Small Velocity Changes Research Articles

Temporal Seismic Velocity Changes Associated With the Mw 6.1, May 2008 Ölfus Doublet, South Iceland: A Joint Interpretation From dv/v and GPS

AbstractIn South Iceland, populated and agricultural areas are at risk of earthquakes due to their location within the South Iceland Seismic Zone (SISZ). In 2008, two moderate‐sized earthquakes (M5.8 and M5.9) occurred in Ölfus, the western end of this highly active transform zone. We analyze temporal seismic velocity variations (dv/v) related to the Ölfus earthquake doublet, using cross‐correlations of ambient noise in the frequency range of 0.1–3.0 Hz. The two mainshocks decrease the average velocity by 0.8% at the nearest stations. The co‐seismic changes are most noticeable from 0.7 to 1.7 Hz and affect a 40 km wide region. We present a first‐time comparison of dv/v to crustal deformation, seismicity, co‐seismic volumetric stress changes and reported PGA distribution for the Ölfus doublet. Ground accelerations caused by mainshocks at intermediate distances suggest that strong shaking‐related damage may contribute to the co‐seismic dv/v decrease. A rapid velocity increase (0.3%) in a month after the co‐seismic drop indicates crustal rock healing. We find 3‐months of post‐seismic decorrelation, followed by a nearly permanent velocity decrease (0.2%) confined to a shallow layer (1 km) until the end of the observation period. Afterslip and pore fluid effects in the near‐source region are likely to influence post‐seismic dv/v. We demonstrate that seismic interferometry can contribute to future fault‐zone monitoring operations in the SISZ by detecting small changes in velocity.

An extended phase graph-based framework for DANTE-SPACE simulations including physiological, temporal, and spatial variations.

The delay alternating with nutation for tailored excitation (DANTE)-sampling perfection with application-optimized contrasts (SPACE) sequence facilitates 3D intracranial vessel wall imaging with simultaneous suppression of blood and CSF. However, the achieved image contrast depends closely on the selected sequence parameters, and the clinical use of the sequence is limited in vivo by observed signal variations in the vessel wall, CSF, and blood. This paper introduces a comprehensive DANTE-SPACE simulation framework, with the aim of providing a better understanding of the underlying contrast mechanisms and facilitating improved parameter selection and contrast optimization. An extended phase graph formalism was developed for efficient spin ensemble simulation of the DANTE-SPACE sequence. Physiological processes such as pulsatile flow velocity variation, varying flow directions, intravoxel velocity variation, diffusion, and effects were included in the framework to represent the mechanisms behind the achieved signal levels accurately. Intravoxel velocity variation improved temporal stability and robustness against small velocity changes. Time-varying pulsatile velocity variation affected CSF simulations, introducing periods of near-zero velocity and partial rephasing. Inclusion of diffusion effects was found to substantially reduce the CSF signal. Blood flow trajectory variations had minor effects, but differences along the trajectory reduced DANTE efficiency in low- areas. Introducing low-velocity pulsatility of both CSF and vessel wall helped explain the in vivo observed signal heterogeneity in both tissue types. The presented simulation framework facilitates a more comprehensive optimization of DANTE-SPACE sequence parameters. Furthermore, the simulation framework helps to explain observed contrasts in acquired data.

Comparing the stretching technique and the wavelet cross-spectrum technique for measuring stress-induced wave-velocity changes in concrete

