Shear Wave Source Research Articles

Seismic Node Arrays for Enhanced Understanding and Monitoring of Geothermal Systems

Abstract Harnessing geothermal energy will likely play a critical role in reducing global CO2 emissions. However, exploration, development, and monitoring of geothermal systems remain challenging. Here, we explore how recent low-cost seismic node instrumentation advances might enhance geothermal exploration and monitoring. We show the results from 450 nodes deployed at a geothermal prospect in Cornwall, United Kingdom. First, we demonstrate how the nodes can be used to monitor the spatiotemporal and size distribution of induced seismicity. Second, we use focal mechanisms, shear-wave source polarities, and anisotropy to indicate how the dense passive seismic observations might provide enhanced insight into the stress state of the geothermal systems. All the methods are fully automated, essential for processing the data from many receivers. In our example case study, we find that the injection-site fracture orientations significantly differ from that of the crust above and the regional stress state. These observations agree well with fracture orientations inferred from independent well-log data, exemplifying how the nodes can provide new insight for understanding the geothermal systems. Finally, we discuss the limitations of nodes and the role they might play in hybrid seismic monitoring going forward. Overall, our results emphasize the important role that low-cost, easy-to-deploy dense nodal arrays can play in geothermal exploration and operation.

Magneto-Acoustic Theranostic Approach: Integration of Magnetomotive Ultrasound Shear Wave Elastography and Magnetic Hyperthermia.

Although magnetically induced hyperthermia has shown great efficiency in the treatment of solid tumors, it is still a challenge to avoid incomplete ablation or overtreatment. In this study, we applied magnetomotive ultrasound shear wave elastography (MMUS-SWE) as a tool for real-time image guidance and feedback in the magnetic hyperthermia (MH) process. We called this new method as magneto-acoustic theranostic approach (MATA). In MATA, a ferromagnetic particle (fMP) was simultaneously used as a thermoseed for MH and a shear wave source for MMUS-SWE. The fMP was excited by a high-frequency magnetic field to induce the heating effect for MH. Meanwhile, the fMP was stimulated by a pulsed magnetic field to generate shear wave propagation for MMUS-SWE. Thus, the changes in elastic modulus surrounding fMP can be used to estimate the therapy effect of MH. The phantom and in vitro experiments were conducted to verify the feasibility of MATA, which has good performance in magnetothermal conversion and treatment efficacy feedback. The shear wave speed of the isolated pork liver changed significantly after the MH process, which varied from about 1.36 to 4.85 m/s. Preliminary results proved that changes in elastic modulus could be useful to estimate the therapy effect of MH. We expect that MATA, which is the integration of MMUS-SWE and MH, will be a novel theranostic method for clinical translation.

A Developed Shear Wave Source for Shallow Seismic Survey

Shear wave velocity is a fundamental parameter in determining geological structures and soil characteristics for geotechnical and earthquake engineering studies. In shallow seismic data acquisition, shear wave is generated by various seismic sources. Although the data quality is affected by the choices of the seismic source, applying highly efficient sources is sometimes limited for some users and by the budget available for a project. Therefore, this work aims to design and develop an effective shear wave seismic source as an alternative for shallow seismic surveys. The developed source consists of 4 main parts, including base plate, activated mass, lifting and shooting system, body and transportation system. It is simply operated by lifting an activated mass with a sling and puller to the armed position. Under this condition, an attached spring is under compression and the maximum potential energy is stored. When the mass is released and horizontally hit the base plate, shear wave is generated by the momentum and energy transferred from the base plate into the ground. To evaluate the source capability, a comparison of the conventional and the developed source was performed at a test site. The recorded data were compared both in qualitative and quantitative manners based on the signal-to-noise ratio, energy and frequency content, signal penetration, and repeatability. It was found that signal energy generated by one blow of the developed source is equivalent to approximately three-fold of the conventional source. There is a remarkably higher repeatability from the developed source than that from the conventional source. Moreover, it is easy to operate, portable, and minimizes required man power. Overall, the new shear source is suitable and has potential application for shallow seismic surveys. HIGHLIGHTS The new shear wave seismic source has been developed and tested as an alternative source for shallow seismic surveys The key advantages of the developed source are that it is simple to operate, reduces required manpower, fairly cost-effective, transportable, safe, and has a minimal environmental impact There is a noticeably higher input signal energy transfer offered by the source leading to the deeper target depth of investigation The better repeatable waveform led to greater accuracy of the 1st break picking in the data processing GRAPHICAL ABSTRACT

Three-Dimensional Shear Wave Elastography Using a 2D Row Column Addressing (RCA) Array.

