Frequency-dependent Amplitude Research Articles

The dissociative role of bursting and non-bursting neural activity in the oscillatory nature of functional brain networks

Abstract The oscillatory nature of intrinsic brain networks is largely taken for granted in the systems neuroscience community. However, the hypothesis that brain rhythms—and by extension transient bursting oscillations—underlie functional networks has not been demonstrated per se. Electrophysiological measures of functional connectivity are indeed affected by the power bias, which may lead to artefactual observations of spectrally specific network couplings not genuinely driven by neural oscillations, bursting or not. We investigate this crucial question by introducing a unique combination of a rigorous mathematical analysis of the power bias in frequency-dependent amplitude connectivity with a neurobiologically informed model of cerebral background noise based on hidden Markov modeling of resting-state magnetoencephalography (MEG). We demonstrate that the power bias may be corrected by a suitable renormalization depending nonlinearly on the signal-to-noise ratio, with noise identified as non-bursting oscillations. Applying this correction preserves the spectral content of amplitude connectivity, definitely proving the importance of brain rhythms in intrinsic functional networks. Our demonstration highlights a dichotomy between spontaneous oscillatory bursts underlying network couplings and non-bursting oscillations acting as background noise but whose function remains unsettled.

Response and reliability analysis of a nonlinear VEH systems with FOPID controller by improved stochastic averaging method and LBFNN algorithm

In engineering, it is an important issue to guarantee the normal working of a vibration energy harvesting (VEH) device to harvest electricity. However, the determination of the system dynamics is a huge challenge due to random excitation, nonlinear structure, and control force. This paper aims to study the response and reliability of a kind of nonlinear and coupled VEH system with an FOPID controller. The stationary response is analyzed by an improved stochastic averaging method, in which an amplitude-dependent frequency is taken into account during the averaging procedure in the case of the strongly nonlinear system. Time-varying reliability in the VEH system is analyzed by using a new LBFNN algorithm, and weight coefficients at each time in this algorithm is obtained through an iteration procedure. Numerical results show that there are essential differences between the weakly nonlinear system and the strongly nonlinear one in terms of averaging procedure and response probability. The fractional orders in the FOPID controller are critical parameters to bring stochastic bifurcations. In addition, strongly nonlinear structure together with lower-order fractional derivative and higher-order fractional integration in FOPID controller are helpful to enhance the reliability performance of the VEH system.

Anelastic Tomography of the Arabian Plate

ABSTRACT In this study, we investigate the upper-mantle structure of the Arabian plate based on anelastic full-waveform inversion. The Arabian plate is composed of two main geological provinces—the Arabian Platform in the east and the Arabian Shield in the west. The Shield is dotted with areas of Cenozoic volcanism (harrats) whose precise tectonic and geodynamic origin is still debated. To help shed light on the deep geophysical properties of the Arabia plate and seismic signatures in the Earth’s upper mantle related to the surface volcanism, we use ∼38,400 three-component waveforms filtered between 25 and 100 s from 64 well-distributed regional earthquakes. We fit frequency-dependent travel-time and amplitude information. We follow a hierarchical inversion strategy in which we initially invert for the elastic structure and subsequently for both the elastic and anelastic parameters simultaneously. We assess the resolution and trade-offs between model parameters by conducting point spread function tests. Our model reveals low-wavespeed anomalies associated with mantle material that may originate from the Afar plume. In addition, we identify significant shear attenuation anomalies in the southern and central regions of the western margin of the Arabian plate. We suggest that these anomalies are linked to either the hot asthenosphere from the Afar plume or partial melting triggered by lithospheric thinning along the Red Sea rift.

