. The latter author suggested that arc migration has actually continued beyond Gibraltar, while other authors have suggested that the motion has stopped retreating in the Gibraltar area [START_REF] Sengör | Some current problems on the tectonic evolution of the Mediterranean during the Cainozoic[END_REF][START_REF] Lonergan | Origin of the Betic-Rif mountain belt[END_REF]). Yet other models suggest that a Northwest dipping slab has subducted beneath Iberia, and has later broken off, producing uplift and extension. It would still be positioned steeply under Andalusia [START_REF] Blanco | The P-wave velocity structure of the mantle below the Iberian Peninsula: evidence for subducted lithosphere below southern Spain[END_REF][START_REF] Zeck | Betic-Rif orogeny:subduction of Mesozoic Tethys lithosphere under eastward drifting Iberia, slab detachment shortly before 22 Ma, and subsequent uplift and extensional tectonics[END_REF].

The alternative set of models explains the anomalous mantle under the Alboran Sea by considering recycling of continental lithosphere by delamination or by convective removal. Fig. 2b illustrates the latter. [START_REF] Platt | Extensional collapse of thickened continental lithosphere: a working hypothesis for the Alboran Sea and Gibraltar Arc[END_REF] have suggested, based on PT-paths of rocks across the Gibraltar arc that convergence has lead to considerable lithospheric thickness, and that the lithosphere became gravitationally unstable and was removed and replaced by low-density asthenospheric material, resulting in uplift and extension. [START_REF] Platt | Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean[END_REF] proposed that removal of the lithosphere occurred at a (shallow) depth of about 60 km. Another way to remove continental lithosphere is by delamination [START_REF] Bird | Continental delamination and the Colorado Plateau[END_REF]. A model of SE migrating delamination has been applied to the Alboran Sea region by [START_REF] Docherty | Evidence for the eastward migration of the Alboran Sea based on regional subsidence analysis: a case for basin formation by delamination of the subcrustal lithosphere?[END_REF] and is coherent with the distribution of seismicity hypocenters [START_REF] Mezcua | Seismological evidence for a delamination process in the lithosphere under the Alboran Sea[END_REF][START_REF] Morales | Active continental subduction beneath the Betic Cordillera and the Alborán Sea[END_REF], and with the variation of seismic velocities found in the mantle below the Alboran Sea (Seber et al., 1996b). It also has been suggested that Iberian and African lithosphere has peeled back from the East to the West (Seber et al., 1996a), this process being initiated by a gravitationally unstable lithospheric root under the Internal zone of the Gibraltar arc.

Proponents of the two types of models, 'oceanic models' versus 'continental models', have lead a debate over the last decade. Arguments for and against each of the two models have been layed out clearly in a comment and reply series [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Gutscher | Reply to comment "Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Platt | Comment to "Evidence for active subduction beneath Gibraltar[END_REF]. The authors had suggested that discriminating tests should be proposed. In the following we present techniques that allow to critically test each of the two models using seismic waves that propagate through the zone. The upper mantle under the Alboran Sea is indeed clearly anomalous. This is most easily seen by inspective the variation of teleseismic station delay times across the region (e.g., Calvert et al., 2000a). Tomographic studies have proposed the presence of a high-velocity anomaly (see below).

We propose that distinguishing the two types of models requires resolving two questions, 1) what is the nature of that high-velocity anomaly, e.g., is it composed of oceanic or of continental lithospheric material?, and 2) what is the shape of the anomaly? We will suggest below that up to recently, neither of these two questions has been well-resolved. We will thus focus on these two questions, and propose techniques that are designed to directly address them. Before that, we address seismic tomography in the area.

Tomography studies have been performed for the area ever since the early 90s [START_REF] Blanco | The P-wave velocity structure of the mantle below the Iberian Peninsula: evidence for subducted lithosphere below southern Spain[END_REF][START_REF] Plomerova | Teleseismic P-residual study in the Iberian Peninsula[END_REF][START_REF] Spakman | Travel-time tomography of the European-Mediterranean mantle down to 1400 km[END_REF]Seber et al., 1996b;[START_REF] Piromallo | Imaging the Mediterranean upper mantle by P-wave travel time tomography[END_REF][START_REF] Bijwaard | Closing the gap between regional and global travel time tomography[END_REF]Calvert et al., 2000a). All of these studies document the presence of a high-velocity anomaly under the Alboran Sea. Most of them used ISC travel times, but Seber et al. (1996b), [START_REF] Gurria | Crustal and upper mantle velocity structure of Southern Iberia, the sea of Alboran, and the Gibraltar arc determined by local earthquake tomography[END_REF]Calvert et al. (2000a) also included data from the Moroccan national and/or Spanish network. There are also studies of crustal structure in the area (e.g., Calvert et al., 2000b;[START_REF] Villasenor | Ambient noise surface wave tomography of the Iberian Peninsula: implications for shallow seismic structure[END_REF][START_REF] Serrano | Seismic anisotropy and velocity structure beneath the southern half of the Iberian peninsula[END_REF]and others).

The resolution of tomographic models depends greatly on the seismic ray coverage at depth. Fig. 3a shows a rose diagram indicating the number of P-waves arriving at one of the seismological stations in the area (Ceuta). Note that there are many arrivals from East and West, but only few from North and South. Ray backazimuthal coverage is clearly not complete. This is accentuated by the sea/land distribution in the area that has necessarily led to a gap in instrumental coverage. Only recently, OBS have been installed in the Alboran Sea and the Gulf of Cadiz to alleviate that lack of coverage.

The incomplete ray coverage in the area indicates that it is necessary to inspect resolution capability of tomographic models obtained for the area. Given the asymmetry of ray density, it will probably be easier to define the EW extent of structures in the subsurface than their NS extent. Calvert et al. (2000a) have taken much care with extracting arrival times from 96 regional stations in Morocco and Spain. Thus they were able to update the ISC arrival time dataset by about 200 regional and teleseismic events. The tomographic inversion was based on a nonlinear maximum likelihood formalism. First, the best-fitting 1D model was constructed that served as initial model for the 3D inversion. Resolution in resulting 3D models can be judged by inspecting the hit count, standard errors from the a-posteriori covariance matrix or from bootstrap resampling, the resolution matrix elements, and all of these tests were performed by Calvert et al. (2000a). The most commonly used test is the 'spike test', where synthetic data are generated by ray tracing through a given velocity model that consists of a regular pattern of high-and low-velocity anomalies. With some noise (random delays) added, these synthetic data are inverted, and the recovered model is compared with the input model.

Regions where the output model matches the input model are regarded as well-constrained, while regions that show substantial difference between the two models are regarded as badly-constrained. There are issues as to the geometry of the synthetic test, and as to the choice of the block size. In particular, a spike test does not give any information on the range of other models that cannot be recovered by the inversion, but these issues are beyond the scope of the present paper.

Fig. 3b shows an East-West profile through the spike test input model, at the latitude of Gibraltar. Ray tracing through that model produced synthetic travel times, to which representative random delays were added. In addition, the block parameterization of the inversion was shifted relative to the input model. Fig. 3b also shows the output model. Comparison with the input model indicates that most of the anomalous blocks show up after the inversion. Resolution is typically better in the deeper layers than the shallower ones, due to the higher number of crossing ray paths. Thus blocks at shallow depth and at the edge of the station coverage are less-well imaged. Such a comparison would typically be judged as a success, since most blocks were recovered. However, as to the interpretation of the shape of the anomaly, there is a caveat. There is significant smearing toward the East, apparently along the paths of the majority of the rays. Such smearing produces tomographic images that resemble an elongated dipping feature, even though the true shape of the feature is perhaps just like that chosen in the input model. Even though individual blocks are well-resolved, the interpretation in terms of geodynamic models is obviously much lesswell constrained. In a way, we are more interested in the validity of the interpretation rather than that of the tomographic model, on which the interpretation is based on. Neither of the above mentioned tests so far has addressed the validity of the interpretation. Clearly, we need more technical development in that direction to assure that we make the best use out of the very valuable information that tomography offers.