Coda wave interferometry (CWI) holds promise as a technique for concrete stress monitoring. This is because the coda, which consists of multiply scattered arrivals, is the result of propagation through the medium over large distances. As such, it is sensitive to both minute structural changes and small velocity changes in that medium. Previous studies focusing on concrete have predominantly utilized the time-domain-based stretching technique to measure travel-time changes. There is, however, a lack of consensus on how to quantify these changes effectively. In this study, we conduct a systematic comparison between two techniques, namely the stretching technique and the wavelet cross-spectrum (WCS) technique, for measuring stress-induced velocity changes in a cylindrical concrete sample. Our comparison focuses on two key aspects: (i) stability against cycle skipping and (ii) consistency in retrieving velocity changes. Experimental results reveal that both the WCS technique and the stretching technique yield consistent velocity changes. In terms of stability, it is challenging to determine which technique performs better, due to differences in the mechanisms triggering cycle skipping. However, when considering waves with frequencies ranging from 50 kHz to 80 kHz, both techniques exhibit comparable performance. Based on our findings, we offer the following recommendations for utilizing these CWI techniques in concrete stress monitoring:For the stretching technique, selecting the time window length based on the wave frequency and the expected magnitude of velocity change.For the WCS technique, operating it in the frequency band where spectral decomposition shows sufficiently high energy in the signal and can accommodate the expected magnitude of velocity change.

A Maneuver of Escape from the Vicinity of a Collinear Libration Point Using Light Pressure Forces

The orbital motion of a spacecraft with a solar sail in the vicinity of the Sun–Earth collinear libration point is studied. The study is carried out within the Hill approximation of the circular restricted three-body problem. The collinear libration point is unstable, but instability can be a positive factor in the tasks of maneuvering in interplanetary space. This is because a small change in velocity near an unstable libration point can lead to a significant change in orbital motion. This property can be used in the development of projects to counteract the comet–asteroid hazard. A scheme for constructing the trajectories of interception of a spacecraft with a potentially hazardous object is proposed. One of the stages of this scheme, an optimal escape from the vicinity of the libration point using light pressure forces, is considered. The results of numerical simulations are presented.

Stress Propagation in the Craniofacial Skeleton on Frontal Impact-A Virtual Simulation Study.

The research paper emphasizes on virtual simulation of craniofacial skeleton to understand its Biomechanics. In this study, a 3D finite element model of the skull was created using CT scan data. All complexities of the skull geometry are simulated using ABAQUS software. The model was validated and then, subjected to frontal impact. Energy plots for the same were obtained. Impact analysis was done, and weak areas susceptible to fracture and hence, failure are identified. One of the emerging areas of applications of computational bio-mechanics is to understand the behavior of the skull during a traumatic injury, such as head impact during accidents. Finite element study is one such part where significant amount of research is being carried out to understand and predict the craniofacial injuries. For the head and brain trauma analysis, there are many software packages available, including ANSYS, LS-DYNA and ABAQUS. Research is being carried out towards developing high-fidelity human models, especially of the human skull to understand the bio-mechanical behavior when subjected to external impact. Impact analysis done in our study showed that a small change in impact velocity can result in a large change in damage that can happen. The study is expected to complement the existing treatment methodologies. Further, appropriate knowledge of fracture biomechanics can be used to design and develop safety measures in automobiles, sport guards, helmets to prevent and reduce facial injuries.

Joint Bayesian Modeling of Velocity Break Points, Noise Characteristics, and Their Uncertainties in GNSS Time Series: Far‐Field Velocity Anomalies Concurrent With Magmatic Activity in Iceland

AbstractEstimating the timing of velocity changes (break points) in Global Navigation Satellite System (GNSS) coordinate time series is required for understanding various Earth processes and how they may couple with each other. We jointly estimate break points, noise parameters and their uncertainties in GNSS time series using Bayesian interference. Synthetic data experiments demonstrate that time‐correlated noise can cause spurious estimates of break points for small velocity change and suggest the lower limits of velocity change for reliable estimates. A case study at the Krafla volcanic system shows increased rift‐perpendicular movementof 7.6–9.8 mm/yr, preceding by 23–77 days the 2014 Bárðarbunga rifting episode that occurred ∼120 km to the south (maximum likelihood estimates). These far‐field velocity anomalies occur in the same period as enhanced near‐field seismicity and deformation before the dike intrusion at Bárðarbunga, possibly indicating the coupling between the two volcanic systems through a deep partial melt zone.