Objective. To develop a 3D shear wave elastography (SWE) technique using a 2D row column addressing (RCA) array, with either external vibration or acoustic radiation force (ARF) as the shear wave source. Impact Statement. The proposed method paves the way for clinical translation of 3D SWE based on the 2D RCA, providing a low-cost and high volume rate solution that is compatible with existing clinical systems. Introduction. SWE is an established ultrasound imaging modality that provides a direct and quantitative assessment of tissue stiffness, which is significant for a wide range of clinical applications including cancer and liver fibrosis. SWE requires high frame rate imaging for robust shear wave tracking. Due to the technical challenges associated with high volume rate imaging in 3D, current SWE techniques are typically confined to 2D. Advancing SWE from 2D to 3D is significant because of the heterogeneous nature of tissue, which demands 3D imaging for accurate and comprehensive evaluation. Methods. A 3D SWE method using a RCA array was developed with a volume rate up to 2000 Hz. The performance of the proposed method was systematically evaluated on tissue-mimicking elasticity phantoms and in an in vivo case study. Results. 3D shear wave motion induced by either external vibration or ARF was successfully detected with the proposed method. Robust 3D shear wave speed maps were reconstructed for phantoms and in vivo. Conclusion. The high volume rate 3D imaging provided by the 2D RCA array provides a robust and practical solution for 3D SWE with a clear pathway for future clinical translation.

Micro-elastography on spheroids using a magnetic pulse as a shear wave source and the impact of cavitation on their mechanical properties

In the context of development of a new treatment against pancreatic adenocarcinoma combining chemotherapy and cavitation, it is interesting to be able to mechanically characterize spheroid tumor models. Spheroids, which are clusters of cancerous cells (KPC) and fibroblasts (iMEF), are made using magnetic nanoparticles which are linked to the cells. Then, by using magnetic cell culture plates, cells aggregate, until forming a spheroid. Hence magnetic nanoparticles are incorporated in the spheroid, and can be used to generate elastic waves inside. A 5-ms magnetic pulse is induced in a coil, allowing to induce vibrations inside the spheroid placed in its center. It is observed using an optical microscope and an ultrafast camera. Then, tracking algorithms allow to measure the displacements at each point of the image, and for each time step. Next, noise correlation algorithms are used to retrieve the local velocity of shear waves inside the spheroid. Therefore, we obtained shear wave velocity maps of spheroids, using a non-destructive method. The influence of trypsine (known for disrupting the bounds between the cells) on shear wave velocity maps has been studied. As expected, a decrease in the wave velocity has been observed, attesting that this method is sensitive to elasticity. Furthermore, the impact of cavitation inside the spheroid has been investigated. Cavitation designates the bubble generation using ultrasound. It has been shown that cavitation also leads to a substantial decrease in the velocity of shear waves.

Time-of-flight and noise-correlation-inspired algorithms for full-field shear-wave elastography using digital holography