Study of high power steam turbine forced oscilla-tions near operating frequency

Problem. Forced oscillations of the turbine unit-foundation-base system near the operating frequency are considered. Goal. The purpose of the work is to study forced vibrations of the turbine unit-foundation-base system near the operating frequency range to assess the vibration state of the system and determine the causes of increased vibrations. The object of the study is the turbine unit-foundation-base system. The system consists of a single reinforced concrete foundation on which a steam turbine and generator are installed. The subject of the study is the amplitude-frequency dependences of forced oscillations of the turbine unit-foundation-base system. Methodology. The study was carried out using vibration methods and the finite element method. Also worth noting is the use of original methods developed directly by the author for constructing models of complex mechanical engineering systems. Results. As a result of the research, three-dimensional finite element models of low-pressure cylinder housings, the foundation and the entire turbine unit-foundation-base system were built. The conducted research allows us to draw a conclusion regarding the reasons for the increased level of vibration near the operating frequency and directions for their prevention. Originality. Regarding the methods for constructing a model of the turbine unit-foundation-base system, it should be noted that they are unique. The features of the applied modeling method make it possible to take into account the variable interaction between the low-pressure cylinder bodies and the foundation. Fixpoints are also taken into account. Previous studies of the turbine unit-foundation-base system did not allow us to determine the causes of increased vibrations of the shaft line supports. Practical significance. The results of the work relate to direct practical application. As a result of the work, conclusions were drawn about the ways to increase the vibration reliability of the turbine unit-foundation-base system and further directions for research.

Viscoelasticity of suspension of red blood cells under oscillatory shear flow

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) for different volume fractions in a wall-bounded, effectively inertialess, small amplitude oscillatory shear (SAOS) flow for a wide range of applied frequencies. The RBCs are modeled as biconcave capsules, whose membrane is an isotropic and hyperelastic material following the Skalak constitutive law. The frequency-dependent viscoelasticity in the bulk suspension is quantified by the complex viscosity, defined by the amplitude of the particle shear stress and the phase difference between the stress and shear. SAOS flow basically impedes the deformation of individual RBCs as well as the magnitude of fluid-membrane interactions, resulting in a lower specific viscosity and first and second normal stress differences than in steady shear flow. Although it is known that the RBC deformation alone is sufficient to give rise to shear-thinning, our results show that the complex viscosity weakly depends on the frequency-modulated deformations or orientations of individual RBCs but rather depends on combinations of the frequency-dependent amplitude and phase difference. The effect of the viscosity ratio between the cytoplasm and plasma and of the capillary number is also assessed.

Exploratory Structural Modification and Nonlinear Based Analysis of Tuned Mass Dampers

This article presents a structural modification-based analysis of tuned mass dampers that motivates to investigate the performance of a cubic stiffness absorber. The rationale behind this investigation is that the analysis of vibration absorber using structural modifications could pave the way for the use of nonlinear mechanical elements in vibration absorption applications. To achieve this, the article reviews the fundamentals of structural modification-based analysis of absorbers. It then examines the effects of using the Duffing oscillator as an absorber. The Duffing oscillator is integrated into the primary system using the describing function and the amplitude-dependent nonlinear frequency response functions are computed. The results of this analysis are compared with those of a linear absorber in order to clarify the differences and point out possible directions for further development.

Lagrangian operator inference enhanced with structure-preserving machine learning for nonintrusive model reduction of mechanical systems

Complex mechanical systems often exhibit strongly nonlinear behavior due to the presence of nonlinearities in the energy dissipation mechanisms, material constitutive relationships, or geometric/connectivity mechanics. Numerical modeling of these systems leads to nonlinear full-order models that possess an underlying Lagrangian structure. This work proposes a Lagrangian operator inference method enhanced with structure-preserving machine learning to learn nonlinear reduced-order models (ROMs) of nonlinear mechanical systems. This two-step approach first learns the best-fit linear Lagrangian ROM via Lagrangian operator inference and then presents a structure-preserving machine learning method to learn nonlinearities in the reduced space. The proposed approach can learn a structure-preserving nonlinear ROM purely from data, unlike the existing operator inference approaches that require knowledge about the mathematical form of nonlinear terms. From a machine learning perspective, it accelerates the training of the structure-preserving neural network by providing an informed prior (i.e., the linear Lagrangian ROM structure), and it reduces the computational cost of the network training by operating on the reduced space. The method is first demonstrated on two simulated examples: a conservative nonlinear rod model and a two-dimensional nonlinear membrane with nonlinear internal damping. Finally, the method is demonstrated on an experimental dataset consisting of digital image correlation measurements taken from a lap-joint beam structure from which a predictive model is learned that captures amplitude-dependent frequency and damping characteristics accurately. The numerical results demonstrate that the proposed approach yields generalizable nonlinear ROMs that exhibit bounded energy error, capture the nonlinear characteristics reliably, and provide accurate long-time predictions outside the training data regime.

