Wave Reverberation Research Articles

Shock response of concrete, fibre concrete and ultrahigh performance concrete

A range of concrete types, representing commercially important formulations, were characterised under shock loading and release, using a large-bore single-stage light-gas gun. The plate impact experiments used a wave reverberation technique and an array of optical velocimetry probes to measure Hugoniot data and the subsequent release states for shocks from 2 to 11 GPa. The use of multiple velocimetry probes enabled quantification of the variation in particle velocity arising from the heterogeneous nature of concrete. Four types of concrete were investigated; a regular concrete, a fibre reinforced concrete using poly-propylene fibres, fibre concrete with steel fibres and, lastly, ultra-high performance fibre-reinforced concrete. Despite the compositional complexity, and range of quasi-static compressive strengths and reinforcement methods, the average Hugoniot data were remarkably similar between compositions.

Effects of the accretionary wedge and sedimentary layers on subduction zone earthquake ruptures and ground motion: 2-D numerical simulations

SUMMARYLow-velocity accretionary wedges and sedimentary layers overlying continental plates are common in subduction zones. These low-velocity sedimentary structures should be considered to accurately model ground motions and estimate coseismic slip on subsurface faults. First, we simulated the rupture process of the 2011 Mw 9.0 Tohoku-Oki earthquake in 2-D dynamic rupture models and found that the co-existence of the accretionary wedge and sedimentary layer can cause an approximately 60 per cent increase in the shallow coseismic slip. Considering the inelastic attenuation, wave reverberations in the accretionary wedge and sedimentary layer significantly amplified offshore ground acceleration at 0.1–0.5 Hz by factors of 1–5 and prolonged ground motion durations. Additionally, high-frequency acceleration at 0.5–2.0 Hz is also amplified on the accretionary wedge with a maximum factor of 5. Our Tohoku-Oki earthquake model reproduces the observed coseismic slip qualitatively, but the model also results in higher scaled energy, due to the overestimation of radiated energy in 2-D models and the inadequate plastic properties in the accretionary wedge and sedimentary layer. We further simulated a suite of earthquake scenarios where the updip rupture terminates at different depths. Our results show that a sedimentary layer enhances coseismic slip in all cases, while an accretionary wedge can decrease the slip when the shallow fault has a strengthening frictional behaviour. Additionally, the effects on slip diminished when the extent of updip rupture becomes deeper. However, offshore ground acceleration at 0.1–0.5 Hz is still amplified due to the dynamic wave effects in the two structures. Furthermore, in the scenarios when earthquake rupture reaches the shallow fault, the existence of an accretionary wedge can cause amplified high-frequency acceleration (0.5–2.0 Hz) near the trench. Our results indicate that compared to those subduction zones with neither an accretionary wedge nor sedimentary layers, subduction zones featuring a co-existence between an accretionary wedge and sedimentary layers are susceptible to host earthquakes with larger shallow slip and amplified offshore ground motions.

Influence of shear wave velocity heterogeneity on SH-wave reverberation imaging of the mantle transition zone

SUMMARY Long-period (T > 10 s) shear wave reflections between the surface and reflecting boundaries below seismic stations are useful for studying phase transitions in the mantle transition zone (MTZ) but shear-velocity heterogeneity and finite-frequency effects complicate the interpretation of waveform stacks. We follow up on a recent study by Shearer & Buehler (hereafter SB19) of the top-side shear wave reflection Ssds as a probe for mapping the depths of the 410-km and 660-km discontinuities beneath the USArray. Like SB19, we observe that the recorded Ss410s-S and Ss660s-S traveltime differences are longer at stations in the western United States than in the central-eastern United States. The 410-km and 660-km discontinuities are about 40–50 km deeper beneath the western United States than the central-eastern United States if Ss410s-S and Ss660s-S traveltime differences are transformed to depth using a common-reflection point (CRP) mapping approach based on a 1-D seismic model (PREM in our case). However, the east-to-west deepening of the MTZ disappears in the CRP image if we account for 3-D shear wave velocity variations in the mantle according to global tomography. In addition, from spectral-element method synthetics, we find that ray theory overpredicts the traveltime delays of the reverberations. Undulations of the 410-km and 660-km discontinuities are underestimated when their wavelengths are smaller than the Fresnel zones of the wave reverberations in the MTZ. Therefore, modelling of layering in the upper mantle must be based on 3-D reference structures and accurate calculations of reverberation traveltimes.

