A tomografia sísmica convencional apresenta baixa resolução em regiões de baixa sismicidade, como consequência, estudos prévios realizados na América do Sul não mapearam com clareza áreas menores do continente, como a região da bacia do Paraná. Para descrever a estrutura da crosta na região da bacia do Paraná, além de dados obtidos através da análise de terremotos andinos, foram utilizadas curvas de dispersão provenientes da correlação do ruído ambiental entre pares de estações sismográficas, técnica conhecida com Ambient Noise Tomography (ANT). Os mapas de velocidade de grupo obtidos correspondem bem com as principais províncias geológicas já conhecidas na América do Sul: baixas velocidades sob os Andes e bacias sedimentares e altas velocidades sob regiões cratônicas. As anomalias de alta velocidade encontradas para o manto superior, concordam com trabalhos prévios que confirmam a presença de um núcleo cratônico sob a bacia. Contudo, não é possível afirmar se tal núcleo está intacto ou não, principalmente devido resultados anteriores que mostram evidências da existência de um núcleo dividido por zonas de sutura. No entanto, a tomografia de ondas de superfície não possui resolução para confirmar este modelo. Como as etapas do processamento de dados são bem definidas e independentes, à medida que novas estações forem instaladas com o avanço do projeto BRASIS, novos caminhos serão adicionadas à base inicial, aumentando a resolução e confiabilidade dos resultados futuros. / Conventional seismic tomography is known to have poor resolution in regions of low seismicity, therefore, studies carried out in South America did not mapped clearly smaller areas of the continent, for example the region of the Parana basin. To describe the of the crustal structure of the Parana basin, in addition to data obtained by analysis of Andean earthquakes, we used dispersion curves from ambient noise correlation between pairs of seismographic stations, a technique known by Ambient Noise Tomography (ANT). The obtained group velocity maps correspond well with the main geological provinces already known in South America: low velocities under the Andes and sedimentary basins and high velocities in cratonic regions. The high velocity anomalies encountered in the upper mantle, agree with previous studies that confirm the presence of a nucleus in the cratonic basin. However, it is not possible to say whether such a nucleus is intact or not, mainly because previous results showing evidence of the existence of a nucleus divided by suture zones. Nevertheless, surface wave tomography has no resolution to confirm this model. The steps of data processing of this work are well defined and independent, thus, as new stations will being deployed with the advance of BRASIS Project, new paths will be added to the database, increasing the resolution and reliability of the future results.

La thèse se concentre sur le suivi temporel des vitesses sismiques, notamment dans des zones de faille actives. En corrélant les signaux générés par le bruit ambiant, il est possible d'estimer la fonction de Green du milieu. Par le suivi continu de ces fonctions, des changements de vitesse dans le milieu peuvent être détectés. Les méthodes de suivi temporel sont appliquées aux données provenant d'une zone de faille active à Parkfield, Californie, ce qui permet de détecter deux chutes de vitesse. Ces dernières coïncident avec des évènements sismiques régionaux, la plus importante concernant un évènement proche des stations. Les deux chutes de vitesse sont suivies d'une récupération postsismique progressive. Pour mieux comprendre la fiabilité des mesures on a effectué des expériences en laboratoire. Un résultat intéressant de ces expériences montre que la reconstruction exacte de la fonction de Green n'est pas nécessaire pour le suivi temporel, ce qui ouvre la voie à de nombreuses possibilités d'applications en sismologie. Grâce à cette connaissance, la série de données de Parkfield a été ré-analysée. En améliorant la résolution temporelle à 1 journée, on montre que la chute de vitesse observée est cosismique avec le séisme de Parkfield. On a établi que les fluctuations de vitesse ne sont pas simplement corrélées aux variations de la distribution de sources du bruit obtenue par formation de voies. Enfin, les méthodes développées sont appliquées à un séisme au Japon. Le réseau étant de taille beaucoup plus grande que celui utilisé pour l'étude de Parkfield, ces données sont analysées pour étudier la dépendance entre la distance stations-séisme et la chute de vitesse mesurée. / The thesis concentrates on monitoring seismic speeds, especially in active fault zones. By correlating signals generated by background noise, one can estimate the Green's function of a medium. When continuously following these functions, wave speed changes in the medium can be detected. Monitoring methods are applied to data from an active fault zone in Parkfield, California, where two wave speed drops, which coincide with regional seismic events, are detected. The largest corresponds to an event close to the stations. Both speed drops are followed by a gradual postseismic relaxation. In order to understand the reliability of the measurements, we perform laboratory experiments. One interesting result of these experiments shows that an exact reconstruction of the Green's function is not necessary for monitoring, which opens up many possibilities of applications to seismology. Armed with this knowledge, the Parkfield data is analysed again. By improving the temporal resolution to 1 day, we show that the observed speed drop is coseismic with the Parkfield event. We establish that the speed fluctuations are not simply correlated to variations in noise source distribution obtained by beamforming. Finally, the developed methods are applied to an event in Japan. Since the array is spatially much larger than the one used at Parkfield, this data is analyzed to study the dependence between station-event distance and the measured seismic speed drop. STAR