A dynamic collision avoidance solution scheme of unmanned surface vessels based on proactive velocity obstacle and set-based guidance

This paper proposes a proactive velocity obstacle (PVO) method through pre-judging whether there are collision risks between an unmanned surface vessel (USV) and its obstacle vessels according to the predicted motion states of the USV by its motion mathematical model to optimize the collision avoidance decision-making. Then integrating the proposed PVO method and the line of sight (LOS) algorithm into the set-based guidance (SBG) framework, we create a dynamic collision avoidance (DCA) solution scheme of USVs and carry out simulations in the cases of single vessel and multiple vessels encounter, respectively. The USV-DCA solution scheme can make the USV successfully avoid obstacle vessels while complying with the international regulations for preventing collisions at sea (COLREGs) and follow the desired path without collisions. The simulation results show that the USV-DCA solution scheme based on PVO and SBG can make USVs avert a series of small velocity changes and make a safer collision avoidance decision than the original SBG framework scheme in the multiple vessels encounter case, which verifies the effectiveness and superiority of our proposed USV-DCA solution scheme.

Numerical Study of Powder Flow Nozzle for Laser-Assisted Metal Deposition

Metal additive manufacturing has received much attention in the past few decades, and it offers a variety of technologies for three-dimensional object production. One of such technologies, allowing large-sized object production, is laser-assisted metal deposition, the limits of which are determined by the capabilities of the positioning system. The already-existing nozzles have either a relatively low build rate or a poor resolution. The goal of this work is to develop a new nozzle with a centered particle beam at high velocity for the laser-assisted metal additive manufacturing technologies. Scientific challenges are addressed with regards to the fluid dynamics, the particle-substrate contact, and tracking of the thermodynamic state during contact. In this paper, two nozzles based on the de Laval geometry with Witoszynski and Bicubic curves of convergence zone were designed; the results showed that the average flow velocity in a Bicubic outlet curve nozzle is around 615 m/s and in Witoszynski this is 435 m/s. Investigation of particle beam formation for the Bicubic curve geometry revealed that small particles have the highest velocity and the lowest total force at the nozzle outlet. Fine particles have a shorter response time, and therefore, a smaller dispersion area. The elasto-plastic particle-surface contact showed that particles of diameter limited up to 3 μm are able to reach experimentally obtained critical velocity without additional heating. For particle sizes above 10 μm, additional heating is needed for deposition. The maximum coefficient of restitution (COR) is achieved with a particle size of 30 μm; smaller particles are characterized by the values of COR, which are lower due to a relatively high velocity. Particles larger than 30 μm are scalable, characterized by a small change in velocity and a rise in temperature as their mass increases.

Vertically Polarized Laser Speckle Contrast Imaging to Monitor Blood Flow in Pulp

This research aimed to study a polarized laser speckle contrast imaging (LSCI) system to assess the pulp blood flow, where the ability of vertically polarized LSCI (vp-LSCI) and the non-polarized LSCI (np-LSCI) systems has been compared through space contrast algorithm and time contrast algorithm. The results showed that the vp-LSCI system could detect a wider contrast dynamic range in the pulp flow, approximately 1.5–2 times than that of the np-LSCI system. The time contrast algorithm detected small velocity changes of 0.03–0.07 mm/s, and its detectability index was 5–10 times than the space contrast algorithm. The vp-LSCI system combined with time contrast algorithm greatly improves the ability of laser speckle to detect pulp blood flow and can detect a wider range of blood flow velocity and smaller changes of blood flow. It indicated that our system may be a promising tool for clinical assessment of pulp blood flow.