As far as our specific objectives are concerned, namely determining the nature of the anomaly, tomography has yet another limitation though: oceanic and continental lithosphere are associated with rather similar velocity anomalies, unless very high resolution can be achieved, and they can hardly be distinguished on that basis. The strength of tomographic constraints for understanding the nature of the anomaly will for that reason always remain limited. Its strength is rather in addressing the second question posed above, that of the shape of the anomaly.

Testing the oceanic model: dispersion of body waves

We will now return to the geodynamic models of Fig. 2, and address tests that can be set up to determine the nature of the anomaly, e.g., whether the anomaly consists of oceanic or of continental lithosphere. The 'retreating subduction' model of Fig. 2a suggests the presence of a 'slab', that is, subducted oceanic lithosphere, under the Alboran Sea.

The typical East-West ray distribution that we have seen in Fig. 3a, which poses a problem for tomography, has the advantage that some rays may propagate more or less parallel to the anomalous feature in the upper mantle. If that feature does indeed represent subducted oceanic lithosphere, it is somewhat likely that it still contains the oceanic crust/mantle series and that it forms thus an inclined layer cake. Waveforms of seismic waves that propagate parallel to such a structure (Fig. 4a) can easily be modelled using seismological modelling tools (e.g. [START_REF] Gubbins | Dispersion of P waves in subducted lithosphere: evidence for an eclogite layer[END_REF][START_REF] Abers | Seismic low-velocity layer at the top of subducting slabs: observations, predictions, and systematics[END_REF][START_REF] Martin | Subduction zone wave guides: deciphering slab structure using intraslab seismicity at the Chile-Peru subduction zone[END_REF]. In fact, waveforms of body waves propagating along the slab may attain similar properties than surface waves, namely a frequencydependent propagation velocity (dispersion) that is due to different frequencies being sensitive differently to the layer. Higher frequency components can propagate within the low-velocity crustal portion of the layer cake, while lower frequencies are more sensitive to average velocities. This should produce a 'normal' dispersion, where high frequencies around 5 Hz are delayed by about a second with respect to lower frequencies around 0.5 Hz (Fig. 4b). Such a dispersion has been observed in several subduction zones around the Pacific [START_REF] Abers | Dispersion of regional body waves at 100-150 km depth beneath Alaska: in situ constraints on metamorphism of subducted crust[END_REF][START_REF] Abers | Seismic low-velocity layer at the top of subducting slabs: observations, predictions, and systematics[END_REF] and in the following, we show evidence indicating that such a phenomenon is also present for waves that propagate below the Gibraltar arc.

Fig. 5 shows waveforms from two teleseismic events observed at two stations, MTE in Portugal and CEU in Ceuta. The latter station has a fortunate location, in that it is positioned along the continuation of the high-velocity anomaly in tomographic models (see for example [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF]. That station is thus at the ideal location for looking for the described dispersion phenomena. On the other hand, the station in Portugal serves as a reference station, so that we can assure that any observed dispersion is really due to the anomalous upper mantle under the Alboran Sea, and not due to effects at larger distance or due to the earthquake source.

Fig. 5c shows P-waves arriving from the East, for an earthquake occurring in Crete. The seismograms were filtered in a series of frequency bands. Waveforms are complicated, and smoothed envelopes are useful for tracing the arrival time across the different frequencies. The envelopes show an interesting difference between the two stations. While all frequencies arrive at about the same time at the reference station MTE, there is a clear dispersion at station CEU. High frequencies arrive up to about one second after the lowfrequency arrivals. This effect is similar to that seen in numerical modelling (Fig. 4b), and also to the observations from known subduction zones around the Pacific [START_REF] Abers | Dispersion of regional body waves at 100-150 km depth beneath Alaska: in situ constraints on metamorphism of subducted crust[END_REF][START_REF] Abers | Seismic low-velocity layer at the top of subducting slabs: observations, predictions, and systematics[END_REF]. The P-waves arriving from the West, in Fig. 5a, do not show such a difference between the two stations. In fact, there is hardly any frequency dependence of arrival times at the two stations, somewhat similar to the arrival from the east at MTE. The anomalous behaviour that requires explanation is the Eastern arrival at CEU that is passing through the upper mantle under the Alboran Sea.

We have studied a larger number of events for their dispersion behaviour. Fig. 6 shows lower hemisphere positions of rays that show dispersion, versus those that don't. In a number of cases we have assigned an 'unclear', when the signal-to-noise ratio was too low to characterize the dispersion either due to an unclear first arrival, or if there was an unclear frequency trend. Note the clear asymmetry in the lower hemisphere under CEU. Many arrivals from the East show dispersion, while most arrivals from the West show clear absence of dispersion. At the reference station MTE, essentially all arrivals show either absence of dispersion or unclear behaviour. There are only two that show dispersion. These events occurred in the Alboran Sea themselves. The different patterns at the two stations, as well as the Fig. 5. Dispersion observations. Waveforms of P-waves arriving at two stations a) from the West (earthquake in Columbia, 15.11.2004, magnitude 7.2, lat. 4.69°, long -77.5°, distance 72.8°) and c) from the East (earthquake in Crete, 17.3.2004, magnitude 6, lat. 34.59°, long. 32.32°, distance 23.45°). Each seismogram is shown filtered in a series of filter bands (from the top: b0.5 Hz, 0.05-0.5, 0.5-1.5, 1.5-2.5, 2.5-3.5, and 3.5-4.5 Hz), and corresponding smoothed envelopes. Amplitudes are given by numbers, and ticks give time intervals of 2 s. The observed dispersion is illustrated by a dashed line. See text for explanation. East-West asymmetry at CEU are coherent with the presence of one or more oceanic slabs under the Alboran Sea.

We have studied alternative explanations such as attenuation, scattering, or multiple phases, and we have not found any simple process that explains these observations other than dispersion due to layering in an oceanic lithosphere (see also [START_REF] Bokelmann | Mantle structure under Gibraltar constrained by dispersion of body waves[END_REF]. We will come back to this in more detail below.

Testing the continental model: anisotropy

The continental model presented in Fig. 2b proposes that a gravitationally unstable continental root has separated from the upper portion of the lithosphere and has foundered into the deeper mantle. The lithospheric root is thus replaced by asthenospheric material, which must be brought in from the side. In idealized theoretical models [START_REF] Houseman | Gravitational (Rayleigh-Taylor) instability of a layer with nonlinear viscosity and convective thinning of continental lithosphere[END_REF], this flow occurs simultaneously from all directions. Obviously, any lateral difference in 'boundary conditions' in the real Earth, as well as heterogeneity in temperature and material properties will perturb this symmetry however. In a real Earth, such a radial replacement flow would probably occur preferentially from one side, especially if the foundering of continental material occurs in the form of 'continental delamination'. In any case, the occurrence of radial flow represents a necessary feature for any continental-type model (also for continental delamination), and we can use it to critically test the continental model.

The induced flow in the upper mantle to replace the material of a foundering overthickened continental lithosphere (Fig. 7a,b) will to some degree resemble a channel flow (Fig. 7c), since it must necessarily come in from the side. We do not know from which side that flow comes in, but it must necessarily be present at least one of the edges of the model. The summary Fig. 9 thus shows a prediction of radial flow. In a channel flow model, we are dealing with simple shear deformation with a horizontal flow direction. Seismic fast azimuths should thus match the (horizontal projection of) flow direction somewhere around the Alboran Sea. This will allow us to directly compare those predicted directions of radial flow with observed fast orientations (see below).