Seismic attributes and spectral decomposition-based inverted porosity-constrained simulations for appraisal of shallow-marine lower-Cretaceous sequences of Miano gas field, Southern Pakistan

Seismic attributes can play an important role in the exploration of hydrocarbon-bearing stratigraphic systems. Incised valley systems are developed during the falling sea, which causes the deposition of coarse-grained sandstone facies inside the low-standing tracts (LST). These regional phenomena constrain the quantitative attributes of ultra-thin-bedded stratigraphic petroleum traps, e.g., vertical and lateral variations in the thickness, accommodation space, lithology, and porosity. This study deals with the application of the continuous wavelet transform (CWT) of a spectral decomposition (SD) tool on a 3D post-stack seismic volume of the Miano gas Field, Lower Indus basin, Pakistan. The results show that the CWT accurately detected the regionally faulted/fractured system and distinguished the frequency-dependent amplitude anomalies. The wedge model resolved a 24-meter-thick gas-bearing resource. Quality control analysis was carried out using CWT-based broadband processing between the designed amplitude spectrum of 17 Hz and 70 Hz. The reservoirs with over 25% porosity that were located within the shale-dominated facies with less than 8% porosity were imaged through the processing of the instantaneous spectral porosity model at the 48-Hz tuning block. Moreover, 190 to 165-m-thick thin-bedded sandstone reservoirs at a 25% porosity zone were resolved using 22-Hz and 28-Hz, which implicates the sea standstill and medium-to-coarse-grained depositional reservoir facies. The ultra-thin-bedded traps inside the laterally continuous stratigraphic lens of 121 m and the prograding clinoform lens of 101-m within the incised valley petroleum system were resolved using 48-Hz, which implicates the falling sea and fine-scaled transgressed erosional facies. These implications suggest that the identified regional stratigraphic traps have development potential for this gas field. The treatment of the inverted model at the highest frequencies can be utilized to investigate the porous stratigraphically trapped facies of LST and can serve as an important analogue for the leading gas field of the Indus Basin and similar basins.

Nonlinear characteristics identification of an impact oscillator with a one-sided elastic constraint

Impacting systems are widely used in many engineering applications, such as self-propelled robots, energy harvesting and percussive drilling, which exhibit rich and complex nonlinear phenomena. Among these applications, predicting nonlinearities and estimating system parameters are of great interest of nonlinear dynamics research community. Backbone curve is an analytical tool that captures the frequency–amplitude dependence of nonlinear systems. In this paper, we estimate the impacting stiffness of a single-degree-of-freedom non-smooth dynamical system qualitatively by using the backbone curve. It was found that an increase of the impacting stiffness may lead to lowering the backbone curve. An adaptive differential evolution algorithm with the Metropolis criterion is proposed to identify the parameters of the impacting system quantitatively using experimental data, which are consistent with our theoretical predictions. Finally, the identified parameters are verified, and the limitations of the backbone curve are drawn. The nonlinear characteristics identification method studied in this paper can be extended to other vibro-impact systems and is potentially applicable to structural health monitoring and robotic sensing.

