Relative Velocity Changes Research Articles

Seasonal Freeze‐Thaw Cycles and Permafrost Degradation on Mt. Zugspitze (German/Austrian Alps) Revealed by Single‐Station Seismic Monitoring

AbstractThawing of mountain permafrost in response to rising temperatures degrades the stability of rock walls and thereby affects infrastructure integrity in Alpine terrain. In this study, we use 15 yr of passive seismic data from a single station deployed near a known permafrost body on Mt. Zugspitze (Germany), to monitor freeze‐thaw processes. The recordings reveal a persistent cultural seismic noise source, which we utilize to compute single‐station cross‐correlations and extract relative seismic velocity changes. We find that parts of the cross‐correlations show seasonal velocity variations (≈3% peak‐to‐peak amplitude) and a long‐term velocity decrease (≈0.1%/yr). Comparison with meteorological data and a previous electrical resistivity tomography study suggests that these velocity changes are caused by freeze‐thaw cycles and by permafrost degradation, respectively. The results demonstrate the potential of passive seismology for permafrost monitoring and suggest that denser instrumentation will provide detailed spatio‐temporal insights on permafrost dynamics in future studies.

Luminous efficiency of meteors derived from ablation model after assessment of its range of validity

Context.The luminous efficiency,τ, can be used to compute the pre-atmospheric masses of meteoroids from corresponding recorded meteor brightnesses. The derivation of the luminous efficiency is non-trivial and is subject to biases and model assumptions. This has led to greatly varying results in the last decades of studies.Aims.The present paper aims to investigate how a reduction in various observational biases can be achieved to derive (more) reliable values for the luminous efficiency.Methods.A total of 281 meteors observed by the Fireball Recovery and InterPlanetary Observation Network (FRIPON) are studied. The luminous efficiencies of the events are computed using an ablation-based model. The relations ofτas a function of the pre-atmospheric meteoroid velocity,ve, and mass,Me, are studied. Various aspects that could render the method less valid, cause inaccuracies, or bias the results are investigated. On this basis, the best suitable meteors were selected for luminous efficiency computations.Results.The presented analysis shows the limits of the used method. The most influential characteristics that are necessary for reliable results for theτcomputation were identified. We study the dependence ofτon the assumed meteoroid’s density,ρ, and include improvedρ-values for objects with identified meteoroid stream association. Based on the discovered individual biases and constraints we create a pre-debiased subset of 54 well-recorded events with a relative velocity change >80%, a final height <70 km, and a Knudsen number Kn < 0.01; this last value indicates that the events were observed in the continuum-flow regime. We findτ-values in the range between 0.012% and 1.1% for this pre-debiased subset and relations ofτtoveandMeof:τ=7.33⋅ve−1.10andτ=0.28⋅Me−0.33.Conclusions.The derived luminous efficiency of meteoroids depends on the assumed material density. Our results indicate that the applied debiasing method improves the analysis ofτfrom decelerated meteoroids. The underlying method is only valid for meteors in the continuum-flow regime. These events tend to have low end heights, large masses, and high deceleration.

Equatorial Nighttime Thermospheric Zonal Wind Jet Response to the Temporal Oscillation of Solar Wind

AbstractThis work investigates the response of the equatorial thermospheric wind jet to the temporal oscillation of the Bz component of the interplanetary magnetic field using Thermosphere Ionosphere Electrodynamics General Circulation Model simulations and CHAllenging Minisatellite Payload observations. The thermospheric wind jet is formed in the magnetic equatorial region with the strongest zonal winds. Both the morning westward jet and the nighttime eastward wind jet at the dip equator decrease during the oscillation periods of IMF Bz. Ion drag dominates the deceleration of the equatorial eastward wind jet in the nighttime, with a minor contribution from the viscous force. The relative motion between the ions and neutrals has a dominant influence on the ion drag at the 60–180°W and 107–180°E geographic longitude; at other longitudes, both relative velocity (∆velocity) and electron density changes (∆Ne) are dominant. Moreover, the decrease in the equatorial eastward wind jet at 20 magnetic local time, which is also driven by ion drag, is significant above 270 km. The generation of the equatorial westward wind jet disturbance is significantly influenced by ∆velocity at all altitudes, whereas ∆Ne becomes important below 450 km.