Flow directions in the mantle can be mapped by anisotropy measurements, e.g. directional dependence of material properties. We will study in the following elastic anisotropy, as seen by seismic waves. Such seismic anisotropy has been much studied over the last twenty years, and it is now well-established as a technique. Seismic anisotropy in the upper mantle results primarily from elastic anisotropy of minerals, particularly olivine, which develop preferred orientations in response to flow and stress (e.g., [START_REF] Nicolas | Formation of anisotropy in upper mantle peridotitesa review[END_REF][START_REF] Mainprice | The seismic anisotropy of the Earth's mantle: from single crystal to polycrystal[END_REF]. Upper mantle seismic anisotropy can be detected from the splitting of teleseismic shear waves: a polarized shear-wave propagating through an anisotropic medium is split into two perpendicularly polarized waves (Fig. 8) that travel at different velocities. From three-component seismic records, two parameters can be measured to quantify anisotropy: the difference in arrival time (δt) between the two split waves, which depends on the thickness and on the degree of intrinsic anisotropy of the medium, and the azimuth Φ of the fast split shear-wave polarization plane that is related to the orientation of the pervasive fabric (foliation and lineation) in the anisotropic structure. Measurement of teleseismic shear-wave splitting is therefore used to probe mantle deformation beneath a station, with a relatively good lateral resolution (tens of kilometers) that can provide crucial information on past and present geodynamic processes that occurred in the upper mantle. This technique has been applied to similar domains as here; see reviews in [START_REF] Savage | Seismic anisotropy and mantle deformation: what have we learned from shear wave splitting?[END_REF] and [START_REF] Silver | Seismic anisotropy beneath the continents: probing the depths of geology[END_REF], and more specifically [START_REF] Walker | Shear-wave splitting to test mantle deformation models around Hawaii[END_REF]Walker et al. ( , 2004aWalker et al. ( ,b, 2005)), [START_REF] Fontaine | Upper mantle flow beneath Polynesia from shear-wave splitting[END_REF], and [START_REF] Barruol | Upper mantle flow beneath the Hangay dome, central Mongolia[END_REF].

If one assumes that most of the anisotropy affecting the vertically propagating shear waves lies within the uppermost 400 km of the Earth (e.g., [START_REF] Savage | Seismic anisotropy and mantle deformation: what have we learned from shear wave splitting?[END_REF][START_REF] Mainprice | The seismic anisotropy of the Earth's mantle: from single crystal to polycrystal[END_REF][START_REF] Mainprice | Pressure sensitivity of olivine slip systems: implications for the interpretation of seismic anisotropy of the Earth's upper mantle[END_REF][START_REF] Sieminski | Finite-frequency sensitivity of body waves to anisotropy based upon adjoint methods[END_REF] and that the crust may contribute to the total observed delay times by only a few tenths of seconds [START_REF] Barruol | 3D seismic velocities calculated from LPOs and reflectivity of a lower crustal sectionexample of the Val Sesia (Ivrea Zone, Northern Italy)[END_REF][START_REF] Godfrey | Anisotropy of schists: contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations[END_REF], then most of the SKS splitting has to be explained by deformation in the upper mantle.

We have analyzed shear-wave splitting at a set of 16 stations around the Alboran Sea. Requiring a reasonable signal-to-noise ratio allowed us to use 267 seismograms from 67 teleseismic events. The SplitLab tool of [START_REF] Wüstefeld | SplitLab: a shear-wave splitting environment in Matlab[END_REF] was used to perform the splitting analysis, resulting in measures of fast orientations and splitting delay times. Details of data and results are given in [START_REF] Buontempo | Seismic anisotropy beneath southern Iberia from SKS splitting[END_REF]. These are coherent with results from earlier studies (e.g., [START_REF] Diaz | Anisotropy beneath the Iberian Peninsula: the contribution of the ILIHA-NARS Broad-band Experiment[END_REF][START_REF] Schmid | Delay times and shear wave splitting in the Mediterranean region[END_REF]. We show weighted-means for each station in Fig. 9. Since fast orientations should be identical with the horizontal flow direction, we can directly compare the predicted flow directions (arrows) and the observed fast orientations. We note that there is little indication, in the available splitting data, of such a radial flow toward the Alboran Sea. Essentially, all stations show fast orientations that are more or less tangential to the Alboran Sea, rather than radial. Particularly in the Betics, the fast split orientations are trending parallel to the crustal large-scale tectonic structures, suggesting some coherency between crustal and mantle deformation. A single station, ANER on the Spanish coast at latitude 4°W, shows a slight rotation toward the arc. Shear-wave splitting measurements at that station showed a variation with event backazimuth, which is an indication of a possible two-layer structure [START_REF] Silver | The interpretation of shear-wave splitting parameters in the presence of two anisotropic layers[END_REF]. We have analyzed the backazimuthal dependence, and determined parameters of the two-layer model that best fits the observations [START_REF] Buontempo | Seismic anisotropy beneath southern Iberia from SKS splitting[END_REF]. We have found fast orientations of 28°in the shallower layer, and 68°in the deeper layer, and delay times were 1 and 0.6 s, respectively. We note that both of these directions are different from the predicted radial-flow direction in Fig. 9. This suggests that a convective removal model is not supported by the SKS splitting data that are available so far. At least on the northern side of the Alboran Sea, Pn waves show a rather similar spatial pattern of fast orientation as SKS (Calvert et al., 2000b;[START_REF] Buontempo | Seismic anisotropy beneath southern Iberia from SKS splitting[END_REF] suggesting some coupling between the subcrustal and the deeper mantle. We note a considerable gap in coverage in Northern Morocco (and in the Eastern Alboran Sea). Does the radial flow 'hide' in those regions? This does not seem likely, also in light of recent geodetic observations in the region of that gap [START_REF] Fadil | Active tectonics of the western Mediterranean: GPS evidence for roll back of a delaminated subcontinental lithospheric slab beneath the Rif Mountains, Morocco[END_REF] that show surface motion in the opposite direction, toward the Southwest. In any case, that question can be addressed using data from the IberArray and PICASSO projects. First results [START_REF] Diaz | SKS splitting in Southern Iberia and northern Morocco; first contributions of the IBERARRAY broadband seismic network[END_REF] from the IBERARRAY broadband seismic network seem to confirm our observations.

Discussion

One way of explaining why predictions from the continental model are not matched by the observations is perhaps that the effect of the flow on seismic anisotropy is too weak to be observable. We test this possibility in the following.

The required radial replacement flow can be modelled, in a rough approximation, as a channel flow (Fig. 7). For a constant-strain profile of flow, the volume of transported material is DHu/2. Equating this to the estimated volume of the root replacement V r , which we estimate as (150 km) 3 , we obtain a maximum displacement of 300 km, choosing for simplicity H = D = 150 km. The strain in the channel is then ε =2u/ H = 4. The depth and temperature range considered here falling clearly into the dislocation creep regime [START_REF] Karato | Rheology of the upper mantlea synthesis[END_REF], a strain of 4 should produce strong anisotropy [START_REF] Mainprice | Interpretation of SKS-waves using samples from the subcontinental lithosphere[END_REF][START_REF] Silver | Mantle structural geology from seismic anisotropy, in: mantle petrology: field observations and high pressure experimentations: a tribute to Francis R. (Joe) Boyd[END_REF] and detectable (one second or more) shear-wave splitting. Distributed over a 150 km thick channel, this flow should be observable even if the channel has a lateral extent comparable to the entire Southern limit, about 600 km length. The observed anisotropy that doesn't indicate any sign of this radial flow thus appears to clearly rule out radial flow, and hence a continental-type model.

While the anisotropy constraint is very strong for ruling out a purely continental model, it is weaker for ruling out or confirming a subduction model. In principle, we expect a trench-parallel flow during the rollback [START_REF] Russo | Trench-parallel flow beneath the Nazca Plate from seismic anisotropy[END_REF][START_REF] Faccenna | Lateral slab deformation and the origin of the western Mediterranean arcs[END_REF][START_REF] Faccenna | Constraints on mantle circulation around the deforming Calabrian slab[END_REF], and such a 'toroidal flow' (tangential) component is rather consistent with the 'trench-parallel' fast orientations that we observe here. On the other hand, fast orientations from known subduction zones around the Earth are typically either trench-parallel or trenchperpendicular [START_REF] Long | The subduction zone flow field from seismic anisotropy: a global view[END_REF]. Some of the complexity seen at the different subduction zones around the Earth may be due to a particular olivine fabric that forms under hydrated conditions [START_REF] Jung | Water-induced fabric transitions in olivine[END_REF], especially in the mantle wedge above the slab. Since teleseismic shear-wave splitting observations give only weak constraints on the depth of the anisotropy, we conclude that our splitting observations are consistent with the presence of a subduction zone, but they do not allow to critically test that model.

