Changes In Acoustic Velocity Research Articles

Basic investigation on acoustic velocity change imaging method for quantitative assessment of fat content in human liver

Fatty liver is a disease caused by the excess accumulation of fat in the human liver. The early diagnosis of fatty liver is very important, because fatty liver is the major marker linked to metabolic syndrome. We already proposed the ultrasonic velocity change imaging method to diagnose fatty liver by using the fact that the temperature dependence of ultrasonic velocity is different in water and in fat. For the diagonosis of a fatty liver stage, we attempted a feasibility study of the quantitative assessment of the fat content in the human liver using our ultrasonic velocity change imaging method. Experimental results showed that the fat content in the tissue mimic phantom containing lard was determined by its ultrasonic velocity change in the flat temperature region formed by a circular warming ultrasonic transducer with an acoustic lens having an appropriate focal length. By considering the results of our simulation using a thermal diffusion equation, we determined whether this method could be applied to fatty liver assessment under the condition that the tissue had the thermal relaxation effect caused by blood flow.

Interpreting direct hydrocarbon indicators of low-API biodegraded oils — A case study from a deepwater South Atlantic Basin

Strong seismic flat spots are typically associated with the oil/water contact (OWC) in many amplitude-supported deepwater South Atlantic Basin (SAB) discoveries, particularly in relatively shallow (∼2000 mbml), high-porosity unconsolidated Eocene-age reservoirs. These reservoirs contain biodegraded oils that have low API gravity, low gas-oil ratios, high viscosities, and fluid acoustic velocities between 4500 and 5000 ft/s. Often, a mismatch in amplitude is noted when comparing an observed seismic-amplitude response with a synthetic forward model generated using sonic logging data. These observations can be attributed to the high viscosity (low API gravity) of biodegraded oil that can lead to wave-induced heterogeneous pore pressure at logging frequencies, thus deviating from a key assumption in Gassmann's fluid-substitution theory. These dispersion effects can lead to significantly slower acoustic velocities of these oil-bearing reservoirs at seismic frequencies than those measured at logging frequencies. Using a generalized fluid/solid-substitution theory, we model this change in acoustic velocity and find that the theory can explain the observed bright-spot response.

Investigating the change in surface acoustic wave velocity due to the change in adhesion of a SU-8 thin film using a SU-8/AlN/Si SAW sensor

This study investigates the change in surface acoustic wave (SAW) phase velocity due to the change in adhesion of a SU-8 thin film using an Aluminum Nitride (AlN)/Silicon (Si) SAW sensor. The variation in the stress field at the SU-8/AlN interface at different levels of SU-8 adhesion is also investigated and correlated to the change in SAW phase velocity. A theoretical model is developed to determine the change in wave dispersion profile due to the change in adhesion of the SU-8 film on the surface of the AlN/Si sensor. The interface of the SU-8/AlN layers is represented by a layer of massless springs with interface stiffness K (N/m3). The results illustrate that as the adhesion of the SU-8 film weakens on the surface of the AlN/Si SAW sensor the velocity dispersion profiles fluctuate. The calculated stress field at the SU-8/AlN interface also fluctuates because the weakened interface cannot sustain the increased stress due to the confinement of the wave near the interface. Therefore, the mode of wave propagation varies between the perfect bond case where the SU-8 layer is perfectly bonded to the AlN/Si surface and when the SU-8 is completely de-bonded. Four SAW sensor designs operating in the frequency range of 84–208MHz are developed to measure the shift in the center frequency values and the corresponding change in SAW phase velocities due to the change in adhesion of the SU-8 layer. The adhesion of the SU-8 film on the surface of the AlN/Si SAW sensor is changed by using different adhesion layers. The adhesion layers used are a gold film and an omnicoat coated gold film. Omnicoat is an adhesion promoter used to improve the adhesion of SU-8. The results illustrate that as the adhesion of the SU-8 film improves for the sensors with omnicoat the wave velocity shifts to a lower value. The wave velocity values are fitted with the wave dispersion profiles for different values of the interface spring stiffness K. The interface spring stiffness values for the SAW sensors with omnicoat coated gold and gold films are 8.1×109N/m3 and 7.95×109N/m3, respectively. The use of omnicoat improves the adhesion of the SU-8 film and corresponds to a higher interface spring stiffness value. The stress and displacement fields generated in the SU-8 layer for different values of the interface spring stiffness K are plotted. The results illustrate that as the adhesion of the SU-8 layer improves the stress transmitted to the SU-8 layer increases and the wave is more confined in the SU-8 layer, which justifies the drop in wave velocity since the Rayleigh wave velocity in SU-8 is much lower in comparison to AlN and Silicon; 1166, 5600, 5000m/s, respectively.