Rigidity Strengthening of Landslide Materials Measured by Seismic Interferometry

Landslides have caused extensive infrastructure damage and caused human fatalities for centuries. Intense precipitation and large earthquakes are considered to be two major landslide triggers, particularly in the case of catastrophic landslides. The most widely accepted mechanistic explanation for landslides is the effective-stress dependent shear strength reduction due to increases in pore water pressure. The Chashan landslide site, selected for the present study, has been intensively studied from geological, geophysical, geodetic, geotechnical, hydrological, and seismological perspectives. Our seismic monitoring of daily relative velocity changes (dv/v) indicated that landslide material decreases coincided with the first half of the rainy period and increased during the latter half of the rainy period. The geodetic surveys before and after the rainy period identified vertical subsidence without horizontal movement. The results from the multidisciplinary investigation enabled us to draw a conceptual model of the landslide recovery process induced by water loading. Where all sliding materials were stable (safety factor > 1.0), unconsolidated landslide colluvium and impermeable sliding surfaces trapped the seepage water to form a water tank, provided that compact forces were acting on the materials below the sliding boundary. The vertical force of compaction facilitates an increase in the cohesion and strength of landslide materials, thereby increasing the landslide materials’ stability. We demonstrated that the recovery process periodically occurs only under the combined conditions of prolonged and intense precipitation and the related stability conditions.

An improved ultrasonic coda wave method for concrete behavior monitoring under various loading conditions

Monitoring of concrete behavior is an important task to evaluate the safety of concrete structures. This paper proposes a new stretching factor accumulation method based on the stepwise coda wave interference (CWI) to accurately calculate the relative velocity change Δv/v. The Δv/v and residual decorrelation coefficient Kd are used to study the concrete behavior in step loading experiment and short-time fixed loading experiment. The results show that: (1) In the loading stage of step loading experiments, both Δv/v and Kd can be used to monitor the load and the microcrack development; (2) In the relaxation stage of step loading experiments, the slow dynamics effect of concrete specimens can be characterized by Δv/v and Kd; (3) In the short-time fixed loading experiments, both Δv/v and Kd increase with the time of fixed loading, and the increasing rate of Δv/v decreases with the increase of concrete strength. This work can be used to promote the development of ultrasonic testing techniques that aid in concrete behavior monitoring.

Subsurface Moisture Regulates Himalayan Groundwater Storage and Discharge

AbstractThrough the release of groundwater, most mountain rivers run year‐round despite their small catchments and sporadic precipitation. This makes mountain ranges important sources of reliable freshwater for downstream populations in many parts of the world. However, due to a lack of ground instrumentation, little is known about groundwater dynamics in mountainous landscapes. Recent research has shown that the amount of moisture trapped in the soil and weathered rocks in the vadose zone can significantly buffer groundwater recharge and runoff but the wider recognition of this effect on major mountain systems has not been yet established. In this study, we test whether the moisture reservoir has an impact on hydrological fluxes in a steep Himalayan catchment during three monsoon seasons. We measured an array of parameters including relative seismic velocity changes from ambient noise correlations. This noninvasive technique allows us to monitor groundwater dynamics in conjunction with classical hydrological measurements. We found that the moisture saturation in the vadose zone controls the onset of groundwater recharge and runoff and therefore determines the annual water availability supplied by monsoon precipitation. We model this dynamic using a surface layer that has a finite storage capacity that controls the connectivity of surface flux to groundwater. The extension of this concept, which is thought to apply widely in flat and undulating landscapes, to steep mountain topography with thin and discontinuous soils underlain by regolith and bedrock has important implications for mountain hydrology.

РУХ ЧАСТИНКИ ПО ВНУТРІШНІЙ ПОВЕРХНІ СФЕРИЧНОГО СЕГМЕНТА, ЯКИЙ ОБЕРТАЄТЬСЯ НАВКОЛО ВЕРТИКАЛЬНОЇ ОСІ

The particle relative motion on a spherical segment rotating about a vertical axis was considered in the article. The differential equations of the relative displacement of a particle were completed and solved by numerical methods. The relative and absolute trajectories of particle motion and graphs of relative and absolute velocity changes were constructed. The regularity of particle motion as it is lifted over the surface was found out. The conducted experimental research has confirmed the received theoretical results.

