The use of geophysical testing methods has considerable potential to be a cost effective and accurate technique to assess near-surface soil conditions. Multi channel analysis of surface waves (MASW) test is a geophysical non-intrusive test that uses the dispersive characteristic of Rayleigh waves to estimate low strain shear modulus and damping coefficient of near-surface soil. Also, this technique is used to detect underground voids. Recently, MASW technique has gained more attention, partly because of its ease of use and partly because of the significant improvements in data acquisition systems. The theories of MASW test consider the effect of horizontal soil layering, though the effect of lateral inhomogeneities (i. e. cavities and voids), inclined layering and inverse layering (i. e. a layered system in which the top layers are stiffer than the bottom ones) are not addressed properly in these theories. <br /><br /> The objective of this dissertation is to investigate the effect of lateral inhomogeneities on the propagation of Rayleigh waves in an elastic half-space excited by a transient loading. The results can be applied to locate underground cavities using MASW test and to improve the MASW analysis techniques. In lieu of theoretical solutions, two and three dimensional numerical models are constructed to simulate the MASW test. To assure the quality of the obtained data, numerical models are calibrated with Lamb solution. Voids with different sizes and embedment depths are inserted in the medium. Responses along the surface as well as inside the medium are recorded and analyzed in time, frequency, spatial and frequency-wave number domains. Different material types and sources are used to generalize the results. Afterwards, the combined effect of void and layered systems on the surface responses are studied. To verify the results, experimental field and laboratory data are presented and the trends are compared to the numerical results. <br /><br /> It is found that the void starts to vibrate in response to the Rayleigh wave excitation. Due to the vibration of the void energy partitioning occurs. Part of the incident energy is reflected in the form of Rayleigh wave. Another part is converted to body waves, and spread into the medium. The transferred part of the energy is attenuated and has smaller amplitudes. Finally, a part of energy is trapped in the void region and bounces back and forth between the void boundaries, until it damps. The trapped energy is associated to higher modes of Rayleigh waves and excited Lamb waves. The effect of trapped energy is seen as a region in the vicinity of the void with concentrated energy, in frequency domain. The extents of this region depends on the void size, and the frequency content of the incident energy. Thus, in some cases it is possible to correspond the size of the model to the extents of the region with energy concentration. <br /><br /> A new technique is proposed to determine the location of a void, and estimate its embedment depth. The technique is called Attenuation Analysis of Rayleigh Waves (AARW), and is based on the observed damping effect of the void on the surface responses. For verification, the results are compared to experimental field and laboratory data. The observations are in good agreement with the observed numerical results. Further, the AARW technique showed to be a promising tool for void detection.

The use of geophysical testing methods has considerable potential to be a cost effective and accurate technique to assess near-surface soil conditions. Multi channel analysis of surface waves (MASW) test is a geophysical non-intrusive test that uses the dispersive characteristic of Rayleigh waves to estimate low strain shear modulus and damping coefficient of near-surface soil. Also, this technique is used to detect underground voids. Recently, MASW technique has gained more attention, partly because of its ease of use and partly because of the significant improvements in data acquisition systems. The theories of MASW test consider the effect of horizontal soil layering, though the effect of lateral inhomogeneities (i. e. cavities and voids), inclined layering and inverse layering (i. e. a layered system in which the top layers are stiffer than the bottom ones) are not addressed properly in these theories. <br /><br /> The objective of this dissertation is to investigate the effect of lateral inhomogeneities on the propagation of Rayleigh waves in an elastic half-space excited by a transient loading. The results can be applied to locate underground cavities using MASW test and to improve the MASW analysis techniques. In lieu of theoretical solutions, two and three dimensional numerical models are constructed to simulate the MASW test. To assure the quality of the obtained data, numerical models are calibrated with Lamb solution. Voids with different sizes and embedment depths are inserted in the medium. Responses along the surface as well as inside the medium are recorded and analyzed