High-resolution seismic velocity analysis as a tool for exploring gas hydrate systems: An example from New Zealand’s southern Hikurangi margin

The existence of free gas and gas hydrate in the pore spaces of marine sediments causes changes in acoustic velocities that overprint the background lithological velocities of the sediments themselves. Much previous work has determined that such velocity overprinting, if sufficiently pronounced, can be resolved with conventional velocity analysis from long-offset, multichannel seismic data. We used 2D seismic data from a gas hydrate province at the southern end of New Zealand’s Hikurangi subduction margin to describe a workflow for high-resolution velocity analysis that delivered detailed velocity models of shallow marine sediments and their coincident gas hydrate systems. The results showed examples of pronounced low-velocity zones caused by free gas ponding beneath the hydrate layer, as well as high-velocity zones related to gas hydrate deposits. For the seismic interpreter of a gas hydrate system, the velocity results represent an extra “layer” for interpretation that provides important information about the distribution of free gas and gas hydrate. By combining the velocity information from the seismic transect with geologic samples of the seafloor and an understanding of sedimentary processes, we have determined that high gas hydrate concentrations preferentially form within coarse-grained sediments at the proximal end of the Hikurangi Channel. Finer grained sediments expected elsewhere along the seismic transect might preclude the deposition of similarly high gas hydrate concentrations away from the channel.

On the Anomalously Strong Dependence of the Acoustic Velocity of Alumina on Temperature in Aluminosilicate Glass Optical Fibers—Part I: Material Modeling and Experimental Validation

The thermo‐acoustic coefficient (TAC, or the change of acoustic velocity with a change in temperature) of the alumina (Al2O3) component in aluminosilicate core silica clad glass optical fibers is anomalously large. It has been speculated previously that this anomaly is not due to a large alumina TAC, but instead due to the thermal expansion mismatch between the aluminosilicate core glass and the pure silica cladding glass. In this new work, it is shown that the acoustic velocity of the aluminosilicate core glass changes much differently than that for a bulk aluminosilicate glass. In the high alumina content compositional range, it is found that the larger core‐region thermal expansion results in a net positive pressure imparted on the silica component in the core. As the acoustic velocity of silica has a strong negative dependence on any applied pressure, this offsets the bulk positive‐valued thermal response of silica. Utilizing a modified form of the Winkelmann–Schott additivity model, the effect of the thermal expansion mismatch between the core and clad glasses is incorporated, and the resultant TAC determined for alumina is fully consistent with data found in the literature for bulk alumina. The consideration and incorporation of the thermal expansion of the core and clad glasses adds another degree of freedom to the design of specialty optical fibers for distributed sensor applications. Complementary to this article, Part II describes an efficient computational route for determining selected physical properties of an amorphous analog to a given crystalline form whose physical properties are known.

The real-time measurement of wear using ultrasonic reflectometry

Ultrasonic reflectometry is commonly used in the fields of non-destructive testing (NDT) for crack detection, wall thickness monitoring and medical imaging. A sound wave is emitted through the material using a piezoelectric transducer. This waveform travels through the host medium at a constant speed and is either partially or fully reflected at an interface. The reflected wave is picked up by the same sensor; the signal is then amplified and digitised. If the speed that sound travels through a host medium is known as well as the time this takes, the thickness of the material can be established using the speed, distance and time relationship.Previous work has concluded that the ultrasonic method is too inaccurate to measure wear due to the errors caused by temperature, vibration and the experimental arrangement. This body of work looks at methods to minimise these errors, particularly the inaccuracies introduced from the change in temperature caused by change of acoustic velocity and the thermal expansion of the material, which can be significant in many applications. Numerous case studies are presented using the technique in both laboratory and industrial environments using low cost retro-fittable sensors and small form electronics.