Determination of nanoscale layer thickness adsorbed water liquid by acoustoelectric methods

It is shown to determine the thickness of the adsorbed liquid layer, it is proposed to use surface acoustic waves (SAW). The relative change in the SAW velocity is proportional to the thickness of the adsorption layer. The constant of proportionality is depending on the acoustic parameters of the solid (adsorbent) and the liquid in the layer. The velocity of sound in an adsorbed liquid depends on the thickness of the layer. For the determination of the velocity of sound in the liquid of the adsorption layer, it is proposed to use an original method based on comparing the parameters of bi-directional (differing in velocity) SAW.

Seismic constraints on rock damaging related to a failing mountain peak: the Hochvogel, Allgäu

AbstractLarge rock slope failures play a pivotal role in long‐term landscape evolution and are a major concern in land use planning and hazard aspects. While the failure phase and the time immediately prior to failure are increasingly well studied, the nature of the preparation phase remains enigmatic. This knowledge gap is due, to a large degree, to difficulties associated with instrumenting high mountain terrain and the local nature of classic monitoring methods, which does not allow integral observation of large rock volumes. Here, we analyse data from a small network of up to seven seismic sensors installed during July–October 2018 (with 43 days of data loss) at the summit of the Hochvogel, a 2592 m high Alpine peak. We develop proxy time series indicative of cyclic and progressive changes of the summit. Modal analysis, horizontal‐to‐vertical spectral ratio data and end‐member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of a 260,000 m3 large, instable rock volume, due to thermal forcing. Relative seismic wave velocity changes also indicate diurnal accumulation and release of stress within the rock mass. At longer time scales, there is a systematic superimposed pattern of stress increased over multiple days and episodic stress release within a few days, expressed in an increased emission of short seismic pulses indicative of rock cracking. Our data provide essential first order information on the development of large‐scale slope instabilities towards catastrophic failure. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

The Degradation of Automotive Radar Sensor Signals Caused by Vehicle Vibrations and other Nonlinear Movements.

As the demands on modern radar systems with respect to accuracy, reliability, and availability increase, a detailed assessment of the influence of nonlinear movements has become necessary. In particular, from the point of view of radar, different types of movements, such as any kind of acceleration, braking situation, or vehicle vibration, are essential parts of any traffic scenario. These unavoidable motions, in which the relative velocity changes within one measurement cycle, are called nonlinear movements. These nonlinearities contribute to intermediate frequencies, which are comparable to the extensively described nonlinearities of a frequency ramp. This additional contribution to the intermediate signal has a direct effect on the signal-to-noise ratio and thus on the accuracy and probability of target detection. This paper presents a study of various types of nonlinear motion and a detailed definition of the resulting parameters based on a variety of vehicle-based measurements. An advanced signal model of frequency-modulated continuous wave (FMCW) radar is introduced and verified in addition to a detailed mathematical description of spectral signal behaviour in sinusoidal motions and linear acceleration. The theoretical and experimental results in idealised point targets are transferred to real complex road users. Furthermore, by applying established automotive signal processing steps in the form of an ordered statistical constant false alarm rate (OS CFAR), the consequences of determining the noise level are also shown. In combination with the already introduced signal behaviour, these results enabled general description of the signal-to-noise ratio of nonlinear movements in complex traffic scenarios.

Field-angle dependence of sound velocity in the Weyl semimetal TaAs

The elastic modulus $c_{44}$ of a single crystal of the Weyl semimetal TaAs was investigated by measuring relative changes in the sound velocity under application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo technique, we studied the shear response of the crystal when the angle between the sound wave propagation and the magnetic field is changed. We observe a broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely related to a longitudinal negative magnetoresistance and therefore might provide evidence of the chiral anomaly, one of the main topological signatures of this class of materials. We also observe quantum oscillations in the sound velocity whose frequencies vary with magnetic field orientation. A fan diagram of Landau level indices reveals topological and trivial Berry phases, depending on the field orientation, indicating a sensitivity to different Fermi surface pockets that do or do not enclose Weyl nodes respectively.