ON THE RECONSTRUCTION OF PRESTRESS FIELDS IN A HOLLOW CYLINDER

The present research is devoted to the development of the theoretical foundations of nondestructive acoustic method for identifying inhomogeneous prestress fields in a hollow cylinder, depending on the probing loading type. A linearized model of steady oscillations of an elastic body in the presence of an inhomogeneous prestress field of arbitrary nature is considered in the standard and weak formulations. On the basis of this model, we formulate a problem for a cantilever-clamped prestressed hollow cylinder that performs steady axisymmetric vibrations under three types of probing loading. A corresponding weak formulation of the problem in the cylindrical coordinate system is presented, in which six independent components of the prestress tensor are taken into account. At that, a case of prestress fields obtained by applying some initial mechanical external static load is considered. In the presence and absence of prestresses of various types, amplitude-frequency dependences are analyzed, and resonant and natural frequencies are found in a wide frequency range. Numerical calculations were carried out using the FEM on a non-uniform grid; mesh refinement is carried out in the vicinity of the boundary points, where the type of boundary conditions changes. Based on the numerical solution of an auxiliary set of direct problems, seven types of prestress fields are constructed, differing in the types of initial loading, most often encountered in practice. To assess the possibility of implementing the procedure for reconstructing prestresses of each of the considered types, a sensitivity analysis was additionally performed, which showed that for some prestress types there are frequencies and types of probing loading for which the presence of prestress is practically not manifested. The sensitivity analysis performed made it possible to implement the optimal method of probing loading when solving the inverse coefficient problem. The statement of the new inverse problem on the restoration of arbitrary inhomogeneous prestress fields in the considered finite hollow cylinder is formulated. When restoring the prestress of a given structure, the inverse problem is reduced to finding a set of parameters from an ill-conditioned algebraic system, which was studied with the help of the A.N. Tikhonov regularization method. Additional data for solving the inverse problem was obtained on the basis of probing both via a single load and via combined probing modes. It has been found that it is most effective to use a combined loading mode and use a sufficiently wide frequency range when selecting sounding frequencies. The results of computational experiments on the reconstruction of six components of the prestress tensor are presented and analyzed, and recommendations are proposed for choosing the optimal modes of acoustic sounding.

Frequency dependent amplitude response of different couplant materials for mounting piezoelectric sensors

Non-destructive testing techniques, such as ultrasonic testing and acoustic emission analysis, commonly employ piezoelectric sensors for monitoring and detecting defects in structures. The quality of data acquired using these sensors is highly dependent on the coupling layer between the transducer and the structure’s surface. In this context, this study compares the signal response of four different couplant materials, namely acrylic adhesive pads, honey, vaseline, and hot glue, applied to a steel surface. For this purpose, experiments were conducted using a laser Doppler vibrometer and acoustic emission analysis to investigate the signal transmission of the couplant materials and their impact on the signal response of a coupled piezoelectric sensor VS30-V. The repeatability of the experiments was statistically analyzed. The findings indicate that acoustic emission measurements with acrylic adhesive pads exhibited the lowest relative standard deviation of 11.4%, followed by honey (13.2%), hot glue (21.9%), and vaseline (32.1%). The investigated couplant materials exhibited different effects on the signal response of the piezoelectric sensor. Specifically, acrylic adhesive pads and hot glue demonstrated more reliable signal transmission in the frequency range of 50 kHz to 65 kHz. In contrast, honey and vaseline had better performance within the frequency range of 65 kHz and 80 kHz. Considering the frequency-dependent characteristics of signal transmission and the ease of application, acrylic adhesive pads and honey are considered the preferred couplant materials for the frequency ranges of 50 kHz to 65 kHz and 65 kHz to 80 kHz, respectively.