Big things come in small packages—Clinical use of endobronchial ultrasound

Endobronchial ultrasound (EBUS) revolutionized bronchoscopy. It first became available for clinical use in 1999 as a mechanical radial probe and was initially used to define airway wall anatomy, but was supplanted by convex probe EBUS bronchoscopes in 2002–now the preferred initial approach for evaluation of mediastinal pathology. Radial EBUS has since proven invaluable for evaluating the peripheral lung and aiding biopsy guidance despite the limitations in ultrasound (US) propagation through air filled structures. Centrally, airways pass through the mediastinum in direct contact with the esophagus, heart, great vessels, and lymph nodes, making the mediastinum amenable to direct US imaging. However, beyond the lung hila, the airways are largely surrounded by air, generally preventing production of usable US images by halting penetration and reverberation of sound waves. The tissue mass that makes up the terminal airway walls, however, does propagate ultrasound waves before encountering reflective air. The ratio of tissue to air varies according to anatomic location and physiologic/disease state; identifying patterns in the varying ratio of tissue to air within the lungs allows the ability to distinguish normal from abnormal lung. This presentation will describe the clinical use of EBUS in modern bronchoscopy.

The formation of impact coesite

Coesite in impact rocks is traditionally considered a retrograde product formed during pressure release by the crystallisation of an amorphous phase (either silica melt or diaplectic glass). Recently, the detailed microscopic and crystallographic study of impact ejecta from Kamil crater and the Australasian tektite strewn field pointed in turn to a different coesite formation pathway, through subsolidus quartz-to-coesite transformation. We report here further evidence documenting the formation of coesite directly from quartz. In Kamil ejecta we found sub-micrometric single-coesite-crystals that represent the first crystallization seeds of coesite. Coesite in Australasian samples show instead well-developed subeuhedral crystals, growing at the expenses of hosting quartz and postdating PDF deformation. Coesite (010) plane is most often parallel to quartz {10–11} plane family, supporting the formation of coesite through a topotactic transformation. Such reaction is facilitated by the presence of pre-existing and shock-induced discontinuities in the target. Shock wave reverberations can provide pressure and time conditions for coesite nucleation and growth. Because discontinuities occur in both porous and non-porous rocks and the coesite formation mechanism appears similar for small and large impacts, we infer that the proposed subsolidus transformation model is valid for all types of quartz-bearing target rocks.

Preliminary Determination of the Optimal Parameters When Using an Ultrasonic Probe to Measure Cavern Geometry Where a Metal Borehole Pipe Is Present

In order to determine the optimal parameters when using an ultrasonic probe to measure cavern geometry when a metal borehole pipe is present, an investigation was firstly carried out on influence of a vertical metal plates with a thickness from 1 mm to 15 mm immersed in water on transmitted and reflected ultrasonic waves. The results obtained will be used as an indicator for the measurement of underground geometry in which the ultrasonic probe is placed inside a metal pipe lining a borehole. These studies were performed both by experiment and computer simulation. The results show that the wavelength of the incident ultrasonic signals should be equal to half the thickness of the metal plate or an integer times smaller than this thickness. When the thickness of the barrier is unknown, an ultrasonic signal with linear frequency modulation (LFM) should be used. Due to the reverberation of the ultrasonic waves inside the pipe for caverns filled with water, the distance from the transducer to the cavern wall can be measured if it is longer than three times of the pipe diameter. Frequency analysis of both the reflected and the transmitted waves enables an optimal frequency of the incident ultrasonic wave to be selected, which can be used in the measurement of cavern geometry in conditions in which the ultrasonic probe is inside a metal pipe.