2P5-13 Investigation on quantitative assessment of fat content in human liver using acoustic velocity-change

2P5-13 Investigation on quantitative assessment of fat content in human liver using acoustic velocity-change

Acoustic velocity measurements for stishovite across the post-stishovite phase transition under deviatoric stress: Implications for the seismic features of subducting slabs in the mid-mantle

Understanding effects of non-hydrostatic pressure on phase transitions in minerals relevant to the Earth’s mantle is important to translate the observable seismic signals to not directly observable mineralogical models for the deep Earth. SiO 2 can occur as a free phase in subducting slabs, which contain sedimentary layers and/or mid-ocean-ridge basalts. In this study, we report on the effect of deviatoric strain on the pressure-induced phase transition in SiO 2 and its consequences on the seismic signal. The acoustic velocity in polycrystalline stishovite across the post-stishovite phase transition was measured by Brillouin scattering in the pure SiO 2 system at room temperature under deviatoric stress. High-pressure synchrotron X-ray diffraction data were also collected at SPring-8. A linear fit to the symmetry-breaking strain values and the pressure of the transverse velocity minimum indicate a transition pressure between 25 and 35 GPa, which is about 20 GPa lower than that under hydrostatic conditions. The transverse velocity dropped by about 3% at around 25 GPa in this study. This is much smaller than the prediction from ab initio calculations that a transverse velocity reduces by ~60% at around 50 GPa under hydrostatic conditions. The results of the present study indicate that the deviatoric stress lowers the transition pressure and reduces the acoustic velocity change associated with the post stishovite phase transition. Sedimentary and mid ocean ridge basalt (MORB) layers in a subducting slab are likely sites for finding stishovite and its high-pressure polymorphs in the deep earth. Seismic observations of deep earthquakes occurring in subducting slabs indicate the existence of considerable stress in down-going slabs. This study suggests that nonhydrostatic deviatoric stress is one of the possible reasons for the absence of general seismic features that can be directly related to the post-stishovite phase transition in subducting slabs at 1500 km depth. The phase transition of stishovite under deviatoric stress, which occurs at shallower depths, can affect the local seismic scattering structures and the rheological behavior of a subducting slab in the mid-lower mantle region.

Experimental study of hydrogen embrittlement on AISI 304 stainless steels and Rayleigh wave characterization

Many failures due to hydrogen embrittlement or hydrogen damage are widely reported in oil and refinery industry. Despite many ultrasonic testing methods have been developed to assess hydrogen embrittlement, they are applied well to serious hydrogen attack instead of earlier degradation. This paper aims to characterize nascent hydrogen embrittlement of AISI 304 austenitic stainless steels under cathodic hydrogenation using Rayleigh wave. After cathodic hydrogen charging of AISI 304 stainless steel, XRD and metallographic examination show that martensite transformation occurs within the subsurface region of the specimens. Microhardness testing indicates that hydrogen leads to hardening of the material. It is found that Rayleigh wave are better to inspect local degradation than bulk waves. Rayleigh wave velocity of 5MHz and 10MHz decreases significantly with cathodic charging time, while longitudinal wave velocity changes not. Acoustic velocity change is due to elastic modulus reduction resulting from hydrogen-induced phase transformation in the subsurface region.

Apparatus for measurement of acoustic wave propagation under uniaxial loading with application to measurement of third-order elastic constants of piezoelectric single crystals

We describe an apparatus for the measurement of acoustic wave propagation under uniaxial loading featuring a special mechanism designed to assure a uniform mechanical load on a cube-shaped sample of piezoelectric material. We demonstrate the utility of the apparatus by determining the effects of stresses on acoustic wave speed, which forms a foundation for the final determination of the third-order elastic constants of langasite and langatate single crystals. The transit time method is used to determine changes in acoustic wave velocity as the loading is varied. In order to minimize error and improve the accuracy of the wave speed measurements, the cross correlation method is used to determine the small changes in the time of flight. Typical experimental results are presented and discussed.

