Seismic wave propagation in heterogeneous limestone samples

International audienceMimic near-surface seismic field measurements at a small scale, in the laboratory, under a well-controlled environment, may lead to a better understanding of wave propagation in complex media such as in geological materials. Laboratory experiments can help in particular to constrain and refine theoretical and numerical modelling of physical phenomena occurring during seismic propagation, in order to make a better use of the complete set of measurements recorded in the field. We have developed a laser Doppler vibrometer (laser interferometry) platform designed to measure non-contact seismic displacements (or velocities) of a surface. This technology enables to measure displacements as small as a tenth of a nanometer on a wide range of frequencies, from a few tenths to a few megahertz. Our experimental setup is particularly suited to provide high-density spatial and temporal records of displacements on the edge of any vibrating material (aluminum, limestone, ...). We will firstly present experiments in cuboid and cylinders of aluminum (homogeneous) in order to calibrate the seismic sources (radiation diagram, frequency content) and identify the wave arrivals (P, S, converted, surface waves). The measurements will be compared quantitatively to a direct 2D numerical elastodynamic simulation (finite elements, Interior Penalty Discontinuous Galerkin). We will then show wave measurements performed in cylindrical heterogeneous limestone cores of typical diameter size around 10 cm. Tomographic images of velocity (figure 2a) in 2D slices of the limestone cores will be derived based upon the time of first arrivals and implemented in the numerical model. By quantifying the difference between numerical and experimental results, the tomographic velocity model will be reciprocally improved and finally compared to a X − ray tomographic image of that slice. A brief overview of the studies Seismic sources We will explore piezo-electric sources of different frequencies (100 kHZ ∼ 5 M Hz) and test the new laser ablation source whose dominant frequency can reach 2 M Hz in aluminium. Avantages and drawbacks of each technology will be discussed in terms of source and wave propagation characterisation. Wave identification in an aluminium cube of side length 280 mm and seismic source at the center of one face We have identified experimentally P, S, head wave, PS, SP and surface waves measured on the cube surfaces. Meanwhile, direct numerical simulations have helped to quantitatively analyze the kinematics of wave fronts. For example, on the surface where the seismic source is excited, a P front, an S front and a PS head wave front are measured by the laser vibrometer right after the initial seismic impulse. These wavefronts can be understood by both the Huygens' Principle and the Snell-Descartes Law. In Figure 1, the seismic source excits simultaneously at time t = 0 a P wave and an S wave. As time evolves, waves propagate inside the volume and a P-wave propagates along the boundary as well: the latter one acts on the boundary as secondary sources which will emit both P and S waves, creating finally a new PS head wave front nicely measured in the experiments. The colours of magenta and green correspond to null amplitudes

On the use of a pulsed-laser source in laboratory seismic experiments

International audienceReproduction of large-scale seismic exploration at laboratory-scale with controllable sources is a promising approach that could not only be applied to study small-scale physical properties of the medium, but also contribute to significant progress in wave-propagation understanding and complex media imaging at exploration scale via upscaling methods. We seek to characterize the properties of a laser-generated seismic source for new geophysical experiments at laboratory scale. This consists in generating seismic waves by pulsed-laser impacts and measuring the displacement wavefield by laser vibrometry. Parallel 2D/3D simulations using Discontinuous Galerkin discretization method and analytic predictions have been done to match the experimental data

Evaluating the impact of groundwater on cotton growth and root zone water balance using Hydrus-1D coupled with a crop growth model

Groundwater is an important factor that needs to be considered when evaluating the water balance of the soil-plant-atmosphere system and the sustainable development of arid oases. However, the impact of shallow groundwater on the root zone water balance and cotton growth is not fully understood. In this study, we have first analyzed the influence of the groundwater table depth on the seasonal maximum leaf area index of cotton, the average seasonal water stress, cotton yield, actual transpiration, actual evaporation, and capillary rise using experimental data collected at the Aksu water balance station, in Xinjiang, northwest of China and the Hydrus-1D variably-saturated soil water flow model coupled with a simplified crop growth model from SWAT. The coupled model has been first calibrated and validated using field observations of soil water content, leaf area index, cotton height, the above ground biomass, and cotton yield comparisons between measured and modeled variables have shown a reasonable agreement for all variables. Additionally, with a validated model, we have carried out numerical experiments from which we have concluded that groundwater is a major water resource for cotton growth in this region. The capillary rise from groundwater contributes almost 23% of crop transpiration when the average groundwater depth is 1.84. m, which is the most suitable groundwater depth for this experimental site. We have concluded that cotton growth and various components of the soil water balance are highly sensitive to the groundwater table level. Different positions of the groundwater table showed both positive and negative effects on cotton growth. Likewise, cotton growth has a significant impact on the capillary rise from groundwater. As a result, groundwater is a crucial factor that needs to be considered when evaluating agricultural land management in this arid region. The updated Hydrus-1D model developed in this study provides a powerful modeling tool for evaluating the effects of the groundwater table on local land management

