Monitoring Local Changes in Granite Rock Under Biaxial Test: A Spatiotemporal Imaging Application With Diffuse Waves

International audienceDiffuse acoustic or seismic waves are highly sensitive to detect changes of mechanical properties in heterogeneous geological materials. In particular, thanks to acousto-elasticity, we can quantify stress changes by tracking acoustic or seismic relative velocity changes in the material at test.In this paper, we report on a small-scale laboratory application of an innovative time-lapse tomography technique named Locadiff to image spatio-temporal mechanical changes on a granite sample under biaxial loading, using diffuse waves at ultrasonic frequencies ( 300 kHz to 900 kHz). We demonstrate the ability of the method to image reversible stress evolution and deformation process, together with the development of reversible and irreversible localized micro-damage in the specimen at an early stage. Using full-field infrared thermography, we visualize stress induced temperature changes and validate stress images obtained from diffuse ultrasound. We demonstrate that the inversion with a good resolution can be achieved with only a limited number of receivers distributed around a single source, all located at the free surface of the specimen. This small-scale experiment is a proof of concept for frictional earthquake-like failure (e.g. stick slip) research at laboratory scale as well as large scale seismic applications, potentially including active fault monitoring

Doctor of Philosophy

dissertationAs part of the EarthScope initiative, the USArray Transportable Array (TA) consisted of seismic sensor platforms that were developed to improve understanding of subsurface processes beneath the North American continent. Atmospheric pressure sensors depl

Improving the knowledge of the 2013 Castor UGS (Spain) seismic sequence from a seismological and geomechanical standpoint

The injection of fluids into the crust can modify the governing stress state and therefore generate seismic activity. The 2013 seismic sequence which took place in the area around the Castor offshore underground gas storage facility in northeastern Spain is studied here; the project aims to assess the anthropogenic seismicity problem. Location of a dataset of 50 earthquakes which are interpreted jointly with the largest 45, and a focal mechanism computation of the 8 strongest events were performed; seismological aspects were completed by studying the frequency-magnitude relationship. The final part of the study focused on earthquake static stress transfer as a trigger, which was quantified through Coulomb stress changes. Results show that relating earthquakes and faults from earthquake location is difficult, but focal mechanism solutions indicate that both NE-SW to NNE-SSW and NW-SE oriented planes could have hosted the earthquakes. Seismicity could have taken place at greater depths than the reservoir bottom. Frequency-magnitude distribution points towards a first phase of the sequence exhibiting a b value of around 1.5, higher than expected for tectonic earthquakes, but lower than the usually reported values for induced seismicity; afterwards, the b parameter drops towards likely values for slip in active fault systems (near to 1.0), although the linearity of the distribution should be argued. Computed static stress transfer due to the 8 largest earthquakes is consistent with the activation of an eastdipping, NE-SW-striking fault, and it should not have significantly altered return periods of characteristic tremors in the area. The question regarding the origin of seismicity cannot be rigorously addressed unless a quantitative model which considers gas injections and fluid diffusion throughout the fracture network, as well as other earthquake-triggering mechanisms, is made.Injectar fluids a l'interior de l'escorça terrestre pot modificar l'estat d'esforços dominant i conseqüentment generar activitat sísmica. Aquest treball estudia la seqüència sísmica de 2013 en l'àrea al voltant del magatzem de gas subterrani Castor al nord-est d'Espanya; l'objectiu és avaluar la problemàtica en relació a la sismicitat antropogènica. Es van realitzar localitzacions d'un grup de 50 terratrèmols que s'interpreten en conjunt amb els 45 de major magnitud, així com el càlcul dels mecanismes focals pels 8 més grans; els aspectes sismològics es van completar a partir de l'estudi de la distribució magnitudfreqüència. La part final de l'estudi es concentra en la transferència d'esforç estàtic per terratrèmols propers com a mecanisme físic desestabilitzant, que es quantifica a partir de canvis d'esforç de Coulomb. Els resultats mostren que relacionar terratrèmols i falles a partir de les localitzacions és difícil, però les solucions dels mecanismes focals indiquen que tant falles amb orientació NE-SW a NNE-SSW com NW-SE podrien haver generat els terratrèmols. La sismicitat podria ser més profunda que la base del reservori. La distribució magnitud-freqüència suggereix una primera fase de la seqüència amb valors del paràmetre b al voltant de 1.5, més alts que el que s'espera per terratrèmols tectònics, però inferiors que els valors normalment trobats per projectes que generen sismicitat induïda; posteriorment, el valor de b cau fins a valors normals en el context de terratrèmols tectònics (al voltant de 1.0), tot i que la linealitat de la relació és discutible. Els valors obtinguts a partir de la transferència d'esforç estàtic degut als 8 terratrèmols de major magnitud és consistent amb l'activació d'una falla d'orientació NE-SW que cabussa cap a l'est, i no hauria d'haver modificat de manera rellevant els períodes de retorn de terratrèmols característics en la zona. La qüestió en referència a l'origen de la sismicitat no es pot discutir de manera rigorosa sense un model que permeti quantificar les injeccions de gas i la difusió del fluid a través de la xarxa de fractures presents, així com altres mecanismes que poden desencadenar terratrèmols

The Importance of Marine Research Infrastructures in Capturing Processes and Impacts of Extreme Events

Extreme events have long been underestimated in the extent to which they shape the surface of our planet, our environment, its ecological integrity, and the sustainability of human society. Extreme events are by definition rarely observed, of significant impact and, as a result of their spatiotemporal range, not always easily predicted. Extremes may be short-term catastrophic events such as tsunamis, or long-term evolving events such as those linked to climate change; both modify the environment, producing irreversible changes or regime shifts. Whatever the driver that triggers the extreme event, the damages are often due to a combination of several processes and their impacts can affect large areas with secondary events (domino effect), whose effects in turn may persist well beyond the duration of the trigger event itself. Early studies of extreme events were limited to opportunistic approaches: observations were made within the context of naturally occurring events with high societal impact. Given that climate change is now moving us out of a relatively static climate regime during the development of human civilization, extreme events are now a function of underlying climate shifts overlain by catastrophic processes. Their impacts are often due to synergistic factors, all relevant in understanding process dynamics; therefore, an integrated methodology has become essential to enhance the reliability of new assessments and to develop strategies to mitigate societal impacts. Here we summarize the current state of extreme event monitoring in the marine system, highlighting the advantages of a multidisciplinary approach using Research Infrastructures for providing the temporal and spatial resolution required to monitor Earth processes and enhance assessment of associated impacts.publishedVersio

Past, Present and Future of a Habitable Earth

This perspective of this book views Earth's various layers as a whole system, and tries to understand how to achieve harmony and sustainable development between human society and nature, with the theme of " habitability of the Earth." This book is one effort at providing an overview of some of the recent exciting advances Chinese geoscientists have made. It is the concerted team effort of a group of researchers from diverse backgrounds to generalize their vision for Earth science in the next 10 years. The book is intended for scholars, administrators of the Science and Technology policy department, and science research funding agencies. This is an open access book