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Mode Conversion (mode + conversion)

Distribution by Scientific Domains

Engineering	50%

Selected Abstracts

High-frequency mode conversion technique for stiff lesion detection with magnetic resonance elastography (MRE)

MAGNETIC RESONANCE IN MEDICINE, Issue 6 2009
Yogesh K. Mariappan
Abstract A novel imaging technique is described in which the mode conversion of longitudinal waves is used for the qualitative detection of stiff lesions within soft tissue using magnetic resonance elastography (MRE) methods. Due to the viscoelastic nature of tissue, high-frequency shear waves attenuate rapidly in soft tissues but much less in stiff tissues. By introducing minimally-attenuating longitudinal waves at a significantly high frequency into tissue, shear waves produced at interfaces by mode conversion will be detectable in stiff regions, but will be significantly attenuated and thus not detectable in the surrounding soft tissue. This contrast can be used to detect the presence of stiff tissue. The proposed technique is shown to readily depict hard regions (mimicking tumors) present in tissue-simulating phantoms and ex vivo breast tissue. In vivo feasibility is demonstrated on a patient with liver metastases in whom the tumors are readily distinguished. Preliminary evidence also suggests that quantitative stiffness measurements of stiff regions obtained with this technique are more accurate than those from conventional MRE because of the short shear wavelengths. This rapid, qualitative technique may lend itself to applications in which the localization of stiff, suspicious neoplasms is coupled with more sensitive techniques for thorough characterization. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source]

90-Degree H -plane bent waveguide using dielectric rods

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 9 2009
Yoshihiro Kokubo
Abstract Conventional rectangular metallic waveguides are seldom used at frequencies higher than twice cutoff because of higher mode propagation. However, single-mode propagation can occur for a metallic waveguide composed of an array of dielectric rods. An efficient way to construct one is to introduce dielectric rods at the bent portions because mode conversion does not occur in straight segments. As an example of the ease of fabrication, holes can be drilled at the top of a waveguide and dielectric rods inserted. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2015,2017, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24573 [source]

Lamb Wave Interactions with Non-symmetric Features at Structural Boundaries

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2008
M. R. Mofakhami
The paper initially describes on a numerical basis how a Lamb wave would have to perform that has been initiated in a pure mode (either symmetric or anti,symmetric) and what the wave would have to anticipate in terms of mode conversion when being reflected at a surface not perpendicular to its traveling direction. The effects of changing in geometric specifications of non,symmetric artificial features like angle of sloping edge or partially sloping edges are studied. The results obtained from these studies are presented as the reflected and converted parts of the incident wave versus angle of the edge or percentage of the sloped edge. It has been further shown that Lamb waves being generated experimentally by a finite size transducer into a plate like structure thus most likely result in a combination of modes. Reflection of these combined modes at structural boundaries will therefore generate an even more complex coupling of modes. This situation is further aggravated if the structural boundary is not purely perpendicular to the traveling wave but has a slightly varying angle such as it might have to be anticipated at a countersunk rivet, a notch or even more extreme a crack in a metallic component. However from understanding the background of Lamb wave generation, mode separation and superposition, a systematic approach can be established that allows complex Lamb waves, such as they are observed when monitoring true structures, to be interpreted and understood. This approach has been explained on the basis of numerical result obtained from finite element analyses first before proving the findings by some fundamental experiments performed with variable angle beam transducers which demonstrates the difficulties in de,coupling Lamb wave modes and how to handle those coupled modes in terms of structural condition monitoring. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

P- and S-wave velocities of consolidated sediments from a seafloor seismic survey in the North Celtic Sea Basin, offshore Ireland

GEOPHYSICAL PROSPECTING, Issue 2 2008
Donna J. Shillington
ABSTRACT A geophysical survey was conducted over a hydrocarbon prospect in the North Celtic Sea Basin using a small array of ocean-bottom seismographs (OBSs). The purpose of this study was to determine the ratio of compressional (P)- to shear (S)-wave velocity of consolidated sedimentary rocks in order to constrain possible subsurface variations in pore-fluid content. The ratio of VP and VS is known to be particularly sensitive to lithology, porosity and pore-fluid content, making it a useful parameter for evaluating hydrocarbon prospects. OBSs offer a relatively cheap and time-effective means of acquiring multi-component data compared with ocean-bottom cables. In this contribution, we demonstrate the ability of an OBS survey comprising three pairs of two OBSs spaced at 1.6 km to recover lateral variations in the VP/VS ratio. A key requirement of this type of study is that S waves will be generated by mode conversions in the subsurface, since they cannot be generated in nor travel through fluids. In this survey, the contrast in physical properties of the hard seabed of the North Celtic Sea Basin provided a means of generating converted S waves. Two-dimensional ray-tracing and forward modelling was used to create both VP and VS models along a profile crossing the Blackrock prospect in the North Celtic Sea Basin. These models comprise four layers and extend to a maximum depth of 1.1 km. The observed northward decrease in the VP/VS ratio at depths of 500,1000 m below the seafloor in the study area is interpreted to represent lateral variation in the amount of gas present in the pore space of Upper Cretaceous chalks and shales overlying the prospective reservoir. [source]