A Built-In Mechanism to Mitigate the Spread of Insect-Resistance and Herbicide-Tolerance Transgenes into Weedy Rice Populations

BACKGROUND: The major challenge of cultivating genetically modified (GM) rice (Oryza sativa) at the commercial scale is to prevent the spread of transgenes from GM cultivated rice to its coexisting weedy rice (O. sativa f. spontanea). The strategic development of GM rice with a built-in control mechanism can mitigate transgene spread in weedy rice populations. METHODOLOGY/PRINCIPAL FINDINGS: An RNAi cassette suppressing the expression of the bentazon detoxifying enzyme CYP81A6 was constructed into the T-DNA which contained two tightly linked transgenes expressing the Bt insecticidal protein Cry1Ab and the glyphosate tolerant 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), respectively. GM rice plants developed from this T-DNA were resistant to lepidopteran pests and tolerant to glyphosate, but sensitive to bentazon. The application of bentazon of 2000 mg/L at the rate of 40 mL/m(2), which is approximately the recommended dose for the field application to control common rice weeds, killed all F(2) plants containing the transgenes generated from the Crop-weed hybrids between a GM rice line (CGH-13) and two weedy rice strains (PI-63 and PI-1401). CONCLUSIONS/SIGNIFICANCE: Weedy rice plants containing transgenes from GM rice through gene flow can be selectively killed by the spray of bentazon when a non-GM rice variety is cultivated alternately in a few-year interval. The built-in control mechanism in combination of cropping management is likely to mitigate the spread of transgenes into weedy rice populations

Two ultraviolet radiation datasets that cover China

Ultraviolet (UV) radiation has significant effects on ecosystems, environments, and human health, as well as atmospheric processes and climate change. Two ultraviolet radiation datasets are described in this paper. One contains hourly observations of UV radiation measured at 40 Chinese Ecosystem Research Network stations from 2005 to 2015. CUV3 broadband radiometers were used to observe the UV radiation, with an accuracy of 5%, which meets the World Meteorology Organization's measurement standards. The extremum method was used to control the quality of the measured datasets. The other dataset contains daily cumulative UV radiation estimates that were calculated using an all-sky estimation model combined with a hybrid model. The reconstructed daily UV radiation data span from 1961 to 2014. The mean absolute bias error and root-mean-square error are smaller than 30% at most stations, and most of the mean bias error values are negative, which indicates underestimation of the UV radiation intensity. These datasets can improve our basic knowledge of the spatial and temporal variations in UV radiation. Additionally, these datasets can be used in studies of potential ozone formation and atmospheric oxidation, as well as simulations of ecological processes

Experimental and numerical studies of seismic wave propagation in carbonate rocks at the laboratory scale