On the other hand, a subduction model makes a clear prediction as to the dispersion of body waves, and we have used that constraint to critically test the subduction zone model. We have found the predicted phenomenon for numerous waves propagating through the mantle under the Alboran Sea. In this study, we have in fact never found that behaviour for rays that do not pass through the anomalous zone beneath the Alboran Sea. It is rather difficult to explain these observations otherwise, without invoking a subduction model (see [START_REF] Bokelmann | Mantle structure under Gibraltar constrained by dispersion of body waves[END_REF]. On the other hand, one might wonder whether the dispersion might be produced by propagation within continental crust that is taken down into the mantle with a piece of continental lithosphere. This seems very unlikely however, since convective removal and delamination models typically propose to separate and founder only deeper portions of the lithosphere. If some crust were to descend nevertheless, it would have to remain nearly unperturbed though, to keep its planar and elongated shape. This seems extremely unlikely. The observed dispersion thus gives an important constraint on the nature of the anomaly (oceanic rather than continental lithosphere). On the other hand, the constraint on shape is relatively weak, and does not go much beyond fixing the general location of the oceanic slab to be under the Alboran Sea.

Other pieces of evidence that point to subduction rather than removal of continental lithosphere as a zeroth-order model for the area are east-dipping reflectors in the Gulf of Cadiz [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Thiebot | The Gibraltar Arc seismogenic zone (part 1): constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic data, gravity and thermal modeling[END_REF]) that suggest the presence of an eastward-dipping accretionary wedge. Block rotations from paleomagnetism in the Betics (mostly clockwise) and in the Rif (mostly counterclockwise) can be explained by westward rollback of an eastdipping subduction zone [START_REF] Turner | Magmatism associated with orogenic collapse of the Betic-Alboran Domain[END_REF]; see also [START_REF] Michard | How does the Alpine belt end between Spain and Morocco?[END_REF], but apparently other mechanisms can explain that as well [START_REF] Hindle | Strain, displacement and rotation associated with the formation of curvature in fold belts; the example of the Jura Arc[END_REF][START_REF] Balanyá | Structural trend line pattern and strain partitioning around the Gibraltar Arc accretionary wedge: insights as to the mode of orogenic arc building[END_REF]. Geochemistry of volcanic rocks in Spain and Morocco has also been proposed to support subduction [START_REF] Duggen | Magmatic evolution of the Alboran region: the role of subduction in forming the western Mediterranean and causing the Messinian Salinity Crisis[END_REF] in that they indicate a contamination of the mantle source with hydrous fluids/melts. However, geochemical evidence from volcanic rocks in the Gibraltar arc has also been interpreted as being due to a combination of both mechanisms, subduction and delamination [START_REF] Duggen | Post-collisional transition from subduction-to intraplate-type magmatism in the westernmost Mediterranean: evidence for continental-edge delamination of subcontinental lithosphere[END_REF][START_REF] Booth-Rea | Crustal types and Tertiary tectonic evolution of the Alborán sea, western Mediterranean[END_REF].

Focal mechanisms [START_REF] Stich | Moment tensor solutions for small and moderate earthquakes in the Ibero-Maghreb region[END_REF], as well as GPS measurements (e.g., [START_REF] Fadil | Active tectonics of the western Mediterranean: GPS evidence for roll back of a delaminated subcontinental lithospheric slab beneath the Rif Mountains, Morocco[END_REF] show that the Alboran Sea is currently opening toward the West or Southwest. Seismicity hypocenters themselves do not show a clear Benioff zone, but there are occasionally deep earthquakes at depths around 600 km [START_REF] Buforn | Intermediate and deep earthquakes in Spain[END_REF][START_REF] Buforn | Source mechanism of intermediate and deep earthquakes in southern Spain[END_REF]. The occurrence of such deep quakes is familiar from several subduction zones around the Earth, and also the focal mechanisms of these events that show East-West P-and T-axes are consistent with a subduction zone origin. Seismicity at intermediate depth is more enigmatic (e.g., [START_REF] Casado | The structure of the Alboran Sea: an interpretation from seismological and geological data[END_REF]. Neither the distribution of hypocenters nor the focal mechanisms are reminiscent of a classical subduction zone that would be oriented in the same direction. P-and T-axes are more frequently in N-S direction, more in line with the crustal stress field (see [START_REF] Stich | Crust-mantle coupling in the Gulf of Cadiz (SW-Iberia)[END_REF][START_REF] Fernández-Ibáñez | Present-day stress field in the Gibraltar Arc (western Mediterranean)[END_REF] and the relative motion (e.g., [START_REF] Stich | Kinematics of the Iberia-Maghreb plate contact from seismic moment tensors and GPS observations[END_REF] of the surface. This depth variation apparently reflects the complex geodynamic situation that results from a subduction zone embedded into an active transpressional plate boundary.

It is also possible that the nature of the subducted lithosphere has changed with time. Surface tectonics suggests that the direction of extension has changed from North-South to East-West between 20 and 14 Ma ago, accompanied by a thermal event. It may be argued that this corresponds to a change from a North-South (continental) subduction to an East-West oceanic subduction. In fact, our dispersion observations indicate a low-velocity crust in the topmost portion of the slab. That portion of the slab will be composed mostly of material younger than 14 Ma, if the subduction velocity is at least several mm/yr, and we will be unable to see a deeper portion where the low-velocity channel is perhaps missing.

Interestingly, Calvert et al. (2000a) have addressed the question as to whether the slab is continuous or not between 200 and 600 km, but they showed in a 'smearing test' that tomography is unfortunately not yet capable of distinguishing a gap between two different slabs from a single continuous slab. The dispersion observations are of interest for this question, since they require a continuous slab over a certain depth range.

Besides the debate about the nature of the anomaly under the Alboran Sea, there is another one that concerns the question whether the subduction is still ongoing (e.g., [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF] or by now extinct [START_REF] Sengör | Some current problems on the tectonic evolution of the Mediterranean during the Cainozoic[END_REF][START_REF] Lonergan | Origin of the Betic-Rif mountain belt[END_REF][START_REF] Iribarren | The structure of the Atlantic-Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia-Africa plate boundary)[END_REF]. No matter which of these two groups of authors is right, the zone is certainly tectonically active, as shown by the seismicity and the geodetic deformation. The open question is whether the zone is active only in the sense of a plate boundary, or whether the embedded subduction zone is still active. In any case, some of the intermediatedepth seismicity (nearly vertical line of seismicity) is indicative of a slab tear that is currently ongoing, and that is at least one sense in which the subduction may still be active. However, the precise relationship of this structure with the low-and high-velocity features still needs to be clarified. GPS velocity vectors and stress field inverted for the region [START_REF] Stich | Crust-mantle coupling in the Gulf of Cadiz (SW-Iberia)[END_REF][START_REF] Stich | Kinematics of the Iberia-Maghreb plate contact from seismic moment tensors and GPS observations[END_REF][START_REF] Stich | Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake: implications for the 1755 Lisbon earthquake[END_REF] do not seem to require an active subduction of the oceanic portion.

There is an interesting similarity with the Calabrian arc in Southern Italy, for which shear-wave splitting results have been presented by [START_REF] Civello | Toroidal mantle flow around the Calabrian slab (Italy) from SKS splitting[END_REF] and more recently by [START_REF] Baccheschi | Seismic anisotropy reveals focused mantle flow around the Calabrian slab (Southern Italy)[END_REF]. These authors find trench-parallel seismic fast orientations also for that region, and they show a spectacular rotation following the curvature of the rollback and of the mountain chain. The mirror symmetry of seismic anisotropy in the two arcs that are at opposite ends of the Western Mediterranean extension system is probably not fortuitous, but rather related to a similar phenomenon, and thus to the large-scale dynamics of oceanic domain opening in the Southwestern Mediterranean. The eastern slab rolled back toward the south-east, inducing the opening of both the Ligurian and Tyrrhenian Sea and letting its flow imprint in the upper mantle along its long retreating route [START_REF] Barruol | Mapping upper mantle anisotropy beneath SE France by SKS splitting indicates a Neogene asthenospheric flow induced by the Apenninic slab rollback and deflected by the deep Alpine roots[END_REF][START_REF] Faccenna | Lateral slab deformation and the origin of the western Mediterranean arcs[END_REF][START_REF] Lucente | Seismic anisotropy reveals the long route of the slab through the western-central Mediterranean mantle[END_REF]. It is now well imaged through the upper mantle beneath present-day Calabrian arc and is probably lying within the transition zone [START_REF] Piromallo | P wave tomography of the mantle under the Alpine-Mediterranean area[END_REF]. The western slab fragment rolled back toward the south west, participating to the opening of the Algerian basin, generating the Alboran Sea and the Betic-Rif orocline.