Energy absorption mechanisms of nanoscopic multilayer structures under ballistic impact loading

We conducted large-scale molecular dynamics (MD) simulations of ballistic impact tests on ultrathin aluminum and polyurea layers and their multilayer arrangements to investigate the energy absorption and deformation mechanisms at the nanoscale. In contrast to the results obtained from experiments conducted at the macroscale, our nanoscale simulations demonstrate that a polymer layer on the strike face is more effective in mitigating impact-induced damage. A polymer layer on the back face is weakened by the shock wave reverberation. The temporal variations of shock pressure and temperature, due to adiabatic heating of the targets, were analyzed to understand the energy dissipation mechanisms under supersonic projectile impacts. The shock pressure and adiabatic heating of polymer films significantly increase as the film thickness decreases. During the perforation, a nanoscopic projectile experiences two substantially different levels of resistance due to the interaction between the incident projectile and the tensile release wave. Moreover, the vertical surface area of a nanoscale projectile contributes significantly to the absorption of impact energy during the perforation.

Acoustic impact of the human skull on transcranial photoacoustic imaging.

With balanced spatial resolution, imaging depth, and functional sensitivity, photoacoustic tomography (PAT) hold great promise for human brain imaging. However, the strong acoustic attenuation and aberration of the human skull (∼8 mm thick) are longstanding technical challenges for PAT of the human brain. In this work, we numerically investigated the impacts of the stratified human skull on photoacoustic wave propagation (i.e., the forward model) and PAT image formation (i.e., the inverse model). We simulated two representative transcranial PAT implementations: photoacoustic computed tomography (PACT) and photoacoustic macroscopy (PAMac). In the forward model, we simulated the detailed photoacoustic wave propagation from a point or line source through a digital human skull. The wave attenuation, refraction, mode conversation, and reverberation were thoroughly investigated. In the inverse model, we reconstructed the transcranial PACT and PAMac images of a point or line target enclosed by the human skull. Our results demonstrate that transcranial PAMac suffers mainly from wave reverberation within the skull, leading to prolonged signal duration and reduced axial resolution. Transcranial PACT is more susceptible to the skull's acoustic distortion, mode conversion, and reverberation, which collectively lead to strong image artifacts and deteriorated spatial resolutions. We also found that PACT with a ring-shaped transducer array shows more tolerance of the skull's adverse impacts and can provide more accurate image reconstruction. Our results suggest that incorporating the skull's geometry and acoustic properties can improve transcranial PAT image reconstruction. We expect that our results have provided a more comprehensive understanding of the acoustic impact of the human skull on transcranial PAT.

Stress Transfer Mechanism of Flange in Split Hopkinson Tension Bar

To reveal the stress transfer mechanism of the flange in a split Hopkinson tension bar, explicit finite element analyses of the impact of the hollow striker on the flange were performed across a range of flange lengths. The tensile stress profiles monitored at the strain gauge position of the incident bar are interpreted on a qualitative basis using three types of stress waves: bar (B) waves, flange (F) waves, and a series of reverberation (Rn) waves. When the flange length (Lf) is long (i.e., Lf > Ls, where Ls is the striker length), the B wave and first reverberation wave (R1) are fully separated in the time axis. When the flange length is intermediate (~Db < Lf < Ls, where Db is the bar diameter), the B and F waves are partially superposed; the F wave is delayed, then followed by a series of Rn waves after the superposition period. When the flange length is short (Lf < ~Db), the B and F waves are practically fully superposed and form a pseudo-one-step pulse, indicating the necessity of a short flange length to achieve a neat tensile pulse. The magnitudes and periods of the monitored pulses are consistent with the analysis results using the one-dimensional impact theory, including a recently formulated equation for impact-induced stress when the areas of the striker and bar are different, equations for the reflection/transmission ratios of a stress wave, and an equation for pulse duration time. This observation verifies the flange length-dependent stress transfer mechanism on a quantitative basis.