Identifying the Prestress Force in Prestressed Concrete Bridges Using Ultrasonic Technology

In this research, ultrasonic tests were performed on a prestressed concrete box-girder model to identify the prestressed force according to the acoustoelastic theory. During the tests, the ultrasonic wave was generated using piezoelectric transducers and emitted to the prestressed concrete bridge model. The concrete bridge model was subjected to three different levels of prestress force (PF), which were limited to about 30%, 50% and 80% of the ultimate tensile strength. The experimental results showed the increase in prestress force level leads to an increase in the relative change in the wave velocity and the amplitude energy of the ultrasonic wave which proved the acoustoelastic effect theory. This study contributes to the knowledge of the acoustoelastic behavior of the prestressed concrete and presents the capability of the ultrasonic system in evaluating the stress state in the prestressed concrete bridge.

Comparison of Experimentally Determined Two-Dimensional Strain Fields and Mapped Ultrasonic Data Processed by Coda Wave Interferometry.

Due to the high sensitivity of coda waves to the smallest structural alterations such as strain, humidity or temperature changes, ultrasonic waves are a valid means to examine entire structures employing networks of ultrasonic transducers. In order to substantiate this ex ante assessment, the viability of measuring ultrasonic waves as a valid point of reference and inference for structural changes is to be further scrutinized in this work. In order to investigate the influence of mechanical strain on ultrasonic signals, a four-point bending test was carried out on a reinforced concrete beam at Ruhr University Bochum. Thus, measurements collected from a network of selected transducer pairings arranged across the central, shear-free segment of the test specimen, were correlated to their respective strain fields. Detected ultrasonic signals were evaluated employing Coda Wave Interferometry. Such analysis comprised the initial non-cracked state as well as later stages with incremental crack depth and quantity. It was to ascertain that the test specimen can in fact be qualitatively compartmentalized into areas of compression and tension identified via Relative Velocity Changes presented in Attribute Maps. However, since results did not entail a zero crossing, i.e., neither positive nor negative values were to be calculated, only relative changes in this work displayed staggered over the height of the object under test, are discussed. Under the given methodological premises, additional information is currently required to make quantitative assertions regarding this correlation of ultrasonic and strain results. This holds true for the comparability of the ultrasonic and strain results for both non-cracked and even the cracked state.

Acute Adolescent Concussion: Cerebrovascular Reactivity, Symptom Burden, And Exercise Response

Cerebral vasoreactivity (CVR) disruptions have been observed following concussion, which may exacerbate concussion-related symptoms. These CVR disruptions may also influence the cerebral blood flow (CBF) responses during aerobic exercise and thus limit exercise capacity post-concussion. PURPOSE: To examine the relationships between concussion-related symptoms, cerebral vasculature’s ability to respond to changes in CO2 (vasoreactivity), and CBF responses during sub-maximal aerobic exercise in adolescents post-concussion. METHODS: : Adolescents less than two weeks post-concussion completed the Post-Concussive Symptom Checklist (PCSC), cerebral vascular assessments, and a modified YMCA exercise protocol. CVR at rest was estimated from the slope (cm/s/mmHg) of the relationship between the increases in breath-by-breath end-tidal CO2 and responses of CBF velocity during an air rebreathing task (i.e., increasing end-tidal CO2). CBF velocity was measured via transcranial Doppler ultrasonography at the M1 segment of the middle cerebral artery unilaterally during the air rebreathing task and during the aerobic exercise test. Only physical and vestibular-related symptoms were included in analysis: headache, nausea, balance problems, dizziness, fatigue, and % activity level. Two stepwise linear regressions were conducted to test (1) if concussion symptoms are related to CVR and (2) if CVR predicted relative change in CBF velocity during the aerobic exercise test. RESULTS: The majority of the variance in CVR was explained by symptoms of nausea, dizziness, as well as current physical activity levels. Age, sex, resting heart rate, balance, and fatigue also contributed to a lesser degree. Exercise caused a significant increase in MCA blood flow (p<0.001). This exercise induced increase in CBF was largely explained by CVR and estimated VO2peak, with resting heart rate, prior concussion(s), and time since injury also contributing. CONCLUSIONS: Our findings suggests that symptom burden is related to disruptions in CVR in adolescents in the early weeks after a concussion. In turn, the cerebral blood flow response to exercise was related to both CVR and aerobic capacity, suggesting that disruptions in CVR may impact exercise tolerance post-concussion.