Conclusions

We have presented two observational constraints that allow to critically test the geodynamic models that are most frequently discussed for the Alboran Sea region (Fig. 2). An oceanic model (retreating subduction) predicts that some P-waves should have a characteristic type of dispersion. Depending on the geometry of rays, this phenomenon may or may not be observed. We have searched for this phenomenon in waves at two stations, and have found that this occurs frequently (and exclusively) for waves propagating through the mantle under the Alboran Sea. This is strongly indicative of the presence of a subduction zone under the Alboran Sea. The second type of constraint was based on seismic anisotropy, which allows to critically test the continental model (convective removal) since that model predicts radial flow to occur towards the Alboran Sea. This phenomenon does not appear in the observations, given the current station distribution. On the other hand, a subduction-induced toroidal flow is quite consistent with the observations. Both constraints thus strongly favour the oceanic subduction model. This resolves the most important question, namely that of the nature of the anomalous feature under the Alboran Sea. The second question, as to the shape of the anomaly is at this point less-well constrained. Ongoing and future experiments in the area will probably shed more light on the shape of the anomalies, and we will be able to learn how these processes occur in more detail. In particular, enhanced imaging techniques based on waveform imaging, use of later phases, and array-seismological techniques will be rather useful.

Fig. 1 .

 1 Fig. 1. Map of the Mediterranean region. The box shows the Gibraltar arc around the Alboran Sea. This map gives the spatial extent of the Alpine chain, and the oceanic as well as extended continental crust (after Comas et al., 1999).

  Fig. 3. a) Azimuthal distribution of P-wave arriving at Ceuta (magnitude N5, distance 0-90°) in a three-year time interval (maximum from NW has 110 events); b) west-east profile (see a) through 3D input model for tomographic resolution test, and resulting output model (after Calvert et al., 2000a). See text for explanations.

Fig

  Fig. 4. a) Illustration of ray propagation parallel to subducted oceanic lithosphere, b) group arrival times as a function of frequency (dispersion) for a 6 km thick crustal low-velocity channel (8 km/s) embedded in a background velocity of 8.5 km/s (redrawn after Abers, 2005).

Fig. 6 .

 6 Fig. 6. Lower hemispheres under the two stations, a) MTE and b) CEU, indicating for which arrival dispersion is present or not, or whether the dispersion observation is unclear (see text). The events of Fig. 5 are shown by "1" and "2".

Fig. 7 .Fig. 8 .

 78 Fig. 7. Model of the radial replacement flow in the context of the continental model: a) overthickened lithosphere that is sinking into the deeper mantle (b). c) and d) show displacement and strain during a channel flow. Dashed lines show the case of a constant strain-rate flow. The height of the channel is H; D is its lateral extent.

Fig. 9 .

 9 Fig.9. Map showing an illustration of the expected radial flow for the continental model (shown by red arrows), towards the Alboran Sea. Also shown is shear-wave splitting at each of the 16 stations: Weighted-mean fast orientations are given by the direction of the line. Splitting delays are given by its length.

Acknowledgements

We thank Juan Carlos Balanyá Roure and an anonymous reviewer for reviews.

We thank Andreas Wüstefeld for use of the SplitLab code (Wüstefeld et al. 2008;Wüstefeld and Bokelmann 2007) and help in the data processing. We wish to thank Marc-André Gutscher and Greg Houseman for interesting discussions. Some figures were created using GMT (Wessel and Smith, 1995). Data were used from the IAG-UGR and ROA-UCM-Geofon networks. We wish to acknowledge support by the CNRS/INSU through SEDIT grants, as well as Actions, and the French Ministry of the Exterieur (CNRS-CSIC project). We are also thankful for the financial support of the Spanish project CGL2008-01830-BTE, Junta de Andalucia RNM104 and the project Topo-Iberia CSD2006.


Identification of thermal properties distribution in building wall using infrared thermography Jordan Brouns [START_REF] Ibos | A comparative study of in-situ measurement methods of a building wall thermal resistance using infrared thermography[END_REF][START_REF] Nassiopoulos | On-site building walls characterization[END_REF] and Jean Dumoulin [START_REF] Ibos | A comparative study of in-situ measurement methods of a building wall thermal resistance using infrared thermography[END_REF][START_REF] Nassiopoulos | On-site building walls characterization[END_REF] (1) French institute of science and technology for transport, development and networks (IFSTTAR), Components & Systems (COSYS), Bouguenais, France (jean.dumoulin@ifsttar.fr), (2) INRIA/IRISA, I4S Team, Campus de Beaulieu, F-35042 Rennes, France

In the construction sector, most of the measurements carried out from IR camera devices are exploited in a qualitative way (e.g. observation of thermal bridges). However, unless a quantitative analysis is realized, it is not possible to assess the impact of the observed phenomena. Most of research efforts and proposed solutions to identify quantified thermal properties (e.g. U-values) have to be completed, adapted to the built environment and validated in experimental and real conditions to allow quantified assessment of materials thermal properties thanks to IR camera devices [START_REF] Ibos | A comparative study of in-situ measurement methods of a building wall thermal resistance using infrared thermography[END_REF]. We still need several steps in terms of scientific and technical developments for such technological progress. The H2020 European Built2Spec research project (http://built2spec-project.eu/) aims at giving highlights on that.

Heat transfer through the walls are generally model by 1D heat equation in the wall depth. The built is composed by a multilayer domain representing the construction process. In this context, the thermal parameters of the wall are piecewise constant space functions. We propose a methodology to recover the vector of the wall thermal properties (conductivity and capacity) from boundary measurements obtained from an IR camera. It formulates as an inverse problem where the unknown are sought as minimizers of a cost function evaluating the gap between the measures and the model response. This optimization problem is non linear, and we solve it with the Levenberg-Marquardt algorithm coupled with the conjugate gradient method [START_REF] Nassiopoulos | On-site building walls characterization[END_REF][START_REF] Brouns | Développement d'outils numériques pour l'audit énergétique des bâtiments[END_REF]. To shorten the time of the identification process, we use the adjoint method coming from the control theory [START_REF] Lions | Contrôle optimal de systèmes gouvernés par des équations aux dérivées partielles[END_REF]. This method fasten the gradient computation by solving an associated model, named the adjoint model. We study the ability of the procedure to reconstruct internal wall constitution from different environmental conditions. Furthermore, we propose a controlled experimental test to evaluate the method in laboratory conditions.


M Marchetti
email: mario.marchetti@cerema.fr

J Dumoulin
email: jean.dumoulin@ifsttar.fr

M Fois

L Ibos
email: ibos@u-pec.fr

N Le Touz

J.-M Piau

Thermographic monitoring of asphalt concrete surface with phase change materials inclusions for icing delays purposes

If de-icers still are the main solution to avoid black ice occurrence and snow accumulation on pavements, some alternatives based on transportation infrastructure modifications have been tested over the past years. They aim at the reduction of environmental impacts and at coping with budgets constraints and cuts, including in winter maintenance. Among existing ones, the implementation of phase changes materials (PCM) into the infrastructure was evaluated to delay the occurrence of water in its solid phase on the surface. Additional results were obtained with an asphalt concrete in simulated winter conditions, indicating a tenuous thermal effect at the surface.