Precise tailoring of acoustic velocity in optical fibers by hydrogenation and UV exposure

Tailoring of acoustic properties in solids has many potential applications in both acoustics, i.e. acoustic gratings and waveguides, and photon-phonon interactions, i.e. stimulated Brillouin scattering (SBS). One immediate application is in the area of SBS suppression in optical fibers. We demonstrate, for the first time, a post-processing technique where hydrogen is diffused in to a fiber core and then locally and permanently bonded to core glass by a subsequent UV exposure. It is discovered that local acoustic velocity can be altered by as much as ~2% this way, with strong potential for much further improvements with an increased hydrogen pressure. It is also found that the large change in acoustic velocity is primarily due to a reduction in bulk modulus, possibly as a result of network bonds being broken up by the addition of OH bonds. It is possible to use this technique to precisely tailor acoustic velocity along a fiber for more optimized SBS suppression in a fiber amplifier. Change in Brillouin Stokes frequency of ~320MHz at 1.064μm was observed.

Tissue-Mimicking Materials Using Segmented Polyurethane Gel and Their Acoustic Properties

Accurate testing of an instrument by phantoms requires a tissue-mimicking material that has the acoustic velocity and density defined in the International Electrotechnical Commission (IEC) standard, and furthermore the tissue-mimicking material must be stable over time. To achieve the tissue-mimicking materials with the desired acoustic velocity and density defined in the IEC standard, new materials have been developed. The form of tissue-mimicking materials reported comprised polystyrene and poly(methyl methacrylate) (PMMA) particles dispersed in segmented polyurethane gel. They were stable over a period of 40 days and the changes in weight and acoustic velocity did not exceed 0.5%.

Estimating Within Tree Spatial Changes in Acoustic Velocity in Felled Douglas-fir Stems

Sorting logs for stiffness in the woods using acoustic devices mounted on harvesters and processors could improve the allocation of logs to their most appropriate processing destinations. Estimating spatial changes in acoustic velocity in felled stems, based on a limited number of measurements, will be essential if they are not to unduly impact harvesting or processing productivity and costs. The effect of five variables (length from butt, bark percentage, moisture content, green density, and oven dry density) and four velocity measurement approaches (time of flight [TOF] across the tree bole, resonance of the full tree length, resonance of the first 3 m butt log segment, and resonance of a log segment between 6 and 9 m above the butt) on the ability to estimate spatial changes in acoustic velocity were evaluated in felled Douglas-fir stems from five stands in southwestern Oregon. Distance from the butt was a significant predictor of acoustic velocity along the stem. TOF acoustic velocity measured across the bole had little value in predicting the longitudinal resonance acoustic velocity of a section of the tree. Full tree length resonance acoustic velocity, or single log acoustic velocity (either 3 m butt log or 3 m log 6 m above the butt) were found to be statistically significant and strongly correlated predictors of acoustic velocity of a section of a tree (R2 ranging from 0.68 to 0.89). The models were found to be stand-dependent, indicating a possible need for calibration for each stand to be harvested.

Numerical Results of Modeling Semiconductor Sensor Layers in SAW Gas Sensors

The paper presents the numerical results of investigations of the layered gas surface acoustic waves sensor. The base electric load of the piezoelectric acoustic line is predicted by the effect of surface acoustic waves velocity changes vs. surface conductivity, which depends on the profile concentration by gas diffused molecules into the porous film. Inside the sensor layer Knudsen’s model of gas diffusion was used.

Analytical Model of Semiconductor Sensor Layers in SAW Gas Sensors

In the paper a new theoretical model for analyzing a surface acoustic wave gas sensor is presented. Basing on the electric load of the piezoelectric acoustic line the effect of surface acoustic wave velocity changes vs. surface conductivity is predicted which depends on the profile concentration of gas molecules diffused into the porous film. Inside the sensor layer Knudsen’s model of gas diffusion was used.

Thermal tuning of phononic bandstructure in ferroelectric ceramic/epoxy phononic crystal

Thermal tuning of phononic bandgaps in megahertz range was demonstrated in ferroelectric ceramic-based phononic crystal structure. Temperature variation across ferroelectric phase transition, accompanied by substantial changes in acoustic velocities, leads to a shift in the phononic bandstructure of a two-dimensional (Ba,Sr)TiO3/epoxy composite sample over a range of 10 °C. Experimental results are supported by modelings based on plane-wave expansion calculations. The high tunability of phononic bandstructure is advantageous for active control of ultrasound transmissions.