Determination of acoustic nonlinearity parameters using thermal modulation of ultrasonic waves

This study presents a test method and its theoretical framework to determine the acoustic nonlinearity parameters (α, β, δ) of material using thermal modulation of ultrasonic waves. Temperature change-induced thermal strain excites the nonlinear response of the material and modulates the ultrasonic wave propagating in it. Experimental results showed a strong correlation between the relative wave velocity change and the temperature change. With a quadratic polynomial model, the acoustic nonlinearity parameters were obtained from the polynomial coefficients by curve fitting the experimental data. Their effects on thermal-induced velocity change were discussed. The parameters α, β, and δ govern the hysteretic gap, average slope, and curvature of the correlation curve, respectively. The proposed theory was validated on aluminum, steel, intact and damaged concrete samples. The obtained nonlinear parameters show reasonable agreement with values reported in the literature. Compared to other nonlinear acoustic methods using vibration or acoustic excitation, the thermal modulation method generates more uniform, slow changing, and larger strain field in the test sample. Employing the thermal effect as the driving force for nonlinearity instead of an undesired influencing factor, this method can measure the absolute values of α, β, and δ with good accuracy using a simple ultrasonic test setup.

Quantifying fatigue-damage and failure-precursors using ultrasonic coda wave interferometry

Accumulation of the subtle impacts of fatigue-induced rock damage may ultimately trigger rock failure. Probing, understanding and quantifying this behavior is crucial in evaluating the mechanical response under dynamic loading. We conduct cyclic uniaxial compression tests with real-time measurement of ultrasonic velocity to probe the evolution of fatigue-induced damage in shale subjected to cycles of increasing-amplitude and constant-amplitude. The stress-strain response shows hysteretic behavior and hardening effect under cyclic compression. Time of first arrival is used to estimate apparent changes in ultrasonic velocity at incremented maximum stresses. Coda wave interferometry (CWI) is applied to detect the more subtle changes in ultrasonic velocity at minimum stress (10 MPa) under fatigue-loading. Despite a continuous increase in ultrasonic velocity measured at incremented maximum stresses the subtle signal of increasing fatigue-induced damage is apparent in the sensitive CWI signal. This is reflected in the descending trend of ultrasonic velocity, especially the S-wave velocity, observed at the minimum stress (10 MPa) – and may ultimately result in failure. The evolution of ultrasonic velocity is controlled by the competing effects of irreversible healing/closure of micro-fractures, dominating at low-stresses/early-cycles, and counteracted by the creation of fatigue-induced micro-fractures, dominating at high-stresses/late-cycles. The development of fatigue-induced micro-fractures is optimally detected by the multiply-scattered coda waves but is absent in the first arrivals. Dynamic modulus and relative changes in effective velocity inferred from CWI are sensitive and discernable indicators of fatigue-induced damage prior to failure while characteristics of the stress-strain curve and the evolution of elastic moduli are inferior indicators.

