Tracking with Stereo-vision System for Low Speed Following Applications

International audienceResearch in adaptative cruise control (ACC) is currently one of the most important topics in the field of intelligent transportation systems. The main challenge is to perceive the environment, especialy at low speed. In this paper, we present a novel approach to track the 3-D trajectory and speed of the obstacles and the surrounding vehicles through a stereo-vision system. This tracking method extends the classical patch-based Lucas-Kanade algorithm [9], [1] by integrating the geometric constraints of the stereo system into the motion model: the epipolar constraint, which enforces the tracked patches to remain on the epipolar lines, and the magnification constraint, which links the disparity of the tracked patches to the apparent size of these patches. We report experimental results on simulated and real data showing the improvement in accuracy and robustness of our algorithm compared to the classical Lucas-Kanade tracker

Cohérence spatiale des mouvements sismiques et structure géométrique du proche sous-sol: un exemple à Argostoli, Grèce

International audienceThe spatial variation of the earthquake ground motion over short distances can significantly affect the dynamic response of large and extended engineered structures, particularly when they are located at sites with lateral heterogeneity. In current practices, it is taken into account in terms of coherency, as a function of frequency and distance, established on an essentially empirical basis, making it difficult to extrapolate at different sites. Hence, in order to improve our understanding of the physical significance of coherency and its relationship to the geometrical and mechanical characteristics of the ground structure, a very dense network was installed in seismically active region of Argostoli-Koutavos (Cephalonia, Greece). Consisting of 21 velocimeters deployed on four concentric circles of radii 5, 15, 40 and 80 m around a central station in a small valley (~50 m thickness, width 1.5 km), the array recorded several hundred events in the period from September 2011 to April 2012. A subset of 46 events having a homogeneous distribution in terms of epicentral distance (0-200 km), back-azimuth (0-360 °) and magnitude (2 to 5) has been selected for a systematic analysis of the coherency of all station pairs observed on 'the most energetic phase' containing the S-wave. The results indicate a lack of a clear dependence of the average coherency on the magnitude, back-azimuth or site-to-source distance of the event, and on the length of chosen signal window provided that it contains the same S-phase energy. The most striking result concerns the influence of the site geometry; the coherency is systematically higher for the pairs aligned parallel to the axis of the valley (2D), and lower for those aligned in the perpendicular direction. This result is consistent with the formation of the wavefield in the valley, dominated by surface waves propagating from one edge to the other. The observed coherency estimates are, however, weakly represented by the existing parametric models.La variabilité spatiale des mouvements sismiques sur de courtes distances peut affecter significativement la réponse dynamique des structures de génie civil de grande taille, en particulier quand elles sont fondées sur des sites latéralement hétérogènes. Elle est souvent prise en compte au travers de fonctions empiriques – peu calibrées et peu extrapolables d'un site à l'autre-décrivant la perte de cohérenceen fonction de la fréquence et la distance. Dans le but d'améliorer la compréhension de la structure de la cohérence et son lien avec les caractéristiques géométriques et mécaniques du sous-sol, un réseau sismologique très dense a été installé dans la région sismiquement très active d'Argostoli-Koutavos (Céphalonie, Grèce). Constitué de 21 vélocimètres déployés sur 4 cercles concentriques de rayons 5, 15, 40 et 80 m autour d'une station centrale au sein d'une vallée de petite taille (épaisseur ~50 m, largeur 1.5 km), ce réseau a enregistré plusieurs centaines d'événements sur la période septembre 2011-avril 2012. Un sous ensemble de 46 événements présentant une distribution homogène en termes de distance épicentrale (0-200 km), back-azimut (0-360°) et magnitude (2 à 5) a été sélectionné pour une analyse systématique de la cohérence observée sur la phase forte comprenant l'onde S de toutes les paires de stations. Les résultats indiquent une absence de dépendance claire de la cohérence moyenne en fonction de la magnitude, du back-azimut de la source, et de la distance hypocentrale, ainsi qu'une grande robustesse vis-à-vis de la fenêtre de signal choisie, pour peu qu'elle contienne la phase S énergétique. Le résultat le plus marquant concerne le contrôle par la géométrie du site : la cohérence est systématiquement plus forte pour les paires alignées parallèlement à l'axe de la vallée (2D), et minimale pour celles alignées selon la direction perpendiculaire. Ce résultat est cohérent avec la constitution du champ d'ondes dans la vallée dominé par des ondes de surface se propageant d'un bord à l'autre de la vallée. Les cohérences observées ne sont par ailleurs que très partiellement expliquées par les modèles existants

Two-dimensional resonances in Alpine valleys identified from ambient vibration wavefields

Although numerical simulations have for long shown the importance of 2-D resonances in site effect estimations of sediment-filled valleys, this phenomenon is usually not taken into account by current hazard assessment techniques. We present an approach to identify the resonance behaviour of a typical Alpine valley by analysis of ambient noise recorded simultaneously on a dense array. The applicability of the method is evaluated further using synthetic ambient noise acquired with current 3-D numerical simulation techniques. Resonance frequencies of the fundamental mode SV and the fundamental and first higher mode of SH are identified from measured data with the reference station method, verifying results of previous studies. Patterns of spectral amplitude and phase behaviour obtained from observed and synthetic noise correlate well with properties expected for 2-D resonance. Application of a frequency-wavenumber technique shows that the noise wavefield is dominated by standing waves at low frequencies (0.25 to 0.50 Hz). The different 2-D resonance modes are creating prominent peaks in horizontal-to-vertical spectral ratios, which can not be interpreted in terms of 1-D resonance. We conclude that ambient noise records measured simultaneously on a linear array perpendicular to the valley axis may be used for identification of resonance modes in sediment-filled valley

Ambient noise energy bursts observation and modelling: Trapping of harmonic structure-soil induced-waves in a topmost sedimentary layer.

We study the nature of energy bursts that appeared in the frequency range 3-5 Hz in ambient seismic noise recorded in the Grenoble basin (French Alps) during a seismological array experiment. A close agreement is found between the identified azimuths of such noise bursts with the location of an industrial chimney. In-situ measurements of the chimney dynamic characteristics show a coincidence between the frequency of the first harmonic mode of the chimney and the fundamental frequency of a thin surficial layer that overlay the deep sediment fill. The interaction between the chimney and the surfical layer is then numerically simulated using simple impedance models, and two geotechnical profiles. Simulations exhibit a satisfactory agreement with observations and suggest that energy bursts result of inertial structure-soil interaction favored by resonance effects between the first harmonic mode of the structure and the fundamental frequency of the topmost layer

Graviton production from D-string recombination and annihilation

Fundamental superstrings (F-strings) and D-strings may be produced at high temperature in the early Universe. Assuming that, we investigate if any of the instabilities present in systems of strings and branes can give rise to a phenomenologically interesting production of gravitons. We focus on D-strings and find that D-string recombination is a far too weak process for both astrophysical and cosmological sources. On the other hand if D-strings annihilate they mostly produce massive closed string remnants and a characteristic spectrum of gravitational modes is produced by the remnant decay, which may be phenomenologically interesting in the case these gravitational modes are massive and stable.Comment: 28 pages, 3 figure

H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations

Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification facto