.Significance: Quantitative stiffness information can be a powerful aid for tumor or fibrosis diagnosis. Currently, very promising elastography approaches developed for non-contact biomedical imaging are based on transient shear-waves imaging. Transient elastography offers quantitative stiffness information by tracking the propagation of a wave front. The most common method used to compute stiffness from the acquired propagation movie is based on shear-wave time-of-flight calculations.Aim: We introduce an approach to transient shear-wave elastography with spatially coherent sources, able to yield full-field quantitative stiffness maps with reduced artifacts compared to typical artifacts observed in time-of-flight.Approach: A noise-correlation algorithm developed for passive elastography is adapted to spatially coherent narrow or any band sources. This noise-correlation-inspired (NCi) method is employed in parallel with a classic time-of-flight approach. Testing is done on simulation images, experimental validation is conducted with a digital holography setup on controlled homogeneous samples, and full-field quantitative stiffness maps are presented for heterogeneous samples and ex-vivo biological tissues.Results: The NCi approach is first validated on simulations images. Stiffness images processed by the NCi approach on simulated inclusions display significantly less artifacts than with a time-of-flight reconstruction. The adaptability of the NCi algorithm to narrow or any band shear-wave sources was tested successfully. Experimental testing on homogeneous samples demonstrates similar values for both the time-of-flight and the NCi approach. Soft inclusions in agarose sample could be resolved using the NCi method and feasibility on ex-vivo biological tissues is presented.Conclusions: The presented NCi approach was successful in computing quantitative full-field stiffness maps with narrow and broadband source signals on simulation and experimental images from a digital holography setup. Results in heterogeneous media show that the NCi approach could provide stiffness maps with less artifacts than with time-of-flight, demonstrating that a NCi algorithm is a promising approach for shear-wave transient elastography with spatially coherent sources.

Lorentz force induced shear waves for magnetic resonance elastography applications

Quantitative mechanical properties of biological tissues can be mapped using the shear wave elastography technique. This technology has demonstrated a great potential in various organs but shows a limit due to wave attenuation in biological tissues. An option to overcome the inherent loss in shear wave magnitude along the propagation pathway may be to stimulate tissues closer to regions of interest using alternative motion generation techniques. The present study investigated the feasibility of generating shear waves by applying a Lorentz force directly to tissue mimicking samples for magnetic resonance elastography applications. This was done by combining an electrical current with the strong magnetic field of a clinical MRI scanner. The Local Frequency Estimation method was used to assess the real value of the shear modulus of tested phantoms from Lorentz force induced motion. Finite elements modeling of reported experiments showed a consistent behavior but featured wavelengths larger than measured ones. Results suggest the feasibility of a magnetic resonance elastography technique based on the Lorentz force to produce an shear wave source.

Coupling resonance effect of interface delamination of laminates excited by horizontal shear wave sources on the surface

Coupling resonance effect of interface delamination of laminates excited by horizontal shear wave sources on the surface is studied by analytical methods based on forced propagation solutions deduced by surface perturbation methods, integral transformation methods and global-matrix methods. The influence of excitation sources on the interface shear stress is investigated. It is found that coupling resonance frequencies of interface shear stress are intrinsic properties of structures and are independent of excitation sources, at which even a very small perturbation can also lead to the interface stratification. The results provide the theoretical basis for layered and/or anti-layered design of laminates.

Shear Wave Elastography Based on Noise Correlation and Time Reversal

Shear wave elastography (SWE) relies on the generation and tracking of coherent shear waves to image the tissue's shear elasticity. Recent technological developments have allowed SWE to be implemented in commercial ultrasound and magnetic resonance imaging systems, quickly becoming a new imaging modality in medicine and biology. However, coherent shear wave tracking sets a limitation to SWE because it either requires ultrafast frame rates (of up to 20 kHz), or alternatively, a phase-lock synchronization between shear wave-source and imaging device. Moreover, there are many applications where coherent shear wave tracking is not possible because scattered waves from tissue’s inhomogeneities, waves coming from muscular activity, heart beating or external vibrations interfere with the coherent shear wave. To overcome these limitations, several authors developed an alternative approach to extract the shear elasticity of tissues from a complex elastic wavefield. To control the wavefield, this approach relies on the analogy between time reversal and seismic noise cross-correlation. By cross-correlating the elastic field at different positions, which can be interpreted as a time reversal experiment performed in the computer, shear waves are virtually focused on any point of the imaging plane. Then, different independent methods can be used to image the shear elasticity, for example, tracking the coherent shear wave as it focuses, measuring the focus size or simply evaluating the amplitude at the focusing point. The main advantage of this approach is its compatibility with low imaging rates modalities, which has led to innovative developments and new challenges in the field of multi-modality elastography. The goal of this short review is to cover the major developments in wave-physics involving shear elasticity imaging using a complex elastic wavefield and its latest applications including slow imaging rate modalities and passive shear elasticity imaging based on physiological noise correlation.

