Co-detection of acoustic emissions during failure of heterogeneous media: new perspectives for natural hazard early warning

A promising method for real time early warning of gravity driven rupture that considers both the heterogeneity of natural media and characteristics of acoustic emissions attenuation is proposed. The method capitalizes on co-detection of elastic waves emanating from micro-cracks by multiple and spatially separated sensors. Event co-detection is considered as surrogate for large event size with more frequent co-detected events marking imminence of catastrophic failure. Using a spatially explicit fiber bundle numerical model with spatially correlated mechanical strength and two load redistribution rules, we constructed a range of mechanical failure scenarios and associated failure events (mapped into AE) in space and time. Analysis considering hypothetical arrays of sensors and consideration of signal attenuation demonstrate the potential of the co-detection principles even for insensitive sensors to provide early warning for imminent global failure

Applications of acoustics in the measurement of coal slab thickness

The determination of the possibility of employing acoustic waves at ultrasonic frequencies for measurements of thicknesses of slabs of coal backed by shale is investigated. Fundamental information concerning the acoustical properties of coal, and the relationship between these properties and the structural and compositional parameters used to characterize coal samples was also sought. The testing device, which utilizes two matched transducers, is described

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

AN IMPROVED METHOD FOR THE IDENTIFICATION AND INVERSION OF MULTI-MODE RAYLEIGH SURFACE WAVE DISPERSION COLLECTED FROM NON-UNIFORM ARRAYS UTILIZING A MOVING SOURCE APPROACH

An improved method using a moving source approach is utilized in the analysis of Rayleigh surface waves for the accurate identification of higher mode propagation used in inversion. Two non invasive surface wave methods, Multi- station Analysis of Surface Waves (MASW) and Refraction Microtremor (ReMi) were used for the construction of composite dispersion curves representing the relationship of Rayleigh phase velocity (VR) with frequency. Multiple tests were executed with source offsets increasing with each successive test in order to account for near field effects and higher mode attenuation levels. The resulting dispersions were combined to form a composite dispersion which effectively maps all participating modes of propagation. The inversion was executed using a genetic algorithm (GA) which takes advantage of the Rayleigh forward problem. The results show good ability to identify intermediate high and low velocity layers and agree well with downhole results

Propagation and attenuation of seismic Rayleigh waves along single paths in Scotland

SIGLELD:D50447/84 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Pavement testing by integrated geophysical methods: Feasibility, resolution and diagnostic potential

This work is focused on the assessment of the diagnostic potential of several geophysical methods when applied to the investigation of a rigid airport pavement. The potential and limit of each technique are evaluated as well as the added value deriving from their integration. Firstly, we reconstruct a high-resolution image of the pavement by a large electromagnetic and georadar screening. An advanced processing of georadar data, implemented through the picking of the arrival times of reflections for each profile, provides a quantitative estimation of the deviation between the design and the as-built thickness of layers. Additionally, electrical tomography has been applied to unequivocally identify the anomalous zones, where higher values of resistivity would be associated to porous zones that are prone to degradation and failure. The seismic tomographic survey had the additional purpose to recover the mechanical properties of the pavement in terms of both P- and S-waves and consequently of elastic constants (Poisson's ratio), whose values were consistent with those recovered in literature. The anomalies detected by each technique are consistent in their indications and they can be correlated to failure phenomena occurring at layer interfaces within the pavement structure or to unexpected variations of the layer thicknesses. The cost-effective geophysical campaign has validated the four-layered system deduced from the original design and has been used to reconstruct a high-resolution map of the pavement in order to discriminate fractures, crack-prone areas or areas where the as-built differs from the original design

Scientific Rationale and Requirements for a Global Seismic Network on Mars

Following a brief overview of the mission concepts for a Mars Global Network Mission as of the time of the workshop, we present the principal scientific objectives to be achieved by a Mars seismic network. We review the lessons for extraterrestrial seismology gained from experience to date on the Moon and on Mars. An important unknown on Mars is the expected rate of seismicity, but theoretical expectations and extrapolation from lunar experience both support the view that seismicity rates, wave propagation characteristics, and signal-to-noise ratios are favorable to the collection of a scientifically rich dataset during the multiyear operation of a global seismic experiment. We discuss how particular types of seismic waves will provide the most useful information to address each of the scientific objectives, and this discussion provides the basis for a strategy for station siting. Finally, we define the necessary technical requirements for the seismic stations