Selecció de materials per a sistemes de generació de so

Treballs Finals de Grau d'Enginyeria Química, Facultat de Química, Universitat de Barcelona, Curs: 2013-2014, Tutora: Segarra Rubí, MercèA bass drum is a percussion instrument made by a cylindrical shell and two membranes (drumheads) stretched over it. The batter drumhead is the vibrator element that produces the sound and the shell and the resonant drumhead are the resonant elements that modify it. Modern music genres seek for certain bass drum’s sound: a sound with a short duration with specific timbral characteristics. In order to achieve this, in recording and live sessions, the resonant drumhead is removed and half of the bass drum is filled with blankets or pillows, to stop the batter drumhead vibration. Also, the sound is processed to achieve the desired sound. The objective of this project is to improve the system’s acoustic sound by modifying the resonant drumhead’s tension and by applying an acoustic material: polyurethane foam with open cells panels. The ideal sound is obtained using actual techniques and it will be used as reference. All three possible configurations are recorded. With resonant drumhead tuned at: A) higher, B) same and C) lower tune than the batter drumhead. Results are compared with the ideal sound’s frequency spectrum. The system’s sound is improved when the resonant drumhead is tuned lower than the batter drumhead. Sound is affected along frequency range from 15 to 500 Hz..

Stress and pore pressure histories in complex tectonic settings predicted with coupled geomechanical-fluid flow models

Most of the methods currently used for pore pressure prediction in sedimentary basins assume one dimensional compaction based on relationships between vertical effective stress and porosity. These methods may be inaccurate in complex tectonic regimes where stress tensors are variable. Modelling approaches for compaction adopted within the geotechnical field account for both the full three dimensional stress tensor and the stress history. In this paper a coupled geomechanical-fluid flow model is used, along with an advanced version of the Cam-Clay constitutive model, to investigate stress,pore pressure and porosity in a Gulf of Mexico style mini-basin bounded by salt subjected to lateral deformation. The modelled structure consists of two depocentres separated by a salt diapir. 20% of horizontal shortening synchronous to basin sedimentation is imposed. An additional model accounting solely for the overpressure generated due to 1D disequilibrium compaction is also defined. The predicted deformation regime in the two depocentres of the mini-basin is one of tectonic lateral compression, in which the horizontal effective stress is higher than the vertical effective stress. In contrast, sediments above the central salt diapir show lateral extension and tectonic vertical compaction due to the rise of the diapir. Compared to the 1D model, the horizontal shortening in the mini-basin increases the predicted present-day overpressure by 50%, from 20 MPa to 30 MPa. The porosities predicted by the mini-basin models are used to perform 1D, porosity-based pore pressure predictions. The 1D method underestimated overpressure by up to 6 MPa at 3400 m depth (26% of the total overpressure) in the well located at the basin depocentre and up to 3 MPa at 1900 m depth (34% of the total overpressure) in the well located above the salt diapir. The results show how 2D/3D methods are required to accurately predict overpressure in regions in which tectonic stresses are important

Towards an integrated restoration/forward geomechanical modelling workflow for basin evolution prediction

Many sedimentary basins host important reserves of exploitable energy resources. Understanding of the present-day state of stresses, porosity, overpressure and geometric configuration is essential in order to minimize production costs and enhance safety in operations. The data that can be measured from the field is, however, limited and at a non-optimal resolution. Structural restoration (inverse modelling of past deformation) is often used to validate structural interpretations from seismic data. In addition, it provides the undeformed state of the basin, which is a pre-requisite to understanding fluid migration or to perform forward simulations. Here, we present a workflow that integrates geomechanical-based structural restoration and forward geomechanical modelling in a finite element framework. The geometry and the boundary kinematics derived from restoration are used to automatically create a forward geomechanical model. Iterative correction may then be performed by either modifying the assumptions of the restoration or modifying the restoration-derived boundary conditions in the forward model. The methodology is applied to two problems; firstly, a sand-box scale benchmark model consisting of sand sediments sliding on silicon leading to the formation of a graben structure; secondly, a field-scale thrust-related anticline from Niger Delta. Two strategies to provide further constraint on fault development in the restoration-derived forward simulation are also presented. It is shown that the workflow reproduces the first order structural features observed in the target geometry. Furthermore, it is demonstrated that the iterative approach provides improved understanding of the evolution and additional information of current-day stress and material state for the Niger Delta Case

