Wave Velocities Of Samples Research Articles

The influence of edge waves in local surface skimming longitudinal wave generation using a focused PVDF transducer

Acoustic microscopy is extensively used for high-frequency imaging and material characterization. In a focused ultrasonic transducer, the presence of edge waves from the edge of the transducer is usually considered a disadvantage. For high-frequency imaging applications, the edge waves adversely affect the quality of the image. This paper discusses edge wave's influence on generating a surface wave in bulk metal samples using a limited aperture PVDF transducer. Acoustic microscopy-based defocusing experiments are conducted on aluminum, stainless steel, copper, and brass samples. A detailed wave-path analysis is done to understand the different wave components in a signal as obtained from defocusing experiments. The travel path of each wave component is analytically obtained and compared with the experimental results. The different wave modes observed in the experiments are identified by overlaying the analytical plots on the experimentally obtained B-scans. A good correlation is obtained between the experimental and analytical results. The surface wave velocity of the samples is calculated using the time-resolved method, and the percentage of error in the measurement is estimated. The challenges for using this method in measuring surface wave velocities in samples with bulk longitudinal velocities <5000m/s are also discussed.

DIASCoPE: Directly integrated acoustic system combined with pressure experiments-A new method for fast acoustic velocity measurements at high pressure.

A new experimental system to measure elastic wave velocities in samples in situ under extreme conditions of pressure and temperature in a multi-anvil apparatus has been installed at Beamline 6-BM-B of the Advanced Photon Source at Argonne National Laboratory. This system allows for measurement of acoustic velocities via ultrasonic interferometry, and makes use of the synchrotron beam to measure sample densities via X-ray diffraction and sample lengths using X-radiographic imaging. This system is fully integrated into the automated software controls of the beamline and is capable of collecting robust data on elastic wave travel times in less than 1 s, which is an improvement of more than one to two orders of magnitude over existing systems. Moreover, this fast data collection time has been shown to have no effect on the obtained travel time results. This allows for more careful study of time-dependent phenomena with tighter snapshots in time of processes that would otherwise be lost or averaged out in other acoustic measurement systems.

Olivine enigma: Why alteration controls the seismic properties of oceanic gabbros

The objective of this study was to determine whether the effects of low Mg content and/or cracks that typically populate olivine grains can explain why P and S wave velocities in samples of oceanic gabbro do not increase with increasing olivine content. The Mg content of olivine in these rocks is known to be appreciably less than Fo90, and optical, SEM, and electron microprobe analyses indicate that the “olivine” is actually an aggregate of olivine and 5–15% crack fill consisting of talc and/or serpentine plus magnetite. Voigt average models show that there is a strong, positive correlation between P wave velocities and Fo90 content but no correlation with Fo73. Because of its Mg content, olivine that is typical of oceanic gabbros does not influence seismic velocities. Adding 11% alteration of olivine to serpentine plus magnetite in the form of crack fill to the Fo73 model yields a nearly exact match between the model and measured P and S wave velocities. The fact that the crack fill consists largely of talc and/or serpentine indicates that these minerals formed by hydrothermal alteration in young, hot (>150°C) lower crust. Hence our results apply to gabbros in situ, as well as to the laboratory samples; seismic velocities in lower oceanic crust produced at high and intermediate spreading rates are controlled principally by the degree of hydrothermal alteration of the gabbros that compose the lower crust.

Frame bulk modulus of porous granular marine sediments

Frame bulk modulus is important for analyzing the acoustic wave propagation in porous water-saturated marine sediments such as sands. Previous measurements of the longitudinal wave velocities in air-saturated glass beads of uniform grain size showed that the longitudinal wave velocity increased with the grain size. This result cannot be explained by using a classical contact theory such as the Hertz-Mindlin model. It was speculated that this phenomenon is due to the effect of air elasticity between the grains. In this study, the longitudinal and shear wave velocities in samples of vacuum-, air-, and water-saturated glass beads as well as beach sands at a lower stress were measured. The results obtained were used to estimate the corresponding values of the frame bulk modulus. In the water-saturated samples, these values are about 109Pa at a frequency of 500kHz and about ten times greater than those in air-saturated samples at a frequency of 11.8kHz. The grain size dependence was also observed. These measurements are explained in terms of the effect of fluid elasticity at the grain-to-grain contact in the context of a modified gap stiffness model.

A simple derivation of the effective stress coefficient for seismic velocities in porous rocks

The effect of confining stress and pore pressure on seismic velocities is important for such geophysical applications as overpressure prediction from seismic data (Eaton, 1975; Dutta, 2002; Huffman, 2002; Sayers et al., 2002) and, more recently, for hydrocarbon production monitoring using time-lapse seismic measurements (Tura and Lumley, 1999; Landr⊘, 2001). The dependence of seismic velocity on pressure has been confirmed for a variety of rocks by laboratory measurements of elastic wave velocities in samples with varying pressure in pore fluids (see, e.g., Wyllie et al., 1958; Todd and Simmons, 1972; Eberhart-Phillips et al., 1989; Prasad and Manghnani, 1997). In general, for a rock subjected to a given confining stress σ c , higher pore pressures P correspond to lower compressional and shear velocities. Confining stress has a similar effect (but with opposite sign) on seismic velocities.

Laboratory measurements of seismic velocities in ocean sediments

Laboratory measurements have been made of the compressional wave velocities in samples of deep-sea sediments using ultrasonic methods. The variation of velocity with compaction pressure has been determined. From the results it has been concluded that a vertical gradient of velocity exists in the sedimentary layer of the ocean.