Introduction

The mobility in adverse weather conditions during winter time relies on many factors, from the ability of winter services to cope with the meteorological phenomenon to the nature of the infrastructure submitted to this event. Despites years of efforts in terms of staff training, evaluations on economical costs [START_REF] Petty | Federal Highway Administration winter road Maintenance Decision Support System (MDSS): Recent enhancements and refinements[END_REF] of winter maintenance, on its environmental impacts, the improvements in weather forecast through numerical models [START_REF] Bouilloud | A coupled model to simulate snow behavior on roads[END_REF][START_REF] Hammond | The validation and improvement of route-based road weather forecasts[END_REF][START_REF] Khalifa | Appreciation of the traffic effects on the RST by infrared thermography[END_REF][START_REF] Marchetti | Methodology to forecast road surface temperature with Principal Components Analysis and Partial Least-Square regression. Application to a urban configuration[END_REF], hundreds of tons of de-icers are still spread over pavements surfaces. One last mean to influence this de-icer trend is to implement a change in the infrastructure itself to avoid black ice occurrence and snow accumulation over the surface submitted to traffic. Such solution has nevertheless to be compliant with maintenance and repairs works on pavements, and is in full opposition and contrast with such changes as ones made on Serso Bridge (Switzerland) for instance. Previous works consisted in evaluation the performance of a multi-layered asphalt concrete structure with a fluid circulating a porous layer, and of phase change materials (PCM) embedded into concrete [START_REF] Marchetti | Infrared thermography applied to surface temperature survey of enhanced pavement sample addressing self-de-icing functionality[END_REF]. Additional tests were then conducted considering another phase change material [START_REF] Cabeza | Materials used as PCM in thermal energy storage in buildings: A review[END_REF] with the appropriate thermodynamic characteristics in the form of inclusions into asphalt concrete, and submitted to controlled winter conditions during several days. The purpose of these PCM was to maintain a stable temperature at the surface for a given duration to ensure the melting of solid water. Results indicated a tenuous effect at the pavement surface, and temperature variations not systematically observed at expected phase transition temperature.

Experimental settings and materials

Simulated winter conditions and temperature monitoring

Atmospheric conditions representative of winter were simulated in a several cubic meters large climatic chamber. Imposed cyclic air temperature variations are as follows: start at 10°C, with over 12h at this temperature to reach a thermal homogeneity, cooling down to -5°C in several steps, finally back to initial temperature. Air temperature was measured with a Pt100 probe. Surface temperature was monitored by thermocouples and a FLIR A655c infrared camera recording images every 10s. The infrared scene included a mirror for further radiative corrections on thermal images, along with surface painted with Nextel 811-21, which emissivity is supposed constant in the selected temperature range.

Asphalt concrete samples and PCM

Four asphalt concrete samples were 50 cm x 18 cm x 14 cm parallelepipeds, manufactured with materials conventionally used in roads. One did not contain any PCM inclusions, while three other ones respectively contained one and two inclusions at depths ranging from 20 mm and as down as 45 mm below the surface. They were installed in the floor of the climatic chamber on a polystyrene foam used as thermal insulator, and their sides were also insulated with polystyrene.

10.21611/qirt.2016.078

Selected PCM was formic acid, either in its pure form or diluted into water (20% v/v water -80% v/v formic acid, 30 % -70 %, and 40 % -60%). In the literature, the liquid to solid temperature transition is given at 7.8°C.The liquid was placed into metallic containers then inserted into the asphalt concrete sample. Thermocouples were attached to these containers, either on the upper lid or at their bottom.

Results and analysis

Temperature profiles from the thermocouples attached to the surface and the PCM containers did indicate variation consistent with the ones of air. There were no indication of stable temperature due to the phase transition of the PCM (figure 1a). When considering thermal images, a temperature variation was monitored during the temperature increase, between -5°C and 2°C (figures 1b, 1c). This effect is quite tenuous, and it is difficult to link the temperature at which it was observed to a potential phase transition. This raises the question of a supercooling occurrence, but also the possibility of molecular organization either of formic acid in its dimer form [START_REF] Fathi | Structural investigation of liquid formic acid by X-ray and neutron scattering, ab initio calculations and molecular dynamics simulations[END_REF], consequently modifying the phase transition temperature.

Conclusion

The possibility to rely on PCM to maintain a positive stable temperature at the surface of asphalt concrete was investigated. It consisted in inclusions of acid formic in pavement samples. Temperatures was monitored by both thermocouples and an infrared camera. Results indicated a tenuous thermal effect at the surface when samples were submitted to a cyclic air temperature variations, raising supercooling questions and possible dimer clusters into the PCM in its liquid phase, affecting the transition temperature.

Fig. 1 .

 1 Air temperature variations in a climatic chamber (a), PCM inclusions (b) in asphalt concrete samples (c)


L Ibos
email: ibos@u-pec.fr

J.-P Monchau

V Feuillet

J Dumoulin

P Ausset

J Hameury

B Hay

Investigation of the directional emissivity of materials using infrared thermography coupled with a periodic excitation

This article concerns the determination of the directional emissivity of materials. Several materials (conducting materials or dielectrics) are investigated and the influence of surface roughness is also considered. The experimental method used for the determination is based on the use of a periodic excitation and the recording of the surface temperature variations of the sample using infrared thermography. Several consecutive measurements are performed for emission angles varying from 0° (for the determinat ion of normal emissivity) to 85°. The experimental device developed (SPIDER instrument) is simple compared to existing devices but the variation of directional emissivity is limited to the spectral bandwith of the camera used. © program to acquire the temperature of the Duralumin plate and to generate the control signal of the power amplifier and the camera synchronization signal.

Introduction

The directional emissivity is particularly important for two applications: the correction of infrared temperature measurements for complex surfaces (varying viewing angles) on one hand and the determination of hemispherical emissivity from normal emissivity measurements (obtained from IR spectroscopy for example) on the other hand. Directional emissivity is generally nearly constant for emission angles lower than 60 degrees from the normal incidence. On the contrary, strong variations of directional emissivity are observed for angles higher than 60 degrees for both dielectric and conducting materials [START_REF] Maldague | Theory and practice of infrared technology for non-destructive testing[END_REF]. Variation of directional emissivity with emission angle can be deduced theoretically from the complex refractive index [START_REF] Kauder | Spacecraft Thermal Control Coating References[END_REF][START_REF] Lorrain | Champs et ondes électromagnétiques[END_REF]. Nevertheless, theoretical relations are only valid for perfectly smooth surfaces and homogeneous materials. Differences between measurements and theory exist for rough or textured materials [START_REF] Sacadura | Influence de la rugosité sur le rayonnement thermique émis par les surfaces opaque : essai de modèle[END_REF][START_REF] Koirala | FTIR-Spectroscopic measurements of directional spectral emissivities of microstructured surfaces[END_REF].

Many experimental devices were developed to determine directional emissivity of materials, for instance for glass [START_REF] Geotti-Bianchini | Total hemispherical emissivity of coated glass[END_REF][START_REF] Janssen | Investigation and development of a method for the measurement of the emissivity of glass[END_REF]. Generally, these devices are quite complex and fail in measuring directional emissivity for angles greater than 80 degrees. Thus, it is still difficult to point out deviations between theoretical and experimental values in this range of emission angles. Moreover, most of the existing devices are usually based on a reflectometric method. In this case, the sample is illuminated with a directional source and the reflected flux is collected by an integrating sphere. Measuring the directional-hemispherical reflectance enables the calculation of the directional emissivity by varying the angle between the incident beam and the normal to the surface of the sample.

Current work concerns the development of a device (SPIDER instrument) allowing determining directional emissivity of dielectric or conducting materials. The method relies on the use of a periodic thermal excitation with different frequencies and the measurement of the luminance of the sample viewed from different angles. First, the experimental protocol is described, then examples of experimental results are presented and future improvements of the device are given.

Experimental protocol

Description of the experimental device

A schematic description and a picture of the experimental test bench are presented in figure 1. This device is an evolution of the one presented in [START_REF] Datcu | Improvement of building wall surface temperature measurements by infrared thermography[END_REF]. The investigated sample is a square thin plate (44×44 mm

2 ) mounted on a Duralumin plate (same cross section, thickness 10 mm) with a K-type thermocouple placed at the center. The Duralumin plate is fixed to a thermoelectric cooler (Peltier module) which applies a periodic thermal excitation. The thermoelectric cooler is fixed to a fin heat sink with fan and powered by a power amplifier controlled by an analog voltage.