Shear Wave Generation by Remotely Stimulating Aluminum Patches With a Transient Magnetic Field and Its Preliminary Application in Elastography.

This article presents shear wave generation by remotely stimulating aluminum patches through a transient magnetic field, and its preliminary application in the cross-correlation approach based ultrasound elastography. A transient magnetic field is employed to remotely vibrate the patch actuators through the Lorentz force. The origin, and the characteristics of the Lorentz force are confirmed using an interferometric laser probe. The shear wave displacement fields generated in the soft medium are studied through the ultrafast ultrasound imaging. The potential of the shear wave fields generated through the patch actuators for the cross-correlation approach based elastography is confirmed through experiments on an agar phantom sample. Under a transient magnetic field of changing rate of 10.44kT/s, the patch actuator generates a shear wave source of amplitude of 100 μm in a polyvinyl alcohol (PVA) phantom sample. The shear wave fields created by experiments agree qualitatively well with those by theory. From the shear wave velocity map computed from 100 frames of shear wave fields, the boundaries of cylindrical regions of different stiffness can be clearly recognized, which are completely concealed in the ultrasound image. Shear wave fields in the level of 100 μm can be remotely generated in soft medium through stimulating aluminum patches with a transient magnetic field, and qualitative shear wave velocity maps can be reconstructed from the shear wave fields generated. The proposed method allows potential application of the cross-correlation approach based elastography in intravascular-based or catheter-based cardiology.

IBIEM Analysis of Dynamic Response of a Shallowly Buried Lined Tunnel Based on Viscous‐Slip Interface Model

A viscous‐slip interface model is proposed to simulate the contact state between a tunnel lining structure and the surrounding rock. The boundary integral equation method is adopted to solve the scattering of the plane SV wave by a tunnel lining in an elastic half‐space. We place special emphasis on the dynamic stress concentration of the lining and the amplification effect on the surface displacement near the tunnel. Scattered waves in the lining and half‐space are constructed using the fictitious wave sources close to the lining surfaces based on Green’s functions of cylindrical expansion and the shear wave source. The magnitudes of the fictitious wave sources are determined by viscous‐slip boundary conditions, and then the total response is obtained by superposition of the free and scattered fields. The slip stiffness and viscosity coefficients at the lining‐surrounding rock interface have a significant influence on the dynamic stress distribution and the nearby surface displacement response in the tunnel lining. Their influence is controlled by the incident wave frequency and angle. The hoop stress increases gradually in the inner wall of the lining as sliding stiffness increases under a low‐frequency incident wave. In the high‐frequency resonance frequency band, where incident wave frequency is consistent with the natural frequency of the soil column above the tunnel, the dynamic stress concentration effect is more significant when it is smaller. The dynamic stress concentration factor inside the lining decreases gradually as the viscosity coefficient increases. The spatial distribution and the displacement amplitudes of surface displacement near the tunnel change as incident wave frequency and angle increase. The effective dynamic analysis of the underground structure under an actual strong dynamic load should consider the slip effect at the lining‐surrounding rock interface.

Characterization of soft marine sediments using vector sensors

Horizontally and vertically polarized interface waves with phase speeds of 45–375 m/s were observed in the North Sea using a horizontal linear array of three-component seismometers [H. Dong et al., J. Acoust. Soc. Am. 134, 176–184 (2013)]. The waves were generated by a low-frequency shear wave source on the seafloor, which operated in the 2–60 Hz band. Dispersion curves of the interface waves exhibit linear dependences between the logarithm of phase speed and logarithm of frequency, with distinct slopes at large and small phase speeds. This suggests a geoacoustic model consisting of two sediment layers with power-law dependences of the shear wave speed on depth below seafloor [O. A. Godin and D. M. F. Chapman, J. Acoust. Soc. Am. 110, 1890–1907 (2001)]. In this paper, dispersion curves of interface waves are inverted for the shear wave speed profile in the sediments. Observations of lateral wave arrivals are used to corroborate the geoacoustic model suggested by the surface wave dispersion and constrain c...