Explicit dynamic approach for unbounded domains in frictional contact with Rate and State laws

The paper explores the ability of an explicit time integration procedure to simulate the dynamics of shear rupture between unbounded elastic blocks on frictional interface, modeled with the finite element method. The behaviour of the interface is governed by Rate and State (RS) friction laws, proposed to describe the rate dependent phenomena observed in experiments on rocks and many other materials in sliding contact. The method for the frictional contact between bodies is integrated in the framework of heterogeneous asynchronous time integrator (HATI). Dual formulation, requiring the introduction of Lagrange multipliers, is adopted by dealing in a weak way with the normal contact conditions as well as the tangential frictional conditions expressed in velocity. Taking advantage of the flexibility of the HATI framework, precise formulations of Perfectly Matched Layers (PML) are also incorporated, enabling us to treat semi-infinite media in frictional contact. Simulations concerning an unbounded elastic block on a rigid flat plane, initially compressed and sheared with remote loadings, are carried out. Advantages of the PML are underlined during the preliminary stages of loading as well as during the nucleation and propagation of the shear rupture along the interface. Numerical oscillations inherent from the high non-linearity of the RS friction law, characterized by significant variations of the friction coefficient with small changes in tangential velocity, are drastically reduced thanks to the HATI method.

Unveiling Vela – variability of interstellar lines in the direction of the Vela supernova remnant – III. Na D and Ca ii K

ABSTRACT High-resolution optical spectra were obtained in 2017–2019 with The Southern African Large Telescope of 15 stars in the direction of the Vela supernova remnant. Interstellar Ca ii H & K and Na i D lines are discussed in this paper. In particular, the line profiles are compared with profiles at a comparable spectral resolution obtained in 1993–1996 by Cha and Sembach. Ten of the lines of sight show changes to one or more of the components in that line of sight. Changes include small changes (1–2 km s−1) in radial velocity and/or increases/decreases in equivalent width over the two decades between the periods of observation. Changes are more obvious in the Ca K line than in the Na D lines. These changes are attributed to gas disturbed by interactions between the supernova ejecta and the surrounding interstellar medium. A representative time-scale may be 20–50 yr. Small-scale variations in line profiles across the face of the remnant suggest, as previously remarked, that a linear scale for interactions is a small fraction of the 40 pc size of the present remnant.

Kinetic Simulation of Nonequilibrium Kelvin-Helmholtz Instability**Supported by the Natural Science Foundation of China under Grant Nos. 91441120, 51806116, 11875001, and 11602162, the China Postdoctoral Science Foundation under Grant No. 2017M620757, the Center for Combustion Energy at Tsinghua University, and the Natural Science Foundation of Hebei Province under Grant Nos. A2017409014, ZD2017001, and A201500111, and the UK Engineering and Physical Sciences

The recently developed discrete Boltzmann method (DBM), which is based on a set of uniform linear evolution equations and has high parallel efficiency, is employed to investigate the dynamic nonequilibrium process of Kelvin-Helmholtz instability (KHI). It is found that, the relaxation time always strengthens the global nonequilibrium (GNE), entropy of mixing, and free enthalpy of mixing. Specifically, as a combined effect of physical gradients and nonequilibrium area, the GNE intensity first increases but decreases during the whole life-cycle of KHI. The growth rate of entropy of mixing shows firstly reducing, then increasing, and finally decreasing trends during the KHI process. The trend of the free enthalpy of mixing is opposite to that of the entropy of mixing. Detailed explanations are: (i) Initially, binary diffusion smooths quickly the sharp gradient in the mole fraction, which results in a steeply decreasing mixing rate. (ii) Afterwards, the mixing process is significantly promoted by the increasing length of material interface in the evolution of the KHI. (iii) As physical gradients are smoothed due to the binary diffusion and dissipation, the mixing rate reduces and approaches zero in the final stage. Moreover, with the increasing Atwood number, the global strength of viscous stresses on the heavy (light) medium reduces (increases), because the heavy (light) medium has a relatively small (large) velocity change. Furthermore, for a smaller Atwood number, the peaks of nonequilibrium manifestations emerge earlier, the entropy of mixing and free enthalpy of mixing change faster, because the KHI initiates a higher growth rate.