Factors controlling the compressive strength and acoustic properties of shales when interacting with water-based fluids

This paper investigates the impact of water-based fluids on the compressive strengths and acoustic velocities of different types of shales. The acoustic velocity (or slowness) and compressive strength of a soft, high water activity, Pierre I shale increase after exposure to different ionic solutions, while for the lower water activity Arco shale, the converse is true. A new gravimetric test that quantitatively determines the flux of water and ions into and out of the shale is utilized to show that changes in compressive strengths and acoustic velocities of shales correlate well with the movement of ions and water into the shale. The results showed that water adsorption weakened, while ion adsorption strengthened the shale. In addition, this study investigated the influence of salt type and salt concentration on the strength and acoustic velocity (slowness) of shales. It was observed that the ionic content of the water-based fluid had a significant effect on the changes in shale properties. These changes in acoustic velocity and compressive strength were found to be highly correlated. This suggests that it may be feasible to use acoustic logging data to determine changes in the mechanical properties of shale. Finally, a 3-D wellbore stability analysis program was used to demonstrate the impact of the reported changes of the mechanical properties of shale on wellbore stability.

The effects of stress paths on acoustic velocities and 4D seismic imaging

Time shifts on 4D seismic images result from two basic causes: (1) acoustic velocity changes due to fluid migration, fluid substitution, fluid composition; and (2) velocity changes resulting from changes in total stresses (i.e., the vertical and horizontal stresses) during depletion. This paper is concerned only with the latter—the effect of changing total stress on rock velocity.

Observations of volcanic tremor during the January–February 2005 eruption of Mt. Veniaminof, Alaska

Mt. Veniaminof, Alaska Peninsula, is a stratovolcano with a summit ice-filled caldera containing a small intracaldera cone and active vent. From January 2 to February 21, 2005, Mt. Veniaminof erupted. The eruption was characterized by numerous small ash emissions (VEI 0 to 1) and accompanied by low-frequency earthquake activity and volcanic tremor. We have performed spectral analyses of the seismic signals in order to characterize them and to constrain their source. Continuous tremor has durations of minutes to hours with dominant energy in the band 0.5–4.0 Hz, and spectra characterized by narrow peaks either irregularly (non-harmonic tremor) or regularly spaced (harmonic tremor). The spectra of non-harmonic tremor resemble those of low-frequency events recorded simultaneously with surface ash explosions, suggesting that the source mechanisms might be similar or related. We propose that non-harmonic tremor at Mt. Veniaminof results from the coalescence of gas bubbles while low-frequency events are related to the disruption of large gas pockets within the conduit. Harmonic tremor, characterized by regular and quasi-sinusoidal waveforms, has duration of hours. Spectra containing up to five harmonics suggest the presence of a resonating source volume that vibrates in a longitudinal acoustic mode. An interesting feature of harmonic tremor is that frequency is observed to change over time; spectral lines move towards higher or lower values while the harmonic nature of the spectra is maintained. Factors controlling the variable characteristics of harmonic tremor include changes in acoustic velocity at the source and variations of the effective size of the resonator.

Search for a relationship between stress wave velocity and internal stresses in eucalypts and radiata pine

Abstract The relation between growth stress and stress wave velocity has been investigated on standing trees of Eucalyptus nitens. Longitudinal growth stress and the stress wave velocity were measured at breast height on 155 trees. Large variation in both growth strain and acoustic velocity was observed. The initial investigation with 34 trees showed good correlation (R2=0.68). Measurements on all 155 trees revealed no reliable relationship (R2=0.05). The influence of internal stress on acoustic velocity has also been investigated on radiata pine boards loaded in bending and tension. It was observed that the resonance frequencies were influenced by the position of loaded supports in bending, but were not affected by the bending load. In boards under tensile stresses, there was no change in acoustic velocity measured by the pulse transit time. The two experimental series, one on standing trees with water-filled lumens and the other on air-dried boards, failed to reveal any dependence of the stress wave velocity on growth stress. No valid basis therefore exists for the use of stress wave velocity to determine growth stress levels in either standing trees or in seasoned wood.