The ocean ambient noise is one of the important parameters in sonar equation. The ocean ambient noise includes diverse and complex sources like waves, marine life, ships, and etc. Using different ways to analyze are needed to understand the complicated properties of ambient noise. Empirical equation obtained from linear regression of wind speed and ambient noise data is a common method to predict noise level. In this article, the ambient noise data were collected from experiments at northeastern sea of Taiwan in 2007, 2008 and 2009. Applying corresponding wind speed data to observed noise level the time series, coefficient of determination is used to estimate how noise fit with wind speed data of regression. The K-S test and Sea States are used to determine the wind speed threshold. Although it is the same sea area in three years, the ocean ambient noise still has variations due to time and variance of sound sources, so it is important to be investigated. This study compares the statistical properties and distribution in ambient noise level and frequencies with corresponding wind speed in same season.

In the traditional analysis of acoustic wave propagation in an ocean waveguide, it's generally assumed that acoustic properties, including density and sound speed profile at seabed are taken to be constant. However, recent experimental data provided by Hamilton~(1980)~ have shown that the sediment layer in the seabed experiences a transitional change in which the density and the sound speed vary continuously from one value at the top to another at the bottom of the layer. The objective of this study is to investigate the surface-generated ambient noise in an isovelocity waveguide with a non-homogeneous fluid sediment layer. The noise model was first proposed by Kuperman and Ingenito~(1980) in the study of surface-generated ambient noise using normal mode approach, and the model proposed by Robins (1993) in the study of the sediment layer change in which the density and the sound speed vary continuously. It is demonstrated that the noise intensity may be affected by the stratification mainly through the continuous spectrum, in that the continuous spectrum is equally important as the normal modes in the present analysis. The continuous variation of the sediment layer reduces the contrast of the interface, which in turn affects the wavenumber spectrum, particularly in the continuous spectrum region. The results show that the horizontal correlation length of the noise field increases as that of the noise random sourse increase, but the vertical correlation length of the noise field decreases as that of the noise random sourse increase.

This article studies ambient noise in the surf zone that was simulated by piston-type wavemaker in the tank. The experiment analyzed the bubbles of breaking wave by using a hydrophone to receive the acoustic signal, and the images of bubbles were recorded by a digital video camera to observe distribution of bubbles. The tank is in College of Marine Sciences, National Sun Yat-sen University, the dimensions of water tank are 35 m ¡Ñ1 m ¡Ñ1.2 m, and the slope of the simulated seabed is 1:5. The studied parameters of ambient noise generates by breaking wave bubbles were wave height, period, and water depth. Short-time Fourier Transform was applied to obtain the acoustic spectrum of bubbles, MATLAB programs were used to calculate mean sound pressure level, and determine number of bubbles. Bubbles with resonant frequency from 0.5 to 10 kHz were studied, counted from peaks in the spectrum. The number of bubbles generated by breaking waves could be estimated by bubbles energy distributions. The sound pressure level of ambient noise was highly related to the wave height and period, with correlation coefficient 0.7. The results were compared with other studies of ambient noise in the surf.