Teleseismic earthquake wavefields observed on the Ross Ice Shelf

AbstractObservations of teleseismic earthquakes using broadband seismometers on the Ross Ice Shelf (RIS) must contend with environmental and structural processes that do not exist for land-sited seismometers. Important considerations are: (1) a broadband, multi-mode ambient wavefield excited by ocean gravity wave interactions with the ice shelf; (2) body wave reverberations produced by seismic impedance contrasts at the ice/water and water/seafloor interfaces and (3) decoupling of the solid Earth horizontal wavefield by the sub-shelf water column. We analyze seasonal and geographic variations in signal-to-noise ratios for teleseismic P-wave (0.5–2.0 s), S-wave (10–15 s) and surface wave (13–25 s) arrivals relative to the RIS noise field. We use ice and water layer reverberations generated by teleseismic P-waves to accurately estimate the sub-station thicknesses of these layers. We present observations consistent with the theoretically predicted transition of the water column from compressible to incompressible mechanics, relevant for vertically incident solid Earth waves with periods longer than 3 s. Finally, we observe symmetric-mode Lamb waves generated by teleseismic S-waves incident on the grounding zones. Despite their complexity, we conclude that teleseismic coda can be utilized for passive imaging of sub-shelf Earth structure, although longer deployments relative to conventional land-sited seismometers will be necessary to acquire adequate data.

A Review of Acoustic Impedance Matching Techniques for Piezoelectric Sensors and Transducers.

The coupling of waves between the piezoelectric generators, detectors, and propagating media is challenging due to mismatch in the acoustic properties. The mismatch leads to the reverberation of waves within the transducer, heating, low signal-to-noise ratio, and signal distortion. Acoustic impedance matching increases the coupling largely. This article presents standard methods to match the acoustic impedance of the piezoelectric sensors, actuators, and transducers with the surrounding wave propagation media. Acoustic matching methods utilizing active and passive materials have been discussed. Special materials such as nanocomposites, metamaterials, and metasurfaces as emerging materials have been presented. Emphasis is placed throughout the article to differentiate the difference between electric and acoustic impedance matching and the relation between the two. Comparison of various techniques is made with the discussion on capabilities, advantages, and disadvantages. Acoustic impedance matching for specific and uncommon applications has also been covered.

Linear Frequency Modulated Reverberation Suppression Using Non-Negative Matrix Factorization Methods, Dechirping Transformation and Modulo Operation

In this study, we propose a reverberation suppression algorithm for linear frequency-modulated (LFM) pulse sonar systems using a non-negative matrix factorization (NMF) method. Because conventional NMF-based reverberation suppression algorithms are only applicable to continuous wave reverberation, we propose two pre-processing methods, namely dechirping transformation and modulo operation, to facilitate application of the NMF method to LFM reverberation. Moreover, we impose additional sparse constraints on the NMF method to improve its performance. To evaluate the proposed algorithm, an experiment involving simulated LFM reverberation is performed. The results thereof show improved detection performance at several signal-to-reverberation ratios and false alarm conditions. Moreover, the proposed algorithm is also applied to sea experiment data. According to the sea experiment analysis, the algorithm is able to suppress the LFM reverberation effectively and improve detection performance in practical LFM pulse sonar systems.

Crust stratigraphy and heterogeneities of the first kilometers at the dichotomy boundary in western Elysium Planitia and implications for InSight lander

