Étude numérique de l'interaction des ondes de surface avec les cavités souterraines

L’effondrement des remblais routiers causé par le développement de cavités souterraines autour des ponceaux pose un risque majeur pour la sécurité des usagers et les installations à proximité. La détection de vides peu profonds est devenue l'une des missions récurrentes difficiles en génie civil à cause de la complexité de la réponse sismique d’un remblai routier en présence d’un ponceau et d’éventuelles cavités. Bien que les méthodes non intrusives basées sur les ondes de surface permettent d’estimer efficacement la vitesse des ondes de cisaillement des dépôts de sol, de nombreux défis sont rencontrés lorsqu'il s'agit de juger de la présence d'une inhomogénéité latérale locale en raison de la résolution limitée des approches géophysiques appliquées. Par conséquent, une étude numérique a été entreprise pour étudier la sensibilité des deux composantes des ondes de Rayleigh (la composante horizontale et la composante verticale désignées X et Z respectivement dans cette étude) et la seule composante des ondes de Love (désignée Y dans cette étude) à un contraste de rigidité (vide) dans différents contextes géologiques. Les accélérations des trois composants sont simulées au moyen du programme de modélisation numérique par éléments finis FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) pour différentes configurations de modèles en présence et en absence de cavité. Les données sismiques sont traitées avec la transformée de Stockwell généralisée (GST) dans le domaine temps-fréquence. Les résultats sont présentés sous forme des tomographies des courbes de dispersion des vitesses de groupe et de phase pour évaluer l'effet de la cavité et la localiser par rapport à la source. La signature de la cavité a également été étudiée à deux différentes profondeurs à partir du modèle parfaitement homogène. Les distributions de vitesse des trois composants ont révélé des changements négligeables après la création de la plus profonde cavité. Les observations numériques ont démontré que les vitesses de phase sont plus sensibles que la vitesse de groupe aux variations latérales de densité. De plus, on peut conclure que les trois composants ont révélé des distributions de vitesse de phase perceptibles et distinctes en présence d’un vide. La composante X s'est également avérée plus efficace pour localiser la cavité. Les résultats de cette étude numérique suggèrent l’acquisition des trois composantes lors des relevés sismiques sur terrain et d’intégrer simultanément ses trois composantes lors de l’analyse pour une plus grande fiabilité.Abstract : A road collapse caused by the development of near-surface cavities surrounding buried culverts poses a major hazard to road users’ safety and nearby facilities. The complexity of the road embankment seismic response has made it a challenging recurring mission in civil engineering to detect shallow voids. Although non-intrusive surface wave methods afford reliable shear wave velocity estimates of the subsurface materials, many challenges are encountered when judging the presence of a local lateral heterogeneity due to the limited resolution of the applied geophysical approaches. Therefore, a numerical study was conducted to investigate the sensitivity of the two Rayleigh waves components (the horizontal and vertical components, designed as X-component and Z-components, respectively in this study) and the only Love waves component (designed as Y-component in this study) to a contrast of rigidity (void) in different geological contexts. The accelerations of the three components are computed using a finite element commercial code FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) for different model configurations both with and without a cavity. The seismic data are processed using the Generalized Stockwell transform (GST) in the time-frequency domain. To evaluate the effect of the cavity and locate it with respect to the source offset, the results are presented in the form of tomography maps and the group and phase velocity dispersion curve variations along the inspected array. The cavity signature was also studied at two depths relying on a perfectly homogeneous model. The velocity distribution change of the three components revealed minor changes after creating the deeper cavity. Moreover, the numerical observations demonstrated that the phase velocity is considerably more susceptible to lateral density variations than the group velocity. It was concluded that the three components revealed perceptible and distinct phase velocity changes in the presence of the void. The X-component was also found to be more effective in localizing the near and far boundaries of the cavity. The results of this numerical study suggest acquiring the three components during field seismic surveys and integrating the three components simultaneously during the analysis procedure for better efficiency