The Great Slave Lake PyroCb of 5 August 2014: Observations, Simulations, Comparisons With Regular Convection, and Impact on UTLS Water Vapor

AbstractThis study analyzes a pyrocumulonimbus (pyroCb) that formed near Great Slave Lake in the Northwest Territories of Canada on 5 August 2014 using multiple satellite‐ and ground‐based data sets, meteorological reanalysis, and a cloud‐resolving model. Passive and active polar‐orbiting instruments and geostationary imagery detail the intense updraft column during the several hours‐long convective stage and capture the entire pyroCb life cycle. CloudSat radar reflectivity profiles and local soundings show the pyroCb cloud base was ≥1 km above the lifting condensation level. Comparisons with profiles through meteorological convection in the same region reaffirm previous evidence for delayed droplet growth within pyroconvective updrafts. The pyroCb penetrated the tropopause, reaching at least 14 km (θ≃ 380 K), and the detrained cirrus/smoke plume was tracked over the following 2 weeks using lagrangian trajectories, the Cloud‐Aerosol Lidar with Orthogonal Polarization and Microwave Limb Sounder. Microwave Limb Sounder ice water content and water vapor mixing ratio (WVMR) observations in the aging plume show ice sublimation occurred in the stratosphere, producing individual WVMR anomalies up to +5 ppmv and plume‐averaged anomalies up to +2 ppmv. Reanalysis indicates the pyroCb formed in favorable convective conditions, but we show the fire itself triggered convection because meteorological triggers were not colocated during initiation. Cloud‐resolving model simulations confirm this result when controlled for surface fluxes, initial thermodynamics, and aerosol loading. Sensitivity tests show surface heat flux is dominant in determining overall pyroCb intensity, but aerosols and moisture inflow can effect small changes in updraft velocity, anvil ice concentration, and detrained WVMR.

Particle size estimation of pulverized fuel in pneumatic pipelines using electrostatic sensing techniques

Particle size measurement of pulverized fuel (PF) provides useful data for efficiency improvement of a pulverizing system, optimization of fuel combustion and emission reduction on a fired power plant. This paper presents an experimental study of particle size estimation of pneumatically conveyed PF through electrostatic sensing. An electrostatic sensor head consisting of two narrow and one wide ring-shaped electrodes is designed and implemented to measure the PF velocity and quantify the features of the sensor signals. Experimental tests using particles with three different size ranges were conducted on the vertical pipe section of a 74 mm bore gas–solid two-phase flow test facility under various flow conditions. Test results show that the root-mean-square magnitude and power spectral density of electrostatic signals vary with particle size. For the same air velocity, the small change in particle velocity is an indication of particle size as the particle velocity decreases when the particle size is larger. The proposed method is capable of estimating PF particle size and has the potential to offer considerable advantages over its counterparts in power plant applications.

Using Sprint Velocity Decrement to Enhance Acute Sprint Performance.

Cochrane, DJ and Monaghan, D. Using sprint velocity decrement to enhance acute sprint performance. J Strength Cond Res 35(2): 442-448, 2021-Acute sled towing has the capability to elicit a potentiation. Currently, sled loading is determined from body mass (BM) percent. However, it may be more relevant to use a percentage of maximal sprint reduction to determine an individual's load. The purpose of this study was to determine if individualizing sled loads at 35 and 55% reduction in velocity would improve 20-m sprint performance. In addition, electromyography (EMG) was assessed to determine if any sprint-related improvements in velocity were due to changes in neural excitation. Twelve senior club male rugby union players performed familiarization and 2 sled towing sessions using a randomized, cross-over, and counterbalanced design. Testing sessions involved: baseline unresisted 20-m sprints, resisted sprint (35 and 55% reduction in velocity), and unresisted 20-m sprints at 2, 4, 6, 8, 12, and 16 minutes. The sled load to reduce maximal velocity by 35% significantly improved 20-m velocity (p <0.05, effect size [ES] = 0.21) compared with the heavier sled load (55% reduction in maximum velocity). A significant decline in sprint velocity occurred at 12 minutes (p < 0.01, ES = -0.61) and 16 minutes (p < 0.01, ES = -0.45) compared with baseline velocity. Other time intervals reported trivial small nonsignificant changes in sprint velocity (p >0.05). There was no significant change in EMG. Reducing sprint velocity provides an alternate method to determining sprint loading. Nonetheless, a greater range of reduced sprint velocity is required to assess if it is more effective than using BM percent procedure.