InSight landed on Mars on November 26, 2018, in western Elysium Planitia. The Mars crust beneath the lander is subject to complex geologic history next to the great topographic and crustal dichotomy of Mars. Understanding this part of the Martian crust in the subsurface would aid future investigations of the internal structure of the planet based on seismic datasets collected by the Seismic Experiment for Interior Structure (SEIS) instrument. Here, we investigate the subsurface structure and composition from the analysis of mineralogy and morphology of exposures in impact craters as well as on scarps and knobs in the general region of Elysium Planitia. Using a combination of orbital datasets, we identify exposures of subsurface materials with distinct composition and physical properties. We find olivine and pyroxene detections associated with small impact craters (1.5–7 km) rim in the vicinity of InSight lander, as well as in the transition unit and Elysium volcanic unit. Fe/Mg phyllosilicates have been identified in the central peak of the 51-km diameter Kalpin crater and on knobs in the transition unit between the dichotomy and the plains. In addition, eroded meter-scale layered unit subject to erosion has been identified in six impact craters to the north-east of the landing site, including Kalpin crater. Massive bedrock and layered, weak materials co-occur in the transition unit, indicating a complex origin. These results together suggest both materials with altered composition and layered deposit occur as distinct geologic units beneath the basaltic lava flow units. Through analogy with terrestrial sedimentary rocks or clay-bearing sediments, we suggest physically weak materials exist beneath the lava flow units in the general region of Elysium Planitia. The spatial distribution and continuity of these materials are unclear due to the lack of exposures within the lava flow unit where the InSight lander is located. These subsurface materials of distinct physical properties may result in increased attenuation or reverberations of seismic waves, to be collected by SEIS. The findings suggest a close investigation of the potential effects of subsurface stratigraphy on seismic data would help inform future data interpretation and understanding of the internal structure of Mars.

Spurious low velocity zones in joint inversions of surface waves and receiver functions

SUMMARYLow-velocity layers within the crust can indicate the presence of melt and lithologic differences with implications for crustal composition and formation. Seismic wave conversions and reverberations across the base of the crust or intracrustal discontinuities, analysed using the receiver function method, can be used to constrain crustal layering. This is commonly accomplished by inverting receiver functions jointly with surface wave dispersion. Recently, the proliferation of model-space search approaches has made this technique a workhorse of crustal seismology. We show that reverberations from shallow layers such as sedimentary basins produce spurious low-velocity zones when inverted for crustal structure with surface wave data of insufficiently high frequency. Therefore, reports of such layers in the literature based on inversions using receiver function data should be re-evaluated. We demonstrate that a simple resonance-removal filter can suppress these effects and yield reliable estimates of crustal structure, and advocate for its use in receiver-function based inversions.

Detection of resonance frequency of both the internal defects of concrete and the laser head of a laser Doppler vibrometer by spatial spectral entropy for noncontact acoustic inspection

In order to detect internal defects of concrete, we have studied noncontact non-destructive acoustic inspection and have detected and visualized internal defects using acoustic vibration excitation on the target surface by aerial sound waves and two-dimensional vibration velocity distribution by a laser Doppler vibrometer. Because a high sensitive scanning laser Doppler vibrometer (SLDV) is used, as the sound pressure increases, the reverberation of sound waves from the surroundings may cause resonance in the galvano mirror system of the laser head of the SLDV. For real concrete structures, for example a tunnel inner wall (distance about 5–10 m), there was no problem because the sound pressure was not so large. In the case of a viaduct, it was measured remotely (about 30 m), but because it is surrounded released space, the influence was not so large. However, in a closed space surrounded by concrete, the sound pressure needs to be increased as the distance increases, and the influence of the reverberation from the surroundings on a laser head cannot be ignored. Therefore, we propose a method to solve them, and at the same time, detect the resonance frequency of internal defects and improve the visualization of the defect.

Field validation of water‐pipe leakage detection through spatial and time‐lapse analysis of GPR wave velocity

ABSTRACTDetection of leakage in a buried water pipe is a crucial issue, as underground pipes become aged and these pipes are often used as pathways for aggressive chemical agents. Non‐destructive geophysical methods for identifying such underground leakages are of vital importance. Leakage detection in buried water pipes is sometimes carried out through specific acoustic methods. In this research, we have used ground‐penetrating radar as an alternative to such acoustic methods, because of the high sensitivity of electromagnetic waves to the presence of water in soil. This paper presents a methodology, illustrated by field‐scale ground‐penetrating radar experiments. Four water leakage points in a buried, ductile, iron, main‐water pipe were pre‐designed within a full‐scale (20 m long × 10 m wide) experimental set up. This facility was paved half by reinforced concrete and half by other pavement blocks. Four ground‐penetrating radar antennae with five nominal frequencies 200, 250, 400, 600 and 900 MHz were tested. Using a modified algorithm for common offset ground‐penetrating radar antennae, in this work, we measure the changes in electromagnetic wave velocity and wave reverberation, which sense upward and downward leakages, respectively. Leakages could be identified most clearly in the 600 MHz ground‐penetrating radar data. Our result suggests two potential applications in terms of detecting and defining the extent of multiple leakages in old water pipes, and testing and commissioning new pipes before and after pressurized tests.