Impact of Acoustic Radiation Force Excitation Geometry on Shear Wave Dispersion and Attenuation Estimates

Shear wave elasticity imaging (SWEI) characterizes the mechanical properties of human tissues to differentiate healthy from diseased tissue. Commercial scanners tend to reconstruct shear wave speeds for a region of interest using time-of-flight methods reporting a single shear wave speed (or elastic modulus) to the end user under the assumptions that tissue is elastic and shear wave speeds are not dependent on the frequency content of the shear waves. Human tissues, however, are known to be viscoelastic, resulting in dispersion and attenuation. Shear wave spectroscopy and spectral methods have been previously reported in the literature to quantify shear wave dispersion and attenuation, commonly making an assumption that the acoustic radiation force excitation acts as a cylindrical source with a known geometric shear wave amplitude decay. This work quantifies the bias in shear dispersion and attenuation estimates associated with making this cylindrical wave assumption when applied to shear wave sources with finite depth extents, as commonly occurs with realistic focal geometries, in elastic and viscoelastic media. Bias is quantified using analytically derived shear wave data and shear wave data generated using finite-element method models. Shear wave dispersion and attenuation bias (up to 15% for dispersion and 41% for attenuation) is greater for more tightly focused acoustic radiation force sources with smaller depths of field relative to their lateral extent (height-to-width ratios <16). Dispersion and attenuation errors associated with assuming a cylindrical geometric shear wave decay in SWEI can be appreciable and should be considered when analyzing the viscoelastic properties of tissues with acoustic radiation force source distributions with limited depths of field.

Nonlinear wave propagation in porous materials based on the Biot theory.

Nonlinearity must be considered with some porous granular media because of the large deformation under seismic waves. In this study, the propagation of nonlinear waves in porous media is studied based on the Biot theory and the governing equations are obtained by the Lagrangian formulation. Three new nonlinear parameters are introduced to consider the coupled nonlinearity between the solid and fluid components in porous media. It is shown that an additional nonlinear wave with a double frequency is generated by the coupling effect of linear fast and slow waves. When only a shear wave is applied at the source, no double-frequency nonlinear wave is predicted and three nonlinear longitudinal waves are generated. On the basis of the practical case studies, the effect of strong nonlinearity is computed under the influence of a one-dimensional single longitudinal wave source and a single shear wave source.

Passive elastography: A shear wave tomography of the human body

Elastography, sometimes referred as seismology of the human body, is an imaging modality recently implemented on medical ultrasound systems. It allows to measure shear waves within soft tissues and gives a tomography reconstruction of the shear elasticity. This elasticity map is useful for early cancer detection. A general overview of this field is given in the first part of the presentation as well as latest developments. The second part, is devoted to the application of time reversal or noise correlation technique in the field of elastography. The idea, as in seismology, is to take advantage of shear waves naturally present in the human body due to muscles activities to construct shear elasticity map of soft tissues. It is thus a passive elastography approach since no shear wave sources are used.

The Diffraction of Rayleigh Waves by Twin Circular Cavities in a Poroelastic Half-Space

ABSTRACTThe diffraction of Rayleigh waves by twin circular cavities in a poroelastic half-space is investigated using the indirect boundary integral equation method (IBIEM). To satisfy the boundary conditions on the free surface, Green’s functions of compressional and shear wave sources in a poroelastic half-space are derived based on Biot’s theory. It is verified that the IBIEM has great accuracy and numerical stability. Then, the influences of the drainage boundary condition, incident wave frequency, porosity, and cavity spacing on the dynamic responses are investigated by numerical examples. The results show that the amplification effect of the twin cavities is greater in the case of undrained boundary conditions, lower frequencies, and smaller cavity spacings. The porosity of the saturated medium also influences the dynamic responses because the velocity of the Rayleigh wave will change with the porosity of the saturated medium.

