An official website of the United States government
A range of ultrasonic techniques associated with the nondestructive evaluation of metals involves the propagation of low-frequency elastic waves. Metals that are isotropic and homogeneous in the macroscopic length scale contain elastic heterogeneities, such as grain boundaries within the microstructures. Ultrasonic waves propagating through such microstructures get scattered from the grain boundaries. As a result, the propagating ultrasound attenuates. The mass density and the elastic anisotropy in each constituent grain govern the degree of heterogeneity in the polycrystalline aggregates. Existing elastodynamic models consider first-order scattering effects from grain boundaries. This paper presents the improved attenuation formulae, for the first time, by including the next order of grain scattering effects. Results from investigating 759 polycrystals reveal a positive correlation between the effects of higher-order scattering from grain boundaries and the degree of heterogeneity. Thus, higher-order grain scattering effects are now known. These results motivate further investigation into higher frequencies and strongly scattering alloys in the future.
more »
« less
Making sense of scattering: Seeing microstructure through shear waves
The physics of shear waves traveling through matter carries fundamental insights into its structure, for instance, quantifying stiffness for disease characterization. However, the origin of shear wave attenuation in tissue is currently not properly understood. Attenuation is caused by two phenomena: absorption due to energy dissipation and scattering on structures such as vessels fundamentally tied to the material’s microstructure. Here, we present a scattering theory in conjunction with magnetic resonance imaging, which enables the unraveling of a material’s innate constitutive and scattering characteristics. By overcoming a three-order-of-magnitude scale difference between wavelength and average intervessel distance, we provide noninvasively a macroscopic measure of vascular architecture. The validity of the theory is demonstrated through simulations, phantoms, in vivo mice, and human experiments and compared against histology as gold standard. Our approach expands the field of imaging by using the dispersion properties of shear waves as macroscopic observable proxies for deciphering the underlying ultrastructures.
more »
« less
- Award ID(s):
- 2308389
- PAR ID:
- 10610887
- Publisher / Repository:
- Science
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 10
- Issue:
- 31
- ISSN:
- 2375-2548
- Format(s):
- Medium: X Other: pdf
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We present a general theory for optical imaging of moving objects obscured by heavily scattering random media. Measurements involve collecting a series of speckle intensity images as a function of the position of a moving object. A statistical average intensity correlation can be formed with the potential to provide access to microscopic and macroscopic information about the object. For macroscopic objects and translation distances that are both large relative to the wavelength, there is a clear method to invert measurements to form an image of the hidden object. Opportunities exist for super-resolution sensing and imaging, with far-subwavelength resolution. Importantly, there is no fundamental limit to the thickness of the background randomly scattering medium, other than the practical requirement of detecting an adequate number of photons and sufficient background scatter for developed Gaussian field statistics. The approach can be generalized to any wave type and frequency, under the assumption that there is adequate temporal coherence. Applications include deep tissue in vivo imaging and sensing in and through various forms of environmental clutter. The theory also provides another dimension for intensity interferometry and entangled state detection to the case with motion of the scatterer or emitter.more » « less
-
The impact of vertical wind shear on the land–sea-breeze circulation at the equator is explored using idealized 2D numerical simulations and a simple 2D linear analytical model. Both the idealized and linear analytical models indicate Doppler shifting and attenuation effects coexist under the effect of vertical wind shear for the propagation of gravity waves that characterize the land–sea-breeze circulation. Without a background wind, the idealized sea breeze has two ray paths of gravity waves that extend outward and upward from the coast. A uniform background wind causes a tilting of the two ray paths due to Doppler shifting. With vertical shear in the background wind, the downstream ray path of wave propagation can be rapidly attenuated near a certain level, whereas the upstream ray path is not attenuated and the amplitudes even increase with height. The downstream attenuation level is found to descend with increasing linear wind shear. The present analytical model establishes that the attenuation level corresponds to the critical level where the background wind is equal to the horizontal gravity wave phase speed. The upstream gravity wave ray path can propagate upward without attenuation as there is no critical level there.more » « less
-
Theoretical explanation of the super-resolution imaging by contact microspheres created a point of attraction for nanoimaging research during the last decade with many models proposed, yet its origin remains largely elusive. Using a classical double slit object, the key factors responsible for this effect are identified by an ab initio imaging model comprising object illumination, wave scattering, and image reconstruction from the diffracted far fields. The scattering is found by a full-wave solution of the Maxwell equations. The formation of super-resolved images relies on coherent effects, including the light scattering into the waves circulating inside the microsphere and their re-illumination of the object. Achieving the super-resolution of the double slit requires a wide illumination cone as well as a deeply sub-wavelength object-to-microsphere separation. The resultant image has a significantly better resolution as compared to that from the incoherent imaging theory.more » « less
-
Abstract Seismicity in the Los Angeles metropolitan area has been primarily attributed to the regional stress loading. Below the urban areas, earthquake sequences have occurred over time showing migration off the faults and providing evidence that secondary processes may be involved in their evolution. Combining high-frequency seismic attenuation with other geophysical observations is a powerful tool for understanding which Earth properties distinguish regions with ongoing seismicity. We develop the first high-resolution 3D seismic attenuation models across the region east of downtown Los Angeles using 5,600 three-component seismograms from local earthquakes recorded by a dense seismic array. We present frequency-dependent peak delay and coda-attenuation tomography as proxies for seismic scattering and absorption, respectively. The scattering models show high sensitivity to the seismicity along some of the major faults, such as the Cucamonga fault and the San Jacinto fault zone, while a channel of low scattering in the basement extends from near the San Andreas fault westward. In the vicinity of the Fontana seismic sequence, high absorption, low scattering, and seismicity migration across a fault network suggest fluid-driven processes. Our attenuation and fault network imaging characterize near-fault zones and rock-fluid properties beneath the study area for future improvements in seismic hazard evaluation.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.adp3363