Cerebrovascular reactivity and disease activity in relapsing-remitting multiple sclerosis

In multiple sclerosis (MS), insufficient blood supply might worsen energy deficiency of the brain tissue. Thus, cerebrovascular reactivity (CVR), which is the capacity of cerebral circulation to match blood supply to metabolic demand, might be important in MS pathology. The objective of this study was to investigate the relationship of CVR to disease activity and neuroimaging markers of disease progression in patients with MS. In 43 patients with relapsing remitting MS (RRMS) in clinical remission, 30 patients with a relapse of MS and 30 healthy controls, we measured CVR with transcranial Doppler as a relative change in flow velocity after breath-holding (breath-holding index) and voluntary hyperventilation (hyperventilation index). All patients in remission underwent brain magnetic resonance imaging at baseline and 33 underwent repeated imaging after 12 months, with various brain volume measurements taken. Cerebrovascular reactivity indices did not differ between patients in remission, patients with a relapse and controls. In patients in remission, CVR did not differ between those with or without contrast-enhancing lesions. In patients with a relapse, glucocorticoids significantly reduced both CVR indices. Cerebrovascular reactivity was not related to brain volume, white matter lesion volume, percent brain volume change, and the change in total white matter lesion volume. In RRMS, CVR appeared normal and unrelated to disease activity. There was no substantial association of CVR to brain atrophy and accumulation of white matter lesions.

The 2018 Geothermal Reservoir Stimulation in Espoo/Helsinki, Southern Finland: Seismic Network Anatomy and Data Features

AbstractA seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time- and frequency-dependent variations of the signal-to-noise ratio of earthquake records from a 260-meter-deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identified by the company. The fault-plane solutions of 14 selected ML 0.6–1.8 events indicate a dominant reverse-faulting style, and the associated SH radiation patterns appear to control the first-order features of the macroseismic report distribution. Beamforming of earthquake data from six arrays suggests heterogeneous medium properties, in particular between the injection site and two arrays to the west and southwest. Ambient-noise cross-correlation functions reconstruct regional surface-wave propagation and path-dependent body-wave propagation. A 1D inversion of the weakly dispersive surface waves reveals average shear-wave velocities around 3.3 km/s below 20 m depth. Consistent features observed in relative velocity change time series and in temporal variations of a proxy for wavefield partitioning likely reflect the medium response to the stimulation. The resolution properties of the obtained data can inform future monitoring strategies and network designs around natural laboratories.

Dynamic Stressing of Naturally Fractured Rocks: On the Relation Between Transient Changes in Permeability and Elastic Wave Velocity

AbstractWe report on laboratory experiments designed to explore the fundamental processes that impact fluid permeability during drilling and fluid injection. Coupled ultrasonic and permeability measurements are presented for Westerly granite samples fractured in situ and used to illuminate the relationship between nonlinear dynamic stiffness and fracture permeability indexed via fracture aperture. We perturb the effective stress field using normal stress and pore pressure oscillations. The velocity of ultrasonic waves transmitted across the fracture is used to infer the evolution of elastic properties and the fracture nonlinearity. Changes in permeability are measured concurrently. We observe that relative changes in wave velocity and permeability, due to both normal stress and pore pressure oscillations, are correlated, such that larger drops in wave velocity (higher nonlinearity) correspond to larger increases in permeability. Our observations suggest that dynamic stressing is more likely to enhance the permeability of fractures that exhibit greater nonlinearity.

Denoising ambient seismic field correlation functions with convolutional autoencoders

SUMMARYSeismic interferometry is an established method for monitoring the temporal evolution of the Earth’s physical properties. We introduce a new technique to improve the precision and temporal resolution of seismic monitoring studies based on deep learning. Our method uses a convolutional denoising autoencoder, called ConvDeNoise, to denoise ambient seismic field correlation functions. The technique can be applied to traditional two-station cross-correlation functions but this study focuses on single-station cross-correlation (SC) functions. SC functions are computed by cross correlating the different components of a single seismic station and can be used to monitor the temporal evolution of the Earth’s near surface. We train and apply our algorithm to SC functions computed with a time resolution of 20 min at seismic stations in the Tokyo metropolitan area, Japan. We show that the relative seismic velocity change [dv/v(t)] computed from SC functions denoised with ConvDeNoise has less variability than that calculated from raw SC functions. Compared to other denoising methods such as the SVD-based Wiener Filter method developed by Moreau et al., the dv/v results obtained after using our algorithm have similar precision. The advantage of our technique is that once the algorithm is trained, it can be apply to denoise near-real-time SC functions. The near-real-time aspect of our denoising algorithm may be useful for operational hazard forecasting models, for example when applying seismic interferometry at an active volcano.