Theoretical basis and application of vertical Z-component in-seam wave exploration

Love seam waves are generally applied to the current in-seam wave explorations. Rayleigh seam waves, however, are rarely used. The dispersion and amplitude features of Rayleigh seam waves are investigated using frequency dispersion analysis and numerical simulations in this study. Firstly, we elucidate the advantages of using the vertical Z-component. Secondly, the theoretical basis standing behind its application to in-seam wave explorations is demonstrated. Furthermore, in order to compare the relative performances of Rayleigh and Love seam wave explorations, three-dimensional geological models with anomalies are numerically simulated. Thus, the feasibilities and actual exploration effects of Z-component in-seam waves are demonstrated for both theoretical models and practical applications. Our studies show that normal modes of Rayleigh seam waves always exist. Furthermore, the energy of the fundamental mode in-seam wave is concentrated in its Z-component for the middle of a coal seam. A compressional wave source primarily generates first-order mode Rayleigh seam waves whereas a shear wave source primarily excites fundamental mode Rayleigh seam waves, whose energy is concentrated in the Airy phase with the lowest velocity. The fundamental mode Z-components at the center of a coal seam are ideal for in-seam wave applications. Actually, fundamental and first-order mode in-seam waves can be recorded in most coal seams. The Z-component receivers can be placed at the location between 1/4 and 1/2 of the thickness of coal seam, where both wave modes could be detected.

Passive elastography: A seismo-inspired tomography of the human body

Elastography, sometimes referred as seismology of the human body, is an imaging modality recently implemented on medical ultrasound systems. It allows to measure shear waves within soft tissues and gives a tomography reconstruction of the shear elasticity. This elasticity map is useful for early cancer detection. A general overview of this field is given in the first part of the presentation as well as latest developments. The second part, is devoted to the application of noise correlation technique in the field of elastography. The idea, as in seismology, is to take advantage of shear waves naturally present in the human body due to muscles activities, heart beating, and arteries pulsatility to construct shear elasticity map of soft tissues such as the liver, the thyroid, or the brain. It is thus a passive elastography approach since no shear wave sources are used.

Elasticity imaging of inhomogeneous media using inverse filtering with multiple shear wave generation

Most diseases like cancer cause to change of tissue mechanical properties described by elasticity. Elastography visualizes these changes and aids in early and differential diagnosis of diseases. Shear wave imaging is one of principles of elastography, and using the speed of shear wave generated by mechanical vibration or the acoustic radiation force impulse. Conventional shear wave imaging uses time-of-flight method; however, it causes artifact by reflection and refraction of shear wave. We proposed a method for shear wave elasticity imaging that combines inverse filtering with multiple shear wave sources induced by acoustic radiation force. Shear waves are induced by pushing pulse and recorded by ultrafast imaging, and repeated at multiple pushing points that are sparsely located. An inverse filter can be applied to virtually focus shear waves on an arbitrary point. Shear wave speed is obtained by a measured shear wavelength and its image is constructed by scanning the focal point. An inverse filter can ...

On the generalization to swirling flows of Lighthill’s eighth-power law. Part II: Laws of intensity of radiation for acoustic-vortical waves

The Lighthill–Proudman theory of sound generation by turbulence, inhomogeneities, and dissipation in a medium at rest is extended to an axisymmetric mean flow with (i) arbitrary unidirectional shear velocity profile and (ii) arbitrary angular velocity of rotation, both depending only on the radius and also (iii) isentropic conditions. The forced acoustic-vortical wave equation through: (a) the wave operator describes the propagation of coupled acoustic-vortical waves (Part I); (b) the forcing terms specifies the compressive, shear, and swirl wave sources that model the generation of acoustic-vortical waves by turbulence, inhomogeneities, and entropy production (Part II). The energy balance is considered for acoustic-vortical waves including the energy density, flux, and production. The dimensional scaling for the wave sources and the energy flux leads to a law of intensity of radiation that generalizes the Lighthill eighth-power law of aeroacoustics by allowing for: (i) compressive, shear, and swirl wave sources; (ii) acoustic and vortical waves and their couplings; (iii) monopoles, dipoles, quadrupoles, and multipoles of any order.