Par la présente étude nous expliquons et illustrons nos travaux sur la propagation d'ondes sismiques en roche carbonatée avec des approches expérimentale et numérique à l'échelle des carottes géologiques au laboratoire. Notre objectif principal consiste à produire des données sismiques qui sont comparables aux données du terrain dans l'optique de transférer au laboratoire les connaissances et techniques acquises et développées pour la sismique de terrain. En retour, les données acquises dans des conditions bien contrôlées à l'échelle du laboratoire peuvent contribuer aux benchmarking tests sur les approches d'imagerie les plus récentes ainsi que les procédures de traitement des données. Sonder les roches naturelles par les ondes ultrasonores est plutôt commun dans de nombreux domaines tels la diagraphie acoustique, le génie civil et la science des matériaux. En termes de sources sismiques, les transducteurs piézoélectriques (PZT) couramment utilisés, ainsi qu'un pulse laser utilisé spécialement dans le cadre de nos mesures, sont appliqués aux configurations expérimentales. En termes de récepteurs, un vibromètre laser monopoint (LDV) est employé dans la plupart des expérimentations. En outre, un vibromètre 3D fut utilisé pour des mesures multi-composantes. L'originalité de ces travaux se trouve dans la caractérisation intégrale de la source laser dans un contexte géophysique et son application sur les carottes carbonatées comme une source sismique Haute Fréquence (HF, Mégahertz), ce qui nous permet de proposer des configurations originales et efficaces pour sonder les carottes carbonatées, aussi bien en expérimentation qu'en simulation. Nous avons construit et validé un prototype comprenant une configuration source-récepteur de type PZT-LDV ou Laser-pulse-LDV. Quant à la simulation numérique, nous avons adapté un code Fortran, initialement dédié au développement à l'échelle du terrain, aux productions numériques à hautes fréquences (HF) à l'échelle du laboratoire. Nous détaillerons les résultats des calibrations sur les outils expérimentaux et numériques ainsi qu'une analyse de la polarisation sur des mesures multi-composantes, les résultats de la caractérisation de la source pulse-laser et ceux de la tomographie en se basant sur les temps des premières arrivées (FAT). Les inversions se font avec un méthode multi-grilles. Parallèlement, des simulations par éléments finis en 2D/3D (code parallélisé) ont été effectuées avec la méthode Galerkin discontinu avec pénalité intérieure. Nous démontrerons également la possibilité d'analyser l'anisotropie sismique d'une carotte à partir du FAT. La dernière génération de méthode d'imagerie sismique fait intervenir à la fois les données observées et les données synthétiques, comme dans la méthode prometteuse de l'inversion de formes d'ondes complètes (FWI), cette méthode pouvant repousser davantage les limites de la résolution de la tomographie classique.The present study is an experimental and numerical work on the propagation of seismic waves in carbonate rocks at the laboratory-scale. We aim at producing seismic data that are comparable with field data so as to transfer the knowledge and techniques acquired and developed in field into the laboratory. In return, well-controlled laboratory-scale data can contribute to benchmarking tests on the latest imaging approaches as well as data processing procedures. Ultrasonic probes on natural rocks are fairly common in numerous engineering fields such as geophysical logging, civil engineering and materials science. In terms of ultrasonic sources, both the conventional piezoelectric transducer (PZT) and the laser-ablation pulse are used as a seismic source in our experimental configurations. In terms of receivers, a single-point Laser Doppler Vibrometer (LDV) is employed in most of the experimental setups. Additionally, a 3D wavefield-scan LDV has been used to perform multi-component measurements. The originality of this work consists in the comprehensive characterization of the laser source in a geophysical context and its use as a point-like megahertz source on the carbonate cores, which enabled us to propose original and efficient core-probing configurations for both experimentation and simulations. We developed and validated an experimental prototype featuring PZT-LDV or Laser-pulse-LDV as source-receiver configurations for high-resolution measurements. As to the numerical simulation, we adapted a Fortran code, initially dedicated to method developments at the field-scale, to the numerical production at the high-frequency (HF) laboratory-scale. We will show the results of calibrations on both the experimental and numerical tools including the polarization analysis on multi-component data, the results of the laser pulsed source characterization and the main tomography results from the First Arrival Travel-time (FAT) Tomography (FATT) featuring multi-grids, along with the parallelized 2D/3D simulations featuring the Interior Penalty Discontinuous Galerkin method (IPDGm). A brief demonstration of anisotropy analyses from FAT will also be given at the end. The latest generation of imaging method involves both the observed and synthetic data, such as the promising Full Waveform Inversion (FWI) that can push further the resolution limits of the classical tomography, which is the key motivation of our studies involving both the experiment and simulation

Precision of DAS simulation for CO2 monitoring and FWI application

International audienceDistributed acoustic sensing (DAS) is one of the latest and prominent measuring instruments based on the development of laser technology. DAS has a wide range of application domains, from geophysics to biology and natural sciences. DAS has gained increasing interest in geophysics during the last decade and the research on it is continuously growing. It has been successfully applied in Vertical Seismic Profile (VSP) for both reservoir and CO2 monitoring. Amplitude-Versus-Offset (AVO) analysis on DAS data may also be beneficial to CO 2 monitoring. Another opportunity is the application of FWI on DAS data. Either for FWI or CO2 monitoring with AVO analysis, the precision of DAS data, in terms of both phase and amplitude, should meet a high standard. DAS is actively researched in project-team Makutu Inria and TotalEnergies for the purpose of seismic imaging and CO2 geological storage monitoring. In the ongoing research, we compare three DAS formulae analytically and numerically. On one hand, this comparison is necessary to validate our numerical tools for DAS simulation; on the other hand, a quantitative study on the precision of DAS simulation will help us ensure a good DAS data quality during a FWI workflow

imaging in laboratory trough laser Doppler vibrometry.

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Pulsed-laser source characterization in laboratory seismic experiments

International audienceThe present study aimed to characterize the properties of a laser-generated seismic source for laboratory-scale geophysical experiments. This consisted of generating seismic waves in aluminum blocks and a carbonate core via pulsed-laser impacts and measuring the wave-field displacement via laser vibrometry. The experimental data were quantitatively compared to both theoretical predictions and 2D/3D numerical simulations using a finite element method. Two well-known and distinct physical mechanisms of seismic wave generation via pulsed-laser were identified and characterized accordingly: a thermoelastic regime for which the incident laser power was relatively weak, and an ablation regime at higher incident powers. The radiation patterns of the pulsed-laser seismic source in both regimes were experimentally measured and compared with that of a typical ultrasonic transducer. This study showed that this point-like, contact-free, reproducible, simple-to-use laser-generated seismic source was an attractive alternative to piezoelectric sources for laboratory seismic experiments, especially those concerning small scale, sub-meter measurements