Transcriptional correlates of frequency-dependent brain functional activity associated with symptom severity in degenerative cervical myelopathy

BackgroundNeuroimaging techniques provide insights into the brain abnormalities secondary to degenerative cervical myelopathy (DCM) and their association with neurological deficits. However, the neural correlates underlying the discrepancy between symptom severity and the degree of spinal cord compression, as well as the transcriptional correlates of these cortical abnormalities, remain unknown in DCM patients. MethodsIn this cross-sectional study, which collected resting-state functional MRI (rs-fMRI) images and the Japanese Orthopedic Association (JOA) score, enrolled 104 participants (54 patients and 50 healthy controls). The frequency-dependent amplitude of low-frequency fluctuation (ALFF) was obtained for all participants. We investigated the ALFF differences between mild-symptom DCM patients and severe-symptom DCM patients while carefully matching the degree of compression between these two groups via both univariate comparison and searchlight classification for three frequency bands (e.g., Slow-4, Slow-5, and Full-band). Additionally, we identified genes associated with symptom severity in DCM patients by linking the spatial patterns of gene expression of Allen Human Brain Atlas and brain functional differences between mild symptom and severe symptom groups. Results(1) We found that the frequency-specific brain activities within the sensorimotor network (SMN), visual network (VN), and default mode network (DMN) were associated with the varying degrees of functional impairment in DCM patients; (2) the frequency-specific brain activity within the SMN correlated with the functional recovery in patients with DCM; (3) a spatial correlation between the brain-wide expression of genes involved in neuronal migration and the brain functional activities associated with symptom severity was identified in DCM patients. ConclusionIn conclusion, our study bridges gaps between genes, cell classes, biological processes, and brain functional correlates of DCM. While our findings are correlational in nature, they suggest that the neural activities of sensorimotor cortices in DCM are associated with the severity of symptoms and might be associated with neuronal migration within the brain.

Geometric nonlinear stiffness and frequency of the traditional spring-suspended free-vibration testing device

Free vibration tests on bridge deck sectional models in wind tunnels have been widely adopted to study the wind-resistant performances of bridges. The traditional spring-suspended free-vibration testing device is suitable for small torsional amplitude vibration tests. When large torsional displacement occurs, a geometry nonlinear will be introduced, and the stiffness and frequency of the traditional device will change. However, quantitative research on this subject is inadequate at present. Based on proper simplifications and assumptions, new analytical formulas are derived that describe how the torsional and vertical stiffnesses of the traditional device vary as the torsional and vertical displacement change for different spring lengths, initial strains, joint spacings, and stiffnesses. The amplitude-dependent torsional frequency is also quantified based on the potential energy function. It is found that the torsional stiffness reduction rate Rα and frequency reduction rate RfAα nonlinearly increase with the torsional displacement α and amplitude Aα, respectively. In the parameter ranges considered, R10° is basically within 4%–6%, R20° is basically within 15%–28%; Rf10° is basically within 1%–2%, Rf20° is basically within 6%–11%. From the view of geometry nonlinear, the maximum torsional amplitude of the traditional device is suggested to be less than 4°. Although Rα can be effectively reduced by increasing the initial strain and length of the upper and lower springs, it cannot be completely eliminated whatever increasing the above parameters. The vertical stiffness reduction rate can be ignored.

THE CONTRIBUTION OF THE RELAXATION OF THE HEAT FLOW TO THE CHARACTERISTICS OF THE FUNDAMENTAL HARMONICS OF A NONLINEAR PHOTOACOUSTIC SIGNAL IN CONDENSED MEDIA

Based on the system of nonlinear heat conduction equations, which takes into account the relaxation of the heat flux for the gas layer, the sample, and the substrate, a system of equations is obtained for temperature fluctuations at the fundamental harmonic (FH) of a nonlinear photoacoustic (PA) signal. It is believed that all three layers in the PA-chamber have their own relaxation times, and the irradiated photon flux is modulated according to the harmonic law. The temperature dependence of thermophysical and optical parameters is represented by thermal coefficients. By solving the boundary problem, general expressions were obtained for the oscillatory component of the temperature perturbation in the exhaust gas, and then, according to a well-known scheme, for pressure fluctuations in the gas layer. The obtained expressions are analyzed. The frequency dependence of the amplitude and phase of this signal is numerically calculated for two possible substrates: the substrate is a thermal insulator and the substrate is an ideal conductor. It has been established that the frequency dependence of the PA-signal amplitude for these cases is antiphase. While, this dependence for the phase of the PA-signal is consistent in phases moreover, the values of the maximum and minimum coincide. The only significant difference is that in the region of zero modulation frequencies, the phase of the signal for a substrate made of a thermal insulator is positive, while for a substrate made of an ideal conductor is negative.