The ins and outs of the BCCAo model for chronic hypoperfusion: a multimodal and longitudinal MRI approach

Cerebral hypoperfusion induced by bilateral common carotid artery occlusion (BCCAo) in rodents has been proposed as an experimental model of white matter damage and vascular dementia. However, the histopathological and behavioral alterations reported in this model are variable and a full characterization of the dynamic alterations is not available. Here we implemented a longitudinal multimodal magnetic resonance imaging (MRI) design, including time- of-flight angiography, high resolution T1-weighted images, T2 relaxometry mapping, diffusion tensor imaging, and cerebral blood flow measurements up to 12 weeks after BCCAo or sham-operation in Wistar rats. Changes in MRI were related to behavioral performance in executive function tasks and histopathological alterations in the same animals. MRI frequently (70%) showed various degrees of acute ischemic lesions, ranging from very small to large subcortical infarctions. Independently, delayed MRI changes were also apparent. The patterns of MRI alterations were related to either ischemic necrosis or gliosis. Progressive microstructural changes revealed by diffusion tensor imaging in white matter were confirmed by observation of myelinated fiber degeneration, including severe optic tract degeneration. The latter interfered with the visually cued learning paradigms used to test executive functions. Independently of brain damage, BCCAo induced progressive arteriogenesis in the vertebrobasilar tree, a process that was associated with blood flow recovery after 12 weeks. The structural alterations found in the basilar artery were compatible with compensatory adaptive changes driven by shear stress. In summary, BCCAo in rats induces specific signatures in multimodal MRI that are compatible with various types of histological lesion and with marked adaptive arteriogenesis

Assessing the implications of tectonic compaction on pore pressure using a coupled geomechanical approach

Overpressure prediction in tectonic environments is a challenging topic. The available pore pressure prediction methods are designed to work in environments where compaction is mostly one dimensional and driven by the vertical effective stress applied by the overburden. Furthermore, the impact of tectonic deformation on stresses, porosity and overpressure is still poorly understood. We use a novel methodology to capture the true compaction phenomena occurring in an evolving 3D stress regime by integrating a fully-coupled geomechanical approach with a critical state constitutive model. To this end, numerical models consisting of 2D plane strain clay columns are developed to account for compaction and overpressure generation during sedimentation and tectonic activity. We demonstrate that a high deviatoric stress is generated in compressional tectonic basins, resulting in a substantial decrease in porosity with continuing overpressure increase. The overpressure predictions from our numerical models are then compared to those estimated by the equivalent depth method (EDM) in order to quantify the error induced when using classical approaches, based on vertical effective stress, in tectonic environments. The stress paths presented here reveal that a deviation from the uniaxial burial trend can substantially reduce the accuracy of the EDM overpressure predictions

A diagenesis model for geomechanical simulations: formulation and implications for pore pressure and development of geological structures

Forward basin modelling is routinely used in many geological applications, with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented. Here, a new, temperature‐dependent, kinetic diagenesis model is formulated and implemented within a hydromechanical framework. The model simulates the macroscopic effects of diagenesis on: 1) porosity loss, 2) sediment strength, 3) sediment stiffness and compressibility, 4) change in elastic properties, 5) increase in tensile strength due to cementation and 6) overpressure generation. A brief overview of the main diagenetic reactions relevant to basin modelling is presented and the model calibration procedure is demonstrated using published data for the Kimmeridge Clay Formation. The calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation. The incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 MPa due to both stress‐independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability. Finally, a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behaviour to be captured due to the increase in strength that results in an over‐consolidated stress state. This is illustrated by comparison of the present day structures predicted by a geomechanical‐only model, where a ductile fold forms, and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted