Dans cet article on propose une nouvelle m´ethode de localisation de la source d’un signal bas´ee sur l’´energie de la source. La m´ethode est d´evelopp´ee et test´ee avec des donn´ees synth´etiques et exp´erimentales. Une bille en acier est lanc´ee sur des points marqu´es sur une plaque en verre (ou en plexiglass selon les exp´eriences). La distance entre la position estim´ee et la position r´eelle a ´et´e enregistr´ee pour plusieurs points. La nouvelle m´ethode de localisation a ´et´e compar´ee avec les m´ethodes pr´e-existantes. On a conclu que la m´ethode propos´ee est polyvalente et qu’elle donne de meilleurs r´esultats que les autres m´ethodes. La m´ethode utilise les signaux arrivant
`a plusieurs capteurs (4 dans le cas pr´esent) et calcule l’´energie de la source grˆace
`a une grille couvrant tout l’espace o`u peut ˆetre positionn´ee cette source. Sur la grille, le meilleur ajustement (best fit) des positions estim´ees par tous les capteurs montre la position finale estim´ee de la source. La qualit´e de l’estimation avec cette m´ethode a ´et´e compar´ee pour diff´erentes fr´equences d’´echantillonnage, pour des plaques dispersives ou non dispersives et avec ou non des ondes r´efl´echies sur les cˆot´es des plaques (plaques infinie ou finie). Il est possible d’adapter cette m´ethode
en 3D dans le cadre d’un futur projet. Cet article a ´et´e publi´e in Review of Scientific Instruments en Septembre 2016 et a ´et´e ajout´e dans le chapitre final de cette th`ese.
Chapter 2 Introduction
This thesis is article based, so the text is intended to wrap up the work and results covered in the papers, making it easier to read the work as a whole. This first chapter gives a general introduction to the topic of flow in transforming porous media in the field of experimental geophysics and physics. The problem is put into context, and various challenges and motivations for studying such processes are introduced.
Chapter 2 introduces basic concepts and analysis methods. Then, the work of this thesis is presented in separate chapters for each paper, including the main questions, methods and results. In the final chapter, future perspectives are discussed. The thesis also has an appendix including the scientific articles where I contributed as a co-author with experiments and discussions.
2.1 Context and motivation
During many subsurface processes in nature and industry, the flow of fluids trans-forms the surrounding porous medium containing them (reservoir), e.g. when fluids flow at a high rate between the grains of soil or chemically react with the solid within pore networks in rocks. Depending on the involved mechanisms, these transforma-tion processes can be fast, lasting only a couple of seconds or less, or slow, lasting from a few hours to several weeks or more. Fast transformation processes of reser-voirs include sudden mechanical deformation, fracturing and channel formation due to sufficient overpressure or flow rates in the pore fluid. Slow transformation pro-cesses of reservoirs include chemical evolution of existing fractures or pore networks due to reactions between the flowing fluid and host medium, i.e. dissolution and precipitation. Reservoir transformation processes further increase or decrease the permeability of the system, which influences the flow of pore fluids and may change the stresses in the material surrounding the deformations. Thus regardless of rate, reservoir transformations due to fluid flow change the boundary conditions of the flow which itself is causing the changes on the host medium. These are cases where the deformations and fluid flow are coupled processes, which makes a system very challenging to fully understand. Even as separate problems, fluid flow in porous media and deformation of granular media are complicated processes, for example granular materials may have a behavior that resemble solid-, liquid-, or gas-like ma-terials depending on the flow regime. This is one of the special properties of granular
materials [8]. In addition, the rheology of dense granular fluids is very complex and is still an open research topic [9]. Another example is immiscible flow in porous media, i.e. flow involving at least two fluids that cannot mix, where one fluid is displacing the other. This adds interfacial forces to the system, which influences the flow path taken by the fluids.
Since flow and deformations in porous media often occur in nature and industry, this is a heavily researched topic in many fields of earth sciences. In some cases, for example, the research is focused on understanding and preventing dangerous situations due to high pressures and deformations, while in other cases it is studied for situations where such processes are desirable to induce in a controlled manner.
In physics, a motivation for studying this topic is to increase the fundamental un-derstanding of these complex systems, and in geology further knowledge of such processes help to explain active natural systems or structures formed in soil and rocks. Furthermore, increased knowledge of the fast transformation processes may have applications in earth science and industry where multiphase flow and solid deformation occur; Several processes in engineering, industry and earth sciences in-volve pneumatic (gas) or hydraulic (liquid) fracturing of the soil, which occurs when fluids in the ground are driven to high enough pressures to deform, fracture and generate porosity in the surrounding soil or rock. For example in environmental engineering, pneumatic or hydraulic fracturing is done to enhance the removal of hazardous contaminants in the vadose zone (soil remediation) [10, 11], for soil stabi-lization injection to ensure a solid foundation for structures [12], or in packer tests for project planning, risk assessment and safe construction of dams and tunnels [13].
In industry, hydraulic fracturing is done to enhance oil and gas recovery [14–16], CO2 sequestration [17], water well- and geothermal energy production [18–20]. Re-lated natural processes such as subsurface sediment mobilization are studied in earth sciences, where sand injectites, mud diapirs and mud volcanoes are formed due to pore-fluid overpressure [21–26]. For example, the Lusi mud volcano in Indonesia is the biggest and most damaging mud volcano in the world [27], having displaced 40 000 people from their homes, and has been active since May 2006. There is an ongoing debate about how it was triggered, i.e. whether it formed naturally by an earthquake or geothermal process [28–31], or that it is a man-made consequence of a nearby drilling operation by a company probing for natural gas [32]. The evolution of faults and fractures at crustal scale can also be affected by fluid flow [33–35] as well as the rheology of fluid saturated faults [36–38].