A FLIR © SC7300 long wave camera (spectral range 7.7-9.2 µm, matrix 320×256) cooled by a Stirling engine is used to measure the temperature of the rear face of the sample. The acquisition of the thermal images is controlled by a digital signal [0-3.3 V] applied to the trigger input of the camera. The experimental setup is controlled by a LabView The thermoelectric cooler allows applying a temperature T1 on the front face of the sample as follows:

( ) ( ) ( )

1 1 1 1 1, 1, 0 0 0 sin 2 sin 2 n n i i i i i i T t T f t T f t π π - - + = = = ∆ = ∆ ∑ ∑ (1)
The excitation signal is then a superposition of n sine waves whose frequencies are successive powers of two of the fundamental frequency f0. For the measurements, the fundamental frequency f0 was chosen between 1/512 Hz and 1/128 Hz and the number of frequencies of the excitation signal was n=5. The data were acquired during nine periods of the excitation signal, that is to say a total duration ranging from 1152 s to 4608 s. This duration was chosen arbitrarily. The first period is finally removed in order to consider only the measurements for which an established periodical regime is reached. The sampling frequency is fe=1 Hz. The measurement is repeated for different observation angles from 0° to 85°, the sample holder being moun ted on a goniometer.

Determination of the directional emissivity

The directional emissivity is determined from the temperature amplitudes of the excitation signal and the amplitudes of the flux emitted by the surface observed at each emission angle. The temperature T2 on the rear face of the sample (surface viewed by the camera) can be expressed as:

( ) ( ) ( ) 1 1 1 1 2 2, 0 1, 0 0 0 sin 2 sin 2 n n i i i i i i i i i T t T f t A T f t π ϕ π ϕ - - + + = = = ∆ - = × ∆ - ∑ ∑ (2)
where Ai is the attenuation of the temperature variation and φi is the phase difference between T1 and T2 at frequency fi. These values depend on the thermophysical properties of the sample, its thickness and the heat exchange with the environment at the rear face of the sample. The temperature of the sample surface observed by the camera can not be determined because its emissivity is unknown. Therefore the apparent temperature T2 o of the surface is used by setting the emissivity equal to 1:

( ) ( ) ( ) 1 1 o o 1 1 2 2, 0 1, 0 0 0 sin 2 sin 2 n n i i i i i i i i i T t T f t C A T f t π ϕ π ϕ - - + + = = = ∆ - = × ∆ - ∑ ∑ (3)
where C is a coefficient depending on the emission angle and independent of the modulation frequency. The advantage of using n modulation frequencies is to obtain n estimates of the directional emissivity in a single experiment, the emissivity of the surface being independent of the excitation frequency. The number of frequencies is fixed to five in order to limit the temperature variation amplitudes of the sample holder.

In addition, it is recalled that the main advantage of using a modulation method is that the reflected flux from the sample surface is not required. Indeed, there is no change of the environment temperature at the modulation frequencies 10.21611/qirt.2016.031 because this temperature remains constant during the experiment. Thus, the flux collected at the modulation frequency comes only from the emission of the sample surface.

For two experiments with two different viewing angles, the orientation of the sample holder relative to the camera lens is modified. For a given angle θ given, we have:

( ) ( ) ( ) { } o 2, 1, 1;... i i i T C A T i n θ θ θ ∆ = × × ∆ ∀ ∈ (4)
If the sample properties do not vary from one experience to another and if the same excitation frequencies are used, the coefficients Ai are invariant. From the measurements of temperature amplitudes on the front and rear faces of the sample at each modulation frequency, we obtain:

( ) ( ) ( ) { } o 2,
1, 1;...;

i i i T C A i n T θ θ θ ∆ × = ∀ ∈ ∆ (5)
By normalizing to the measurement for a direction normal to the surface (θ=0°), we obtain:

( ) ( ) ( ) ( ) ( ) ( ) { } o 2, 1, o 2, 1, 1;
...; 0 0 0

i i i i T T C i n T C T θ θ θ ∆ ∆ = ∀ ∈ ∆ ∆ (6)
The ratio C(θ)/C(0) is independent of the modulation frequency and enables the calculation of the directional emissivity normalized to normal emissivity ε(θ)/ε(0) if the two measurements are made with the same camera. In practice, due to the presence of measurement noise, we obtain an estimate of the ratio C(θ)/C(0) for each modulation frequency and we must compute the average of these estimates.

The normalized directional emissivity is defined as the ratio of the luminance of the sample in the direction θ to the luminance in the direction normal to the surface. In the case of a modulation method, we will focus on the amplitude of the luminance at the different modulation frequencies fi:

( ) ( )

0 0 i i L f L f θ θ ε ε = % % (7)
The luminance measured by the camera is considered to be proportional to T 4 according to the Stefan-Boltzmann law [START_REF] Datcu | Improvement of building wall surface temperature measurements by infrared thermography[END_REF]. Figure 2 shows the relationship between the luminance and the apparent temperature (calibration curve) provided by data extracted from Altair © software for an experience performed with a highly emissive sample with θ=0°, that is to say for the most important luminanc e variations. One can observe that the chosen approximation is valid. This adjustment comes from the relationship:

In the case where the amplitudes of temperature variation at each modulation frequency are low (a few °C), we obtain after simplification:

( ) ( ) ( ) ( ) ( ) ( ) 3 4 o o 2, 2, 2 3 o o 4 0 2, 2 2, ~0 0 0 i i i i θ θ θ ε ε ∆ = = × ∆ (9)
In order to take into account variations of the excitation signal between two different experiences, the ratio of the temperature amplitudes is replaced in eq. ( 9) by the ratio C(θ)/C(0). So if the average temperature of the surface does not change from one experiment to another (acceptable hypothesis for the same material and the same excitation) and if the amplitudes of temperature variation are low, a simple relationship is obtained:

( ) ( ) 0 0 C C θ θ ε ε = (10)

Raw signals and data processing

Examples of thermal images for two viewing angles are shown in figure 3. The temperature evolutions measured for θ=0° and θ=85° are shown in figure 4. Fast Fourier Transform (FFT) computes the amplitudes of temperature variation at the front and rear faces for each excitation frequency. The corresponding spectra are presented 10.21611/qirt.2016.031 in figure 4. A much higher attenuation of temperature changes measured with the camera on the rear face of the sample is observed for grazing incidence than for normal incidence. The ratios of the temperature amplitudes between front and rear faces of the sample, that is to say the attenuation coefficients C(θ)×Ai, are then calculated using equation ( 5) for the different frequencies and emission angles θ. The average and standard deviation for 5 values obtained at each emission angle are then calculated. The directional emissivities normalized to the normal emissivities are finally calculated from equations ( 7) to (10).

Measurements

Three materials were characterized by this method: PVC, float glass, aluminium alloy. The aluminium sample was polished prior to measurement. The surface roughness, measured using a profilometer VEECO © NT-1100, allows considering these surfaces as smooth in the investigated wavelength range (see table 1). The directional emissivity could be determined up to θ=85° for the three samples.