Impacts of the murine skull on high-frequency transcranial photoacoustic brain imaging.

Non-invasive photoacoustic tomography (PAT) of mouse brains with intact skulls has been a challenge due to the skull's strong acoustic attenuation, aberration, and reverberation, especially in the high-frequency range (>15 MHz). In this paper, we systematically investigated the impacts of the murine skull on the photoacoustic wave propagation and on the PAT image reconstruction. We studied the photoacoustic acoustic wave aberration due to the acoustic impedance mismatch at the skull boundaries and the mode conversion between the longitudinal wave and shear wave. The wave's reverberation within the skull was investigated for both longitudinal and shear modes. In the inverse process, we reconstructed the transcranial photoacoustic computed tomography (PACT) and photoacoustic microscopy (PAM) images of a point target enclosed by the mouse skull, showing the skull's different impacts on both modalities. Finally, we experimentally validated the simulations by imaging an in vitro mouse skull phantom using representative transcranial PAM and PACT systems. The experimental results agreed well with the simulations and confirmed the accuracy of our forward and inverse models. We expect that our results will provide better understanding of the impacts of the murine skull on transcranial photoacoustic brain imaging and pave the ways for future technical improvements.

Remote focusing of elastic waves in complex structures using time reversal of acoustic waves

Remotely assessing the state of damage of thin plates or beams within a complex structure is a burning issue in many industrial problems. A time-reversal technique is used to focus a short and localized elastic impulsion on a thin plate using a network of loudspeakers. The generated plate vibration is measured using a laser vibrometer. We built an experimental setup to mimic a complex structure, with several thin plates attached close to each others inside a cylinder. We show that the acoustic method allows us to put a targeted plate in vibration inside the cylinder, without exciting the other plates in the same structure. The time-reversal technique takes advantage of the strong wave reverberation caused by the presence of the complex structure around the plate, which creates virtual sound sources, and improves the focusing on the plate compared to that obtained when the plate has nothing around it. The ratio of plate vibration energy over emitted acoustic energy is about 1%. Finally, we create a small defect on a plate and assess whether the defaults can be detected from changes in the plate vibration modes. Our experimental results are compared with finite elements simulations.

High speed photography technique for measuring impact strength of porous concrete

In this study, an impact strength measurement method based on high speed photography, that has been developed for testing porous concrete, was introduced. In the experiments, a drop weight impact test set-up instrumented with a high speed camera was used. The impact strength analyses were conducted using impedance mismatch method, where the wave reverberations were investigated in detail. The measurement configuration carries the advantage of being a fast, non-contact and accurate experimental method where only the known material properties and particle velocity data of the drop weight are required. The target cementitious material itself is not actually involved in the measurements or the subsequent analyses. The method also facilitates the observation of the crack patterns throughout the experiment. The measurement method was verified to be accurate and consistent by testing different types of porous concretes and by comparing the results with those from other dynamic monitoring techniques, such as laser Doppler velocimetry and direct stress gauge measurements.

Modeling acoustic wave propagation and reverberation in an ice covered environment using finite element analysis

A three-dimensional, longitudinally invariant finite element model of acoustic propagation and reverberation in an ice-covered shallow water waveguide has been developed. The ice is modeled as both an elastic medium and a pressure release surface. Transmission loss levels are calculated and compared for both assumptions of ice. Using Fourier synthesis, the time-harmonic acoustic pressure results are transformed into the time domain, and reverberation levels are then compared for both models. Finally, using a fully three-dimensional version of the finite element model, compressional-to-shear wave conversion at the elastic ice and water interface is characterized to inform propagation mechanisms of acoustic waves in Arctic ice sheets. [Work supported by ONR, Ocean Acoustics and the Robert W. Young Award for Undergraduate Student Research in Acoustics.]