Sound wave has much better transmission in ocean environment than electromagnetic waves, therefore sonar systems are widely applied in underwater investigations. However, not only the target signal is received by the sonar but also the noise from different directions. The noise will affect the performance of the sonar, so the understanding of ocean ambient is an important issue both in academic study and military applications. The ambient noise data of this research was collected by a passive acoustic recording system deployed in the southwest sea of Taiwan, along with the information of wind velocity in the experimented area. The influence on noise level fluctuations by the variation of the wind velocity was first discussed in light of correlation analysis. The fluctuations were expressed in terms of statistic distribution, mean value, standard deviation in different time series. As results, 500 Hz and 1.5k Hz were saturated by high levels signal from unknown sources in spring and summer, so the average sound levels were higher than in fall and winter, about 10 dB and 5 dB higher for 500 Hz and 1.5k Hz respectively. In seasonal analysis, 2.4k and 3.6k Hz have quite stable the mean levels and their standard deviations were around 3 dB. Especially, the noise level of 3.6 Hz has the least fluctuation throughout the year than any other frequencies analyzed. It was also observed that the noise level was decreased with the increase of frequency. Calculated by linear regression, this research worked out the estimation equation for the ambient noise level at high wind speed. However, the estimated values are higher than the measured data, it is due to the distribution of wind velocity. The wind data in this study was skewed towards the lower velocity, consequently the predicted values were overestimated.

Ocean ambient noise is one of the important parameters which affect sonar system performance. If the ocean ambient noise is estimated accurately, the prediction accuracy of sonar system performance can be promoted significantly. Ocean ambient noise includes various and diverse sources, so the characteristics of the ocean ambient noise should be analyzed by long-term observations and statistical methods. Therefore, ocean ambient noise database was developed to facilitate management, preservation, and application of these datasets which increase with time. There were two datasets of acoustic and three datasets of wind speed in this database at this point. To develop this database systematically, this study applied Entity-Relationship Model to describe the relationship between different data and Relational Model to design the required categories. The database was constructed based on Microsoft Office Access, and user-friendly graphical interfaces based on MATLAB were provided for users: wind speed regression, time series, spectrogram, and spectrum tendency for users to query the database. As the results of analysis, the intermittent, unknown, and high-level sources at southwestern sea of Taiwan in spring and summer made the noise level of low frequency about 8 to 10 dB higher in the night-time than that in the day-time. According to wind speed regression analysis, if there were sufficient data, the method would be practicable to pick a wind speed data nearby the location of acoustic data for noise estimation. As the results of the typhoon effects analysis, the noise level of 1 to 8 kHz was increased significantly by typhoons. Besides, the noise level didn¡¦t decrease immediately and significantly after the pass of typhoon because of southwestern air current caused typhoon. After the database was developed, new acoustic data will keep being measured and collected, and the network capability will be integrated into the database to make the database more accessible to users.

Ocean Ambient Noise covers wide range except target signal in the sonar equation and is an influential parameter in sonar performance. Empirical equation obtained from linear regression of wind speed and ambient noise data is a common method to predict the noise level. Both ambient noise and wind speed data collected from experiments in southwest and northeast Taiwan sea were analyzed in statistics and time series. Experiment data was also used for prediction equations and further analysis. Coefficient of determination (r2) and F-test for the slope of the regression line were used to estimate how noise fit with wind speed data and the credibility of the regression. The result of the analysis was that the distribution of r2 changes with regions. The values of r2 calculated from northeast experiment data are higher than southwest because of the high percentage of high wind speed. The data from the northeast experiment is considered more appropriate for the prediction of noise level because the higher value of r2. All results of F-test showed the correlation between wind speed are statistically significant except the winter data in the southwest experiment. By using these two indicators, the credibility of the prediction equation can be realized and the prediction performance of sonar is promoted.

Cette thèse s'intéresse au développement des méthodes d'auscultation sismique passive pour l'imagerie et la surveillance des mines profondes. Les résultats marquants sont 1/ la possibilité d'imager en profondeur les structures géologiques d'intérêt et 2/ la possibilité de suivre dans le temps les propriétés mécaniques des roches qui subissent les sollicitation associés à l'exploitation minière. Ce travail ouvre des perspectives quant à l'amélioration de la sécurité dans les mines profondes. / This work focus on using passive noise-based seismic methods to image and monitor the rock mass in underground mines. The main results show that it is possible to gain benefit from the diffuse ambient seismic field in mines to 1/ image the rock mass and 2/ monitor its mechanical property changes over time. This work opens a way to improve safety in deep underground mines.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第24963号 / 理博第4988号 / 新制||理||1712(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 澁谷 拓郎, 教授 久家 慶子, 教授 大見 士朗 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM