•A numerical scheme is proposed for the direct numerical simulation of complex solid-liquid-gas three-phase flows.•A coupled LBM-DEM model ensuring mass conservation is proposed.•The upward migration ...of the bubbles through a brine-filled sediment column is successfully simulated at various conditions.•Simulation results indicate three different flow regimes: connected finger flow, transition flow, and dispersed bubbly flow.•The dimensionless EO and WE numbers can identify the three different flow regimes.
In this paper, we develop a numerical model for simulating the solid-liquid-gas three-phase flow in unconsolidated particle layers. Based on the discrete element method (DEM) and the multiphase fluid model in the framework of the lattice Boltzmann method (LBM), a multiphase fluid-solid two-way coupling algorithm is proposed. In this model, the fluid-fluid interface is tracked using a phase-field method, and the multiphase fluid-particle interaction is tackled by a combination of the momentum exchange method for the flow field and the immersed boundary method for the phase field. We applied the method to simulate the upward migration of the leaked gas bubbles through a brine-filled sediment column at the seafloor, and investigated the influences of the leak flow rate and the interfacial tension on the bubble rising behavior. The results indicate three different flow regimes: connected finger flow, transition flow, and dispersed bubbly flow. These flow regimes can be distinguished by the dimensionless Eötvös and Weber numbers. The proposed numerical method can accurately characterize various multiphase interaction mechanisms at the mesoscopic scale and has powerful advantages in simulating complex fluid-particle coupling problems.
Carbon dioxide (CO2) capture and storage (CCS) is an important climate change mitigation option along with improved energy efficiency, renewable energy, and nuclear energy. CO2 geosequestration, that ...is, to store CO2 under the subsurface of Earth, is feasible because the world’s sedimentary basins have high capacity and are often located in the same region of the world as emission sources. How CO2 interacts with the connate water and minerals is the focus of this Account. There are four trapping mechanisms that keep CO2 in the pores of subsurface rocks: (1) structural trapping, (2) residual trapping, (3) dissolution trapping, and (4) mineral trapping. The first two are dominated by capillary action, where wettability controls CO2 and water two-phase flow in porous media. We review state-of-the-art studies on CO2/water/mineral wettability, which was found to depend on pressure and temperature conditions, salt concentration in aqueous solutions, mineral surface chemistry, and geometry. We then review some recent advances in mineral trapping. First, we show that it is possible to reproduce the CO2/water/mineral wettability at a wide range of pressures using molecular dynamics (MD) simulations. As the pressure increases, CO2 gas transforms into a supercritical fluid or liquid at ∼7.4 MPa depending on the environmental temperature. This transition leads to a substantial decrease of the interfacial tension between CO2 and reservoir brine (or pure water). However, the wettability of CO2/water/rock systems depends on the type of rock surface. Recently, we investigated the contact angle of CO2/water/silica systems with two different silica surfaces using MD simulations. We found that contact angle increased with pressure for the hydrophobic (siloxane) surface while it was almost constant for the hydrophilic (silanol) surface, in excellent agreement with experimental observations. Furthermore, we found that the CO2 thin films at the CO2–hydrophilic silica and CO2–H2O interfaces displayed a linear correlation, which can in turn explain the constant contact angle on the hydrophilic silica surface. In view of the literature and our study results, a few recommendations seem necessary to construct a molecular system suitable to study wettability with MD simulations. Future work should be conducted to determine the influence of brine salinity on the wettability of minerals with high cation exchange capacity. Mineral trapping is believed to be an extremely slow process, likely taking thousands of years. However, a recent pilot study demonstrated that CO2 mineralization occurs within 2 years in highly reactive basalt reservoirs. A first-principles MD study has also shown that carbonation reactions occur rapidly at the surface oxygen sites of a reactive mineral. We observed carbonate ions on both a newly cleaved quartz surface (without hydrolysis), and a basalt andesine surface after hydrolysis in a CO2-rich environment. Future work should consider the influence of water, gas impurities, and mineral cation type on carbonation.
SUMMARY
We present a crustal shear wave (S-wave) velocity model for central Japan that accurately captures the previously mapped geology and lithology of the region. We perform a full-waveform ...tomographic inversion using a large seismic data volume that was recorded by the dense, permanent seismic monitoring network that spans the Japan Islands to resolve the seismic structure beneath central Honshu Island. The inversion reduces the time–frequency phase misfit by 16.4 and 6.7 per cent in the 20–50-s and 10–30-s period ranges, respectively. We infer that the resolved seismic velocity anomalies in our inversion reflect a range of subsurface features, including volcanic fluids, dehydration fluids from the subducted crust and sedimentary basins. In contrast to previous S-wave velocity models of the same region, which have been based primarily on first-arrival tomography, our S-wave velocity model is based on the explicit computation of the full seismic wavefield. This approach makes our model more suitable for modelling seismic wavefields in the 10–50-s period range and enables high-resolution imaging of the subsurface.
We complete the theory of the local
p
-adic Simpson correspondence developed by Faltings for rational coefficients. It asserts that there is an equivalence between the category of small
Q
p
...-generalized representations of the geometric fundamental group and that of small
Q
p
-Higgs bundles for a certain kind of log smooth affine scheme over a complete discrete valuation ring of mixed characteristic (0,
p
) with perfect residue field. The difficulty lies in the construction of the latter from the former, and we give it via a generalized Sen’s theory for the log smooth affine scheme, which depends on Faltings’ almost purity theorem. Inspired by a recent work by R. Liu and X. Zhu, we also give a formulation of local
p
-adic Simpson correspondence for
Q
p
-generalized representations of the arithmetic fundamental group, and a characterization of Hodge-Tate generalized representations in terms of the correspondence.
Active‐source seismic surveys have resolved the fine‐scale P‐wave velocity (Vp) of the subsurface structure in subduction forearcs. In contrast, the S‐wave velocity (Vs) structure is poorly resolved ...despite its usefulness in understanding rock properties. This study estimates Vp and Vs structures of the Nankai Trough forearc, by applying transdimensional inversion to high‐frequency teleseismic waveforms. As a result, a thin (∼1 km) low‐velocity zone (LVZ) is evident at ∼6 km depth beneath the sea level, which is located ~3 km seaward from the outer ridge. Based on its high Vp/Vs ratio (∼2.5) and comparison to an existing seismic reflection profile, we conclude that this LVZ reflects a high pore pressure zone at the upper portion of the underthrust sediment. We infer that this overpressured underthrust sediment hosts slow earthquake activities and that accompanied strain release helps impede coseismic rupture propagation further updip.
Plain Language Summary
Many geophysical surveys have investigated the subsurface structures of shallow subduction zones by estimating the propagation speed of compressional waves emitted from artificial explosive sources. Although shear and compressional wave speeds are necessary to understand rock properties (e.g., water pressure) of the subsurface, it has been difficult to constrain the shear wave speed with a high‐resolution. In this study, we estimate both compressional and shear wave speeds by applying an advanced technique to earthquake waveforms recorded at ocean‐bottom seismometers deployed at the Nankai subduction zone, Japan. The results show a sufficiently high spatial resolution to detect a thin (~1 km) layer, which we interpret as water‐rich subducted sediment. This water‐rich zone may promote slow slips on the megathrust fault and work as a barrier against rupture propagations during large earthquakes.
Key Points
We applied transdimensional inversion of teleseismic waveforms to seafloor cabled stations at the Nankai subduction zone
The resultant high‐resolution velocity structures allow a direct comparison to active source surveys
A low‐velocity zone beneath the outer ridge is evident, which is interpreted as an overpressured portion of underthrust sediment
To investigate the mechanism of tectonic tremor and the factors that influence its occurrence and fluctuations, we use seismic reflection data to map in detail a shear zone associated with the plate ...boundary fault in the Nankai accretionary prism, southwest Japan. The shear zone can be identified from its low seismic velocity and low reflection amplitude, which may signify the presence of high pore pressure, around the transition boundary from frontal outer wedge to inner wedge. A detailed comparison of the mapped shear zone and the distribution of tectonic tremor demonstrate that tremor tended to occur in the thick part of the shear zone, where high pore fluid pressure may weaken faults and thereby facilitate slow earthquakes. Furthermore tremors align on the oblique strike-slip faults that caused variation of the shear zone thickness as well as fluctuation of stress and pore pressure. The oblique strike-slip faults that control the thickness of the shear zone and tremor distributions are interpreted as features formed by the collision of an accretionary prism against a landward tectonic backstop.
•A shear zone is associated with high pore pressure is identified around the plate interface.•The thickness of the shear zone is mapped based on 2D/3D seismic data.•The shear zone thickness mainly controls source and migration process of slow slip events.•The shear zone thickness is influenced by strike-slip faults formed by backstop collision.
Pulsed laser melting in liquid (PLML) has been developed to fabricate crystalline spherical submicrometer particles (SMPs) by irradiating lasers onto raw particles dispersed in liquid. This technique ...differs from highly energetic pulsed laser ablation in liquid for nanoparticle (NP) fabrication and is based on the photothermal processing of particles dispersed in liquid that is developed from the well-studied NP reshaping of noble metals, such as Au and Ag. However, this phenomenon can be extended beyond Au and Ag using appropriate raw particles and laser fluences for various materials (semiconductors, oxides, carbides, etc.) to reshape or melt agglomerated raw particles. Since well-defined dense and smooth crystalline particles in this size range are difficult to fabricate by other particle fabrication methods, the generated spherical SMPs are useful for realizing miniaturized bulk functionality and submicrometer-specific functionality. This review comprehensively summarizes the historical background, the basic mechanism behind the process, parameters affecting particle size and morphology, possible application examples, specialized issues for characterization and mass production, and future directions of this method. This review will be helpful, especially for those seeking well-defined spherical SMPs with various functionalities.
•Investigate pore heterogeneity on the dissolution pattern and permeability evolution.•Use the Euler characteristic to characterize the pore heterogeneity in porous media.•Influence of pore ...heterogeneity on permeability change presents non-linear behavior.•Dissolution is dependent on pore heterogeneity at high Péclet and Damköhler numbers.•Permeability-porosity curve is influenced by heterogeneity at high Damköhler number.
The influence of pore space heterogeneity on mineral dissolution and permeability evolution in porous media was investigated using a numerical approach. Artificial porous media were generated by the linear Boolean model, and pore heterogeneity was evaluated using the Euler–Poincaré characteristic (i.e., Euler number). We applied the lattice Boltzmann method with dual particle distribution functions to simulate mineral dissolution under the combined effect of fluid flow and a diffusion process. Simulations were conducted to investigate dissolution patterns for a wide range of Péclet (Pe) and Damköhler (Da) numbers and various pore geometries. Six dissolution regimes were observed, and two types of transition phenomena between these dissolution regimes could be characterized. At high Pe and Da, the dissolution patterns strongly depended on the pore heterogeneity. In addition, four types of porosity–permeability relationship were observed. These relationships were influenced by the pore heterogeneity at high Da numbers.
Abstract
Seismic ambient noise with frequencies > 1 Hz includes noise related to human activities. A reduction in seismic noise during the COVID-19 pandemic has been observed worldwide, as ...restrictions were imposed to control outbreaks of the SARS-CoV-2 virus. In this context, we studied the effect of changes in anthropogenic activities during COVID-19 on the seismic noise levels in the Tokyo metropolitan area, Japan, considering time of day, day of the week, and seasonal changes. The results showed the largest reduction in noise levels during the first state of emergency under most conditions. After the first state of emergency was lifted, the daytime noise reverted to previous levels immediately on weekdays and gradually on Sundays. This was likely because economic activities instantly resumed, while non-essential outings on Sundays were still mostly avoided. Furthermore, the daytime noise level on Sundays was strongly reduced regardless of changes on weekdays after the second state of emergency, which restricted activities mainly at night. Sunday noise levels gradually increased from the middle of the second state of emergency, suggesting a gradual reduction in public concern about COVID-19 following a decrease in the number of infections. Our findings demonstrate that seismic noise can be used to monitor social activities.
Earthquakes are known to precede volcanic activity, including long‐period or very long period (VLP) volcanic seismicity. However, the relationships among earthquakes, VLP seismicity, and volcanic ...eruptions are not well understood. Here we present the locations of VLP seismicity at Aso volcano, Japan, between January 2015 and December 2016, a period that includes the Mw 7.0 Kumamoto earthquake and phreatomagmatic eruptions. By using a differential‐time backprojection method developed in this study to accurately locate VLP events, we clearly identified two distinct VLP clusters. Whereas the eastern cluster was active during eruptions, the western cluster displayed intense VLP seismicity only for a few months after the earthquake. The western cluster may be associated with opening of new fractures during the earthquake. This study explores the mechanisms that can relate earthquake to volcanic activities and provides a new approach to analyze the dynamic behaviors inside volcanoes that may yield useful information for hazard evaluation.
Plain Language Summary
The relationship between earthquake and volcanic seismicity is studied by analyzing continuous seismic data recorded between January 2015 and December 2016, a period that includes the Mw 7.0 Kumamoto earthquake and volcanic eruptions at Aso. This study focuses on analyzing very long period (VLP) seismic event with a period of 15 s that can be explained by pressure fluctuation within hydrothermal systems. The intense activity of long‐period event could be an indication of impending eruptions. By using our developed localization method, we revealed two clusters of VLP sources, namely, eastern and western clusters. The eastern cluster was always active during an active period of Aso volcano. However, the western cluster showed intense VLP seismicity only for a few months after the earthquake. The western cluster could be associated with opening of new cracks triggered by the Kumamoto earthquake. This study suggests a strong relationship between distant large earthquake with volcanic seismicity that could be useful for disaster mitigation. Moreover, monitoring of temporal and spatial evolution of VLP sources may be considered as a tool to study the response of volcanic activity to ground shaking from large earthquake.
Key Points
Backprojection technique based on differential‐time and phase weighted stacking is used to map clusters of very long period seismicity
Clusters of very long period seismicity indicate clear relationship between distant earthquake and volcanic activities
The 2016 Kumamoto earthquake suddenly induced very long period seismicity at west of Aso Caldera lasting for five months
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