Geometrically nonlinear analysis of compliant mechanisms using a dynamic beam constraint model (DBCM)

Compliant mechanisms with intermediate-deformation ranges are increasingly applied in large-stroke precision manipulators and other actuators. The nonlinearities of axially-loaded stiffening and kinematics-arching effects (P-Δ effects) have a pronounced influence on the kinetostatic and dynamic behaviors of such a class of compliant mechanisms. A dynamic beam constraint model (DBCM) with the pseudo-static characteristic is reported for the nonlinear kinetostatics and large-amplitude vibration analyses of intermediate-deformation compliant mechanisms. Firstly, the DBCM of flexure beams is derived in a closed-form equation on the frequency domain, including the P-Δ effects. A matrix-based modeling methodology with a step-by-step iteration procedure is then introduced to calculate the nonlinear performance of general planar compliant mechanisms with serial-parallel configurations avoiding inner force analysis and kinematics calculation. Except for the static constraint behaviors in the BCM, the amplitude-dependent resonance frequency is captured with the DBCM. Theoretical, numerical and experimental comparisons for a double parallel guiding flexure pivot and Scott-Russell compliant amplifying mechanism verify the feasibility of the DBCM. In summary, the DBCM enables both the nonlinear kinetostatics and large-amplitude vibration analyses of moderately large-stroke compliant mechanisms in a straightforward way.

An investigation into the interaction between a nonlinear resonance and a fixed anti-resonance in a harmonically excited system

The study of linear mechanical structures with discrete nonlinear attachments is a widespread field in structural dynamics as it is important in many areas of engineering. A common type of nonlinearity is a hardening spring, which can be described by a cubic polynomial representing the force-deflection relationship. The frequency response of a system consisting of a hardening spring and a mass is characterized by an amplitude-dependent resonance frequency, that increases as the displacement of the mass increases. The way in which the resonance of a nonlinear attachment interacts with the resonance of a linear host structure has already been discussed in the literature, however little attention has been paid to the interaction of the nonlinear resonance with a fixed anti-resonance of the host structure. This paper aims to fill this gap by addressing the physical aspects of the interaction between an anti-resonance and a resonance due to a nonlinear resonant attachment. A two-degrees-of-freedom system representing a general structure with two modes of vibration is used as a host structure to which a nonlinear resonant device is attached. The host structure has a fixed anti-resonance prior to the nonlinear device being attached to the structure, and the influence of this device on the anti-resonance is investigated. This is achieved by using the harmonic balance method and a numerical continuation algorithm. In the study, it was found that the anti-resonance interacts with the nonlinear resonance, where some of the peaks bend toward higher frequencies because of the hardening stiffness. Further increasing the nonlinearity, results in an isola being formed inside the main frequency response curve. When the motion is extremely high, quasi-periodic or chaotic-like responses are observed in specific frequency regions.

Frequency response variability in friction-damped structures due to non-unique residual tractions: Obtaining conservative bounds using a nonlinear-mode-based approach

Under static dry friction, the tangential friction force (residual traction) is non-unique within the Coulomb’s limits. The actual value of the residual traction depends on the load history and is generally unknown. This phenomenon can lead to a large variability of the vibration response if at least one but not all contacts are permanently sticking and there is coupling between tangential displacements and normal forces. In the present work, a method is developed that allows obtaining conservative upper and lower bounds of the frequency response. Focusing on primary resonances with well-separated modes, a Nonlinear-Mode-based approach is pursued. First, bounds of the nonlinear modal properties, specifically the amplitude-dependent modal frequency, damping ratio and deflection shape are computed. Then, interval arithmetic is applied to the closed-form expression governing the amplitude-frequency curves for a given harmonic excitation force. The method is numerically demonstrated for a planar structure coupled with two Jenkins-type contact elements. It is shown that for this system, the bounds of the modal properties can be simply obtained from the frictional limit cases (barely sticking contact), so that a potentially costly numerical optimization is not needed. It is concluded that the obtained frequency response bounds are indeed conservative but not overly conservative, and the method is computationally highly efficient as it relies on analytical developments. The developed method of Nonlinear-Mode-based forward propagation seems attractive also for other sources of uncertainty and experimentally identified bounds.