in time, frequency, spatial and frequency-wave number domains. Different material types and sources are used to generalize the results. Afterwards, the combined effect of void and layered systems on the surface responses are studied. To verify the results, experimental field and laboratory data are presented and the trends are compared to the numerical results. <br /><br /> It is found that the void starts to vibrate in response to the Rayleigh wave excitation. Due to the vibration of the void energy partitioning occurs. Part of the incident energy is reflected in the form of Rayleigh wave. Another part is converted to body waves, and spread into the medium. The transferred part of the energy is attenuated and has smaller amplitudes. Finally, a part of energy is trapped in the void region and bounces back and forth between the void boundaries, until it damps. The trapped energy is associated to higher modes of Rayleigh waves and excited Lamb waves. The effect of trapped energy is seen as a region in the vicinity of the void with concentrated energy, in frequency domain. The extents of this region depends on the void size, and the frequency content of the incident energy. Thus, in some cases it is possible to correspond the size of the model to the extents of the region with energy concentration. <br /><br /> A new technique is proposed to determine the location of a void, and estimate its embedment depth. The technique is called Attenuation Analysis of Rayleigh Waves (AARW), and is based on the observed damping effect of the void on the surface responses. For verification, the results are compared to experimental field and laboratory data. The observations are in good agreement with the observed numerical results. Further, the AARW technique showed to be a promising tool for void detection.

L’objectif principal de cette thèse est de développer et d’optimiser les outils d’acquisition de la technique de Sismique réflexion haute résolution (SHR) afin d’améliorer ses performances pour la détection des cavités souterraines. Il est communément admis que l’imagerie SHR est d’autant plus complexe que la profondeur de la cible est petite. Les travaux menés dans le cadre de cette thèse devraient remédier à certains problèmes les plus critiques identifiés lors des applications de la SHR.L’utilisation des sources vibratoires présente des avantages indéniables (non destructivité, contrôle du signal émis...) par rapport aux sources « classiques » (i.e. impulsionnelles, destructives) mais leur application optimale nécessite un choix correct du signal émis.Ainsi, les travaux de recherche réalisés ont permis de (1) développer un système complet de pilotage par ordinateur d’une mini-source vibratoire destinée à l’imagerie SHR, (2) développer une méthode de génération de signaux émis. En établissant un lien entre le signal d’entrée et l’image sismique obtenue, cette procédure offre la possibilité à l’utilisateur de choisir le signal émis en fonction des conditions de terrain, et des objectifs des mesures, (3) tester le fonctionnement du système développé avec plusieurs mini-vibrateurs.Le système développé ainsi est testé et validé dans les tests à petite échelle. Ensuite, il a été utilisé dans les conditions réelles avec l’objectif « détection des cavités » dans le contexte salin (anciennes mines de sel en Lorraine, profondeur : 160 m - 180 m) et les marnières de Haute Normandie (anciennes carrières de craie, profondeur : 15 m - 45 m) / The main objective of this thesis is to develop and optimize the acquisition tools for High Resolution Reflection Seismic (HRS) technique in order to improve its performances in the detection of underground cavities. It is commonly admitted that HRS imaging becomes more complicated with when the depth of interest is decreased. The work carried out in the frame of this thesis aims to bring solutions to some of the most critical problems identified in application of the HRS.The vibratory sources show undeniable advantages (non destructivity, controllable output signal) over “classic” (impulsive, destructive) sources. However, the optimal use of these sources depends on the proper choice of emitted signal.Thus, the research work carried out resulted in (1) development of a complete, computer-based vibrator control system allowing piloting small vibratory source intended to use for HRS surveys, (2) development of a method for generating the source signal. The proposed procedure links the entry signal with seismic image and thus allows the choice of the signal in terrain conditions and with regard to the measurement goals, (3) extensive testing of the developed system with several portable vibratory sources.The developed system was tested and validated in small-scale tests. Afterwards it was used in real conditions with the goal of “cavity detection” in salt-mining context (old salt mines in Lorraine region at depths between 160 m and 180 m) as well as in chalk-mining area (ancient marl-pit quarries in the Normandy region at depths 15 m - 45 m)