Overburden 4D time shifts — Indicating undrained areas and fault transmissibility in the reservoir

Production-induced geomechanical stress changes cause velocity changes in the overburden that might be detected as 4D seismic time shifts. The strength of the velocity changes depends on the degree of pressure changes and the elastic properties of the reservoir and overburden layers. Even small velocity changes (less than 1%) might accumulate into detectable seismic time shifts at the top reservoir, since the overburden thickness typically ranges from one to several kilometers. Reservoir pressure changes inducing seismic time shifts are observed in the overburden of the Snorre, Heidrun, and Statfjord fields, all located on the Norwegian Continental Shelf. The strong correlation between overburden time shifts, geomechanics, and reservoir pressure changes is used to indicate undrained areas and transmissibility across faults, which is useful information for increased oil recovery, well planning, and reservoir model updating. 4D geomechanical models are built with input from simulated reservoir pressures. Geomechanical strain and velocity changes are linked through a “dilation” factor, R. The Snorre, Heidrun, and Statfjord fields indicate an average R value of about 15 for the overburden, when combining modeled vertical strain with observed seismic time shifts. However, this study also shows strong vertical variation in R, implying that R might be layer dependent. For the Statfjord Field, seabed subsidence measurements from gravity and GPS monitoring are used to calibrate the geomechanical model. The Snorre Field results show that both reservoir pressure depletion and pressure buildup can be identified by the use of overburden time shifts. The properties of the reservoir formations and surrounding layers of the investigated fields are typical for many fields on the Norwegian Continental Shelf. This implies that pressure-induced time shifts might be expected for many producing fields, not only chalk or high-pressure, high-temperature reservoirs but also sandstone reservoirs close to hydrostatic pressure.

Monitoring the stability of tailings dam walls with ambient seismic noise

Tailings dams are massive structures designed to contain the waste slurry remaining after processing ore at open pit and underground mines. These structures fail far more regularly than normal water-storage dams. In recent years, catastrophic tailings dam failures have occurred, causing significant damage to the environment and even loss of life. To mitigate these catastrophic events in the future, there is an urgent need to develop cost-effective methods to monitor the structural stability of these constructions over time. The lack of current cost-effective monitoring methods prompted us to investigate whether ambient seismic noise can be used to detect internal changes in a tailings dam wall during a period of heavy rainfall. We recorded three weeks of continuous seismic data with 10 short-period geophones at a tailings dam in Tasmania, Australia. Seismic interferometry was used on ambient noise to create virtual seismic sources. With these virtual source signals, small changes in seismic velocity were measured daily and compared to rainfall, seepage flow rates, and fluid pore pressure. The observed velocity changes were driven by fluid saturation, ground water level, increased loading from increased dam water level, and a sudden increase in fluid pore pressure in a section of the dam wall. The results suggest that this relatively inexpensive method can be used to monitor and locate small changes in the interior of the tailings dam wall, providing a valuable tool to remotely monitor the structural stability of tailings dam walls over time.

Spin-down in a rapidly rotating cylinder container with mixed rigid and stress-free boundary conditions