NONLINEAR EFFECTS IN CRYSTALLINE SOLIDS WITH SATURATION OF AMPLITUDE-DEPENDENT INTERNAL FRICTION, DECREASING WITH IN-CREASING FREQUENCY

As a result of modification of quasi-static elastic and inelastic hysteresis, dynamic hysteresis equations of state of crystalline solids with frequency-dependent saturation of amplitude-dependent internal friction decreasing with increasing frequency are proposed. The nonlinear propagation of initially harmonic longitudinal elastic waves in rods made of such materials is investigated by the perturbation method. Numerical, graphical and comparative analysis of the obtained solutions is carried out and characteristic amplitude-frequency dependenc-es of nonlinear wave effects are revealed. A method for determining the type of dynamic hysteresis for crystal-line solids with frequency-dependent saturation of amplitude-dependent internal friction is proposed.

Wet mantle transition zone beneath the Indian shield: Constraints from P-to-S receiver functions analysis

Indian plate passed over four plumes on its way toward the Eurasian plate. These interactions with plumes certainly affected the upper mantle and crust of the Indian plate as manifested by its thinned lithosphere and relatively low shear velocities both in the crust and upper mantle, but the depth extent of these effects of plume-lithosphere interaction remains ambiguous. In this study, we investigate the mantle transition zone beneath the Indian shield using P receiver functions computed at 23 stations covering the entirety of the Indian shield to investigate the depth extent of the imprints of the plume-lithosphere interaction as well as to study the lateral variations of transition zone beneath a stable intraplate setting like the Indian shield. Our results show good agreement with the results of previous studies as well as with the tomographic models in terms of the average apparent depths of the 410 and 660 discontinuities and the transition zone thickness. However, unlike previous studies, we find compelling evidence of a persistent mid-transition zone discontinuity beneath all the stations and a low-velocity layer beneath some regions. We also investigated the frequency dependence of amplitudes of receiver functions and found that most of the stations showed a strong dependence of amplitudes on frequency. Based on the evidence from our investigation, we demarcate the regions potentially containing relatively more weight percentage of water inside the otherwise considered as a ‘dry’ mantle transition zone. These regions should be further investigated in detail by a dense seismic network and a realistic 3-D velocity model.

Ion/Water Network Structural Dynamics in Highly Concentrated Lithium Chloride and Lithium Bromide Solutions Probed with Ultrafast Infrared Spectroscopy

The structural dynamics of highly concentrated LiCl and LiBr aqueous solutions were observed from 1-4 to 1-16 water molecules per ion pair using ultrafast polarization-selective pump-probe (PSPP) experiments on the OD stretch of dilute HOD. At these high salt concentrations, an extended ion/water network exists with complex structural dynamics. Population decays from PSPP experiments highlight two distinct water components. From the frequency-dependent amplitudes of the decays, the spectra of hydroxyls bound to halides and to water oxygens are obtained, which are not observable in the FT-IR spectra. PSPP experiments also measure frequency-dependent water orientational relaxation. At short times, wobbling dynamics within a restricted angular cone occurs. At high concentrations, the cone angles are dependent on frequency (hydrogen bond strength), but at higher water concentrations (>10 waters per ion pair), there is no frequency dependence. The average cone angle increases as the ion concentration decreases. The slow time constant for complete HOD orientational relaxation is independent of concentration but slower in LiCl than in LiBr. Comparison to structural MD simulations of LiCl from the literature indicates that the loss of the cone angle wavelength dependence and the increase in the cone angles as the concentration decreases occur as the prevalence of large ion/water clusters gives way to contact ion pairs.