Determination of the normal emissivity by a spectroscopic method

The calculation of the directional emissivity requires to know a particular value of the emissivity, for example the normal emissivity. This parameter was determined by IR spectrometry using a Perkin-Elmer © Frontier spectrometer equipped with an integrating sphere Pike © . A diffusing gold surface (SpectraGold © ) was used as reference and its reflectance was measured by an independent method at LNE [9]. A correction of the sphere factor was applied to raw measurements to determine the directional-hemispherical reflectance. The characterized samples being opaque, normal emissivity is then calculated for each wavelength from the following relationship:

/ / 1 λ λ ε ρ ∩ = - (11)
Normal spectral emissivities of the samples obtained are plotted in figure 5. Normal emissivities in the band wavelength ∆λ of the camera (indicated in dotted lines in figure 5) are reported in table 1 and computed by weighting the spectral emissivity by the Planck curve for a black body at 300 K:

( ) ( ) 0, 0, o o L T d L T d λ λ λ λ λ λ ε λ ε λ ∆ ∆ ∆ = ∫ ∫ (12)
× + × -                             (13)
The directional emissivity ε θ is the arithmetic mean of ε// and ε ⊥ without polarization. A comparison of the measured directional emissivities with these theoretical values was performed. An estimate of the real and complex parts (respectively n0 and χ) of the refractive index was performed by inverse method by minimizing the sum of squared residuals between measurement and theoretical values. The values identified for each material are shown in table 1 and a comparison of experimental and theoretical data is presented in figure 6. It is now possible from the previous results to determine the hemispherical emissivity:

( ) / / 2 0 / 2 0 d d π π ε θ θ ε θ ∩ = ∫ ∫ (14)
The values obtained are shown in table 1. For the three investigated materials, experimental results are in accordance with theory. Results for the PVC sample show a typical behavior of a dielectric material: high emissivity almost constant up to 60°. A real refractive index is obtained after id entification. The corresponding dielectric permittivity (2.2) is a typical value for this material. For the glass sample, a typical behavior is also observed for this type of material [START_REF] Geotti-Bianchini | Total hemispherical emissivity of coated glass[END_REF]. Aluminium alloy behaves as a conventional metallic material. Note that it is necessary to perform measurements beyond 85° to observe the significant decrease of directional emissivity for angles near grazing incidence.

Conclusion and perspectives

The use of a modulated thermal excitation avoids to measure the absolute temperature of the surface of the sample and to take into account the reflections on the surface from the environment because only the temperature and luminance changes of the surface for the excitation frequencies are considered. Thus the proposed method has the advantage to require a very simple experimental device over existing methods and can characterize both highly emissive surfaces (glass, polymers) and surfaces with low emissivity (metals). The use of a thermal camera as a detector makes it easy to work at angles close to grazing incidence. Furthermore, this also allows having the spectral emissivity to correct thermal imaging temperature fields.

However, this measurement is limited to the spectral range of the camera. The use of mono-element or multiband detectors is planned for further development of the device. Finally, only the relative variations of emissivity according to the emission angle are obtained because the surface temperature is not known. The calculation of emissivity for all angles requires additional measurement at a particular angle. In its final version, the instrument will allow measurements for emission angles between 0 and 89 degrees in different spectral bands.

Fig. 1 .

 1 Fig. 1. Schematic description and picture of the experimental test bench (SPIDER instrument)

Fig. 2 .Fig. 3 .Fig. 4 .

 234 Fig. 2. Luminance measured by the camera according to apparent temperature (normal incidence to the surface)

Fig. 5 .

 5 Fig. 5. Normal emissivities of the studied materials

Fig. 6 .

 6 Fig. 6. Directional emissivities of the studied materials (markers refer to measurements) and comparison with theory (solid lines)

Table 1 .

 1 Properties of surfaces investigated (roughness, emissivity and refractive index)

	10.21611/qirt.2016.031
	PVC	Rq (µm)	Emissivity ε0, ∆λ 0.95	ε ∩ ∆λ 0.86	Refractive index n0 (∆n0) χ (∆χ) 1.47 (0.14) 4 × 10 -14 ≈ 0
	Float glass	< 1	0.90	0.75	1.05 (0.11) 0.68 (0.06)
	Aluminium alloy	1.5	0.025	0.036	2.92 (0.52) 21.2 (2.2)

Directional emissivity of the studied materials

The directional emissivity for all investigated viewing angles is determined from the relative changes in directional emissivity and the independent measurement of the normal emissivity. Furthermore, for smooth and homogeneous materials, directional emissivity can be determined from theoretical relationships and the complex refractive index n = n0 + jχ [START_REF] Maldague | Theory and practice of infrared technology for non-destructive testing[END_REF][START_REF] Kauder | Spacecraft Thermal Control Coating References[END_REF]. In the case of a planar diopter plane, emissivities for two polarizations are defined as follows [START_REF] Kauder | Spacecraft Thermal Control Coating References[END_REF]:


Chengyi Shen

Daniel Brito

Valier Poydenot

Julien Diaz

Stéphane Garambois

Clarisse Bordes

Seismic wave propagation in heterogeneous limestone samples

Keywords: Wave propagation, laser ablation source, piezo source, numerical modelling, limestone core

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Mimic near-surface seismic field measurements at a small scale, in the laboratory, under a wellcontrolled environment, may lead to a better understanding of wave propagation in complex media such as in geological materials. Laboratory experiments can help in particular to constrain and refine theoretical and numerical modelling of physical phenomena occurring during seismic propagation, in order to make a better use of the complete set of measurements recorded in the field.

We have developed a laser Doppler vibrometer (laser interferometry) platform designed to measure non-contact seismic displacements (or velocities) of a surface. This technology enables to measure displacements as small as a tenth of a nanometer on a wide range of frequencies, from a few tenths to a few megahertz. Our experimental set-up is particularly suited to provide high-density spatial and temporal records of displacements on the edge of any vibrating material (aluminum, limestone, ...).

We will firstly present experiments in cuboid and cylinders of aluminum (homogeneous) in order to calibrate the seismic sources (radiation diagram, frequency content) and identify the wave arrivals (P, S, converted, surface waves). The measurements will be compared quantitatively to a direct 2D numerical elastodynamic simulation (finite elements, Interior Penalty Discontinuous Galerkin). We will then show wave measurements performed in cylindrical heterogeneous limestone cores of typical diameter size around 10 cm. Tomographic images of velocity (figure 2a) in 2D slices of the limestone cores will be derived based upon the time of first arrivals and implemented in the numerical model. By quantifying the difference between numerical and experimental results, the tomographic velocity model will be reciprocally improved and finally compared to a X -ray tomographic image of that slice.

A brief overview of the studies

Seismic sources

We will explore piezo-electric sources of different frequencies (100 kHZ ∼ 5 M Hz) and test the new laser ablation source whose dominant frequency can reach 2 M Hz in aluminium. Avantages and drawbacks of each technology will be discussed in terms of source and wave propagation characterisation.

Wave identification in an aluminium cube of side length 280 mm and seismic source at the center of one face

We have identified experimentally P, S, head wave, PS, SP and surface waves measured on the cube surfaces. Meanwhile, direct numerical simulations have helped to quantitatively analyze the kinematics of wave fronts. For example, on the surface where the seismic source is excited, a P front, an S front and a PS head wave front are measured by the laser vibrometer right after the initial seismic impulse. These wavefronts can be understood by both the Huygens' Principle and the Snell-Descartes Law. In Figure 1, the seismic source excits simultaneously at time t = 0 a P wave and an S wave. As time evolves, waves propagate inside the volume and a P-wave propagates along the boundary as well: the latter one acts on the boundary as secondary sources which will emit both P and S waves, creating finally a new PS head wave front nicely measured in the experiments. The colours of magenta and green correspond to null amplitudes.

Tomography and numerical simulation in the carbonate cylinder

We have performed a V p tomography of a cylindrical slice of the core carbonate based upon the first P wave arrivals. Figure 2a shows the velocity model derived by using the software SARDINE. 6 sources were uniformly distributed on the surface (from 0 • to 360 • ) while 73 uniformly distributed receivers were located along the semicircle on the other side of the corresponding source. That velocity model is then used in our direct numerical elastodynamic simulation to compute wave propagation within the cylinder slice. Figure 2b compares an experimental seismogram to a numerical one for one location of the source. In terms of first arrival times, the simulated data reproduce relatively well the measurements; the simulated first arrivals are however much smoother in terms of amplitudes than the measured ones. Some discontinuities in experimental data are not retrieved by the simulation as well thus we expect to improve the tomographic model by multiplying the number of sources and use efficiently the confrontation experiments vs numerics.

Figure 1 :

 1 Figure 1: View from above of a 2D horizontal slice of the aluminum cube of side length 280 mm. The seismic source (red cross) emits an initial impulse at time t = 0. Until t = 22 µs, P and S wavefronts propagates within the cube as well as a head wave originating from a PS conversion as explained by Huygens' principle.

  Tomographic velocity model. (b) Comparison between experimental and numerical seismograms, the amplitudes of displacements given in nm.

Figure 2 :

 2 Figure 2: Tomography of the carbonate slice and direct numerical simulation.