A comprehensive study of the classical linear spin-down of a constant-density viscous fluid (kinematic viscosity $\unicode[STIX]{x1D708}$) rotating rapidly (angular velocity $\unicode[STIX]{x1D6FA}$) inside an axisymmetric cylindrical container (radius $L$, height $H$) with rigid boundaries, which follows the instantaneous small change in the boundary angular velocity at small Ekman number $E=\unicode[STIX]{x1D708}/H^{2}\unicode[STIX]{x1D6FA}\ll 1$, was provided by Greenspan &amp; Howard (J. Fluid Mech., vol. 17, 1963, pp. 385–404). For that problem $E^{1/2}$ Ekman layers form quickly, triggering inertial waves together with the dominant spin-down of the quasi-geostrophic (QG) interior flow on the $O(E^{-1/2}\unicode[STIX]{x1D6FA}^{-1})$ time scale. On the longer lateral viscous diffusion time scale $O(L^{2}/\unicode[STIX]{x1D708})$, the QG flow responds to the $E^{1/3}$ sidewall shear layers. In our variant, the sidewall and top boundaries are stress-free, a set-up motivated by the study of isolated atmospheric structures such as tropical cyclones or tornadoes. Relative to the unbounded plane layer case, spin-down is reduced (enhanced) by the presence of a slippery (rigid) sidewall. This is evidenced by the QG angular velocity, $\unicode[STIX]{x1D714}^{\star }$, evolution on the $O(L^{2}/\unicode[STIX]{x1D708})$ time scale: spatially, $\unicode[STIX]{x1D714}^{\star }$ increases (decreases) outwards from the axis for a slippery (rigid) sidewall; temporally, the long-time $(\gg L^{2}/\unicode[STIX]{x1D708})$ behaviour is dominated by an eigensolution with a decay rate slightly slower (faster) than that for an unbounded layer. In our slippery sidewall case, the $E^{1/2}\times E^{1/2}$ corner region that forms at the sidewall intersection with the rigid base is responsible for a $\ln E$ singularity within the $E^{1/3}$ layer, causing our asymptotics to apply only at values of $E$ far smaller than can be reached by our direct numerical simulation (DNS) of the linear equations governing the entire spin-down process. Instead, we solve the $E^{1/3}$ boundary layer equations for given $E$ numerically. Our hybrid asymptotic–numerical approach yields results in excellent agreement with our DNS.

DHIGLS: DRAO H i INTERMEDIATE GALACTIC LATITUDE SURVEY

ABSTRACT Observations of Galactic H i gas for seven targeted regions at intermediate Galactic latitude are presented at angular resolution using data from the DRAO Synthesis Telescope (ST) and the Green Bank Telescope (GBT). The DHIGLS data are the most extensive arcminute-resolution measurements of the diffuse atomic interstellar medium beyond those in the Galactic plane. The acquisition, reduction, calibration, and mosaicking of the DRAO ST data and the cross calibration and incorporation of the short-spacing information from the GBT are described. The high quality of the resulting DHIGLS products enables a variety of new studies in directions of low Galactic column density. We analyze the angular power spectra of maps of the integrated H i emission (column density) from the data cubes for several distinct velocity ranges. In fitting power-spectrum models based on a power law, but including the effects of the synthesized beam and noise at high spatial frequencies, we find exponents ranging from −2.5 to −3.0. Power spectra of maps of the centroid velocity for these components give similar results. These exponents are interpreted as being representative of the three-dimensional density and velocity fields of the atomic gas, respectively. We find evidence for dramatic changes in the H i structures in channel maps over even small changes in velocity. This narrow line emission has counterparts in absorption spectra against bright background radio sources, quantifying that the gas is cold and dense and can be identified as the cold neutral medium phase. Fully reduced DHIGLS H i data cubes and other data products are available at www.cita.utoronto.ca/DHIGLS.

Velocity model estimation by full waveform inversion of time-lapse 4D passive seismic array data

SUMMARY In passive-source monitoring, an accurate velocity model is important to precisely estimate microseismic source locations, and to understand changes in reservoir properties. In this study, we employ frequency-domain full waveform inversion in order to obtain a high- resolution velocity model by exploiting full wavefields. We demonstrate the feasibility of the method for a surface geophone array by inverting for time-lapse 4D velocity changes in a realistic subsurface model. Our method successfully estimates the small velocity changes of a few percent within layers of 10s of meters, even for a single passive seismic source event. The analysis of wavepaths and gradients suggests that keys for the successful inversion are the use of full wavefields (both first and scattered arrivals), the vicinity of velocity changes to the source, and the wide-aperture surface array.