Airborne and Underwater Response of Acoustic Structures

Acoustics is a vast subject that has been utilised in many forms for millennia. Recent work has, amongst other things, explored the control of sound using geometric structure to complement inherent material properties. In this thesis, structured plates and surfaces are exploited to engineer specific acoustic responses. The acoustic transmittance and reflectance of these systems is explored in air and underwater to further understanding and develop structures that possess tailored acoustic properties. Original investigations are presented across six chapters. The first three investigations explore the transmittance of periodically perforated plates in air. The fourth investigation considers a non-resonant mechanism of obtaining complete transmission by varying the fluid environment and the fluid in the apertures of a periodically perforated plate is explored. The fifth investigation considers the transmittance through a slit in an acoustically soft plate underwater. Finally, the surface waves supported on periodically structured surfaces are explored by observing the reflectance of the surface. An acoustic field incident upon a perforated plate is partly transmitted. However, at frequencies dictated by the thickness of the plate, the acoustic field is completely transmitted. Stacking two plates with a small separation creates a resonant cavity between the plates that is the origin of a narrow acoustic stop-band at the frequency of the resonance. By varying the offset of the stacked plates and by varying the gap between the plates the frequency of this acoustic stop band is controlled. Altering the geometry of the plate surface within the gaps allows the gap to behave like an array of Helmholtz resonators, in doing so the frequency of the acoustic stop-band is significantly lowered. Varying the acoustic properties of the fluid contained within the apertures of a periodically perforated plates changes how sound is transmitted through the structure. By careful choice of the fluid environment and aperture media, it is demonstrated numerically that broadband total transmittance can be obtained. Acoustic tunnelling is demonstrated through an acoustically soft-walled slit underwater. The slit exhibits a cut-off frequency below which no propagating waves can exist, in contrast to a rigid-walled slit where propagating waves exist down to zero frequency. Resonant acoustic tunnelling is observed through two closely spaced slits in a series connection, at a frequency below the cut-off frequency of the lowest supported propagating mode. A preliminary study of pseudo surface acoustic waves on periodically structured surfaces observes the excitation of surface waves in reflection. A long pitch grating, added to the surface allows diffractive coupling of incident acoustic radiation to the surface wave. However, the height of the grating above the sample is shown to strongly affect the frequency at which the surface wave is detected. All the structures investigated may be designed to provide a desired response by careful choice of the geometry and materials.EPSRCSonardyne International Lt

Investigating the build-up of precedence effect using reflection masking

The auditory processing level involved in the build‐up of precedence [Freyman et al., J. Acoust. Soc. Am. 90, 874–884 (1991)] has been investigated here by employing reflection masked threshold (RMT) techniques. Given that RMT techniques are generally assumed to address lower levels of the auditory signal processing, such an approach represents a bottom‐up approach to the buildup of precedence. Three conditioner configurations measuring a possible buildup of reflection suppression were compared to the baseline RMT for four reflection delays ranging from 2.5–15 ms. No buildup of reflection suppression was observed for any of the conditioner configurations. Buildup of template (decrease in RMT for two of the conditioners), on the other hand, was found to be delay dependent. For five of six listeners, with reflection delay=2.5 and 15 ms, RMT decreased relative to the baseline. For 5‐ and 10‐ms delay, no change in threshold was observed. It is concluded that the low‐level auditory processing involved in RMT is not sufficient to realize a buildup of reflection suppression. This confirms suggestions that higher level processing is involved in PE buildup. The observed enhancement of reflection detection (RMT) may contribute to active suppression at higher processing levels

Development of acoustic sensor and signal processing technique.

Sewer flooding incidents in the UK are being increasingly associated with the presence of blockages. Blockages are difficult to deal with as although there are locations where they are more likely to occur, they do occur intermittently. In order to manage sewer blockage pro-actively sewer managers need to be able to identify the location of blockages promptly. Traditional CCTV inspection technologies are slow and relatively expensive so are not well suited to the rapid inspection of a network. This is needed if managers are to be able to address sewer blockages pro-actively. This thesis reports on the development of low-cost, rapidly deployable acoustic base sensor that will be able to survey live sewer pipes. The sensor emits short coded acoustic signals which are reflected from any defects of the wall of the underground pipes and recorded for future processing. The processing algorithms are based on the temporal windowing, deconvolution, Fourier, and intensity analysis so that the response can be linked directly to the location and property of the of the pipe deformation. The sensor was tested in a full scale sewer pipe in the laboratory and in few sites in UK, Austria and Netherlands and it was shown that it is able to discriminate between blockages and structural aspects of a sewer pipe such as a manhole and lateral connection. The anticipated cost is orders of magnitude lower than any current technique

Acoustic measurements of flowing and quasi-static particulate suspensions

Flowing suspensions of solid particles in gas can be found in various industrial applications, as a method for transporting powdered solids (known as "pneumatic conveying"). The problem of measuring the mass concentration of the solid fraction has not yet been satisfactorily resolved. This thesis explores acoustic techniques to measure the particle concentration. Controlled suspensions -- both flowing and quasi-static - were generated in cylindrical tubes, and their acoustic properties were measured over three frequency ranges, requiring a variety of different measuring techniques: Plane wave region (200 -4 kHz): the attenuation of plane waves travelling along the flow tube was measured. A simple method of measuring the characteristic impedance of the suspension was also devised and preliminary measurements were made. Reverberant region (4 - 20 kHz). Three parameters were measured: the decay rate of the reverberant field in certain frequency bands; the level of actively-excited steady state sound; and the frequency of transverse resonant modes of the pipe. Ultrasonic region (40 - 75 kHz): the attenuation of ultrasound was measured across the pipe diameter. The measurements were compared with theoretical predictions. They showed the predicted linearity of acoustic attenuation with concentration, although the frequency dependence was less well predicted. In general, the larger particle sizes produced the greatest discrepancy; an explanation is proposed. Ultrasonic measurements showed significant differences from the predicted frequency dependence. A method of isolating acoustic transducers from the flow with a column of clean air is described. However, measurements may be complicated by interactions at the orifice into the flow pipe. Further work is needed in this area. It is concluded that acoustic methods could be used to measure particle concentration. However, to remain insensitive to changes in the properties of the particles - size in particular - measurements must be made at more than one frequency