Young-Fo Chang - Academia.edu (original) (raw)

Papers by Young-Fo Chang

Journal of Materials in Civil Engineering, 2018

AbstractConcrete is a composite material composed mainly of water, sand, aggregate, and cement. T... more AbstractConcrete is a composite material composed mainly of water, sand, aggregate, and cement. The characteristics of high attenuation of concrete for ultrasound degrade the performance of ultraso...

Journal of Nondestructive Evaluation, 1997

The detection of the size and the location of existing three dimensional cracks in a concrete str... more The detection of the size and the location of existing three dimensional cracks in a concrete structure is an important topic in civil engineering. In this paper, a multisource, multireceiver method that considers the travel times diffracted by a crack tip is introduced, to backcalculate a 3-D image of the crack tip of a surface opening crack. The possible location

Applied Physics Letters, 2006

A potential piezoelectric crystal LiAlO 2 with ͑100͒ orientation is grown by means of the Czochra... more A potential piezoelectric crystal LiAlO 2 with ͑100͒ orientation is grown by means of the Czochralski pulling method. The as-grown crystal is identified as a single phase with good uniformity by x-ray diffraction pattern. ͑001͒ Transmission electron microscope image showed a unique cross-hatched pattern which reveals a superlattice structure. Several cubic LiAlO 2 specimen, 10.0 mm ϫ 10.0 mmϫ 10.0 mm were manufactured to characterize its elastic properties. The time-based pulse-echo transmission technique was employed to measure the acoustic velocities of longitudinal and transverse modes. The elastic constants of LiAlO 2 were extracted from the acoustic velocity measurements at different propagation directions. It was found that the acoustic velocities of LiAlO 2 are much higher than the current piezoelectric crystals, including quartz, LiNbO 3 , and Langasite family materials.

Earth Sciences, 2019

The existence of subsurface fractures provides not only space for the residence of petroleum but ... more The existence of subsurface fractures provides not only space for the residence of petroleum but also paths of migration. Therefore, subsurface fractures are of great interest to exploration geophysicists. In reflection seismology, a reservoir of vertically aligned fractures is often considered to possess azimuthal anisotropy, or Horizontal Transverse Isotropy (HTI), in terms of seismic anisotropy. The characteristics and information of this specific type of reservoir are widely obtained using seismic attributes, including the azimuthal variation in the P-wave amplitude and velocity, and the fractional difference of split S-waves. Essentially, a converted (C-) wave is initiated by a downward traveling P-wave, which is converted on reflection to upcoming S-waves. Hence, it combines the behaviors of P-and S-waves in theory. Using a forward model study, this study demonstrates the behaviors of a C-waves in a HTI medium, instead of the behaviors of P-or S-waves. Reflections are facilitated on the horizontal symmetry-axis plane of a scaled HTI model along seven different azimuths using end-on shooting arrangement. Using a P-type transducer as a source and an S-type transducer as a receiver, the behaviors of C-waves in a HTI medium are observed. In the acquired profiles, reflections of P-, PS 1-(C 1-), and a mixture of PS 2-(C 2-) and S 1-waves were detected. The phenomenon of C-wave splitting is also observed because of the behavior of an S-wave in a Transversely Isotropic Medium (TIM), and it could be easily identified in the azimuths near the fracture plane. The reflectivity strengths obtained using a Hilbert transform show that the azimuthal variation in the Amplitude Versus Offset (AVO) for both P-and C 1-waves are consistent, but the C 1-wave amplitude variation depends more significantly on the azimuth than that of the P-wave. Furthermore, the percentage anisotropy of the C-wave computed from acquired data falls right between those of P-and S-waves. By incorporating C-wave splitting and azimuthal AVO variation into traditional signature analyses, our results show that the fracture orientation is more pronounced when the potential reservoir has vertically aligned fractures.

Terrestrial, Atmospheric and Oceanic Sciences, 1996

Seismologists customarily use coda wav�� to study the inhomogeneous characteristics of the earth.... more Seismologists customarily use coda wav�� to study the inhomogeneous characteristics of the earth. Howe'\'•er, P coda waves ha'\'•e seldom been stud ied due to their weak signals and short duration time when the epicenter distance is less than 100 km. In order to manifest the characteristics of the P coda, a measurement of 2-D physical modeling was made by systemati cally changing the scattering attenuation and the intrinsic attenuation of the medium. Based on the experimental results, the decay rate of the P coda with elapse time is independent of the scattering attenuation; in contrast, the excitation level of the P coda is proportio11al to the scattering attenuation. Moreo\7er, the amplitude decay with the elapse time of P coda waves can he satisfactorily interpreted by the n1odel (.>f energy transfer (Shang and Gao, 1988).

$Research paper thumbnail of A multi-azimuth seismic refraction study for a horizontal transverse isotropic medium: physical modelling results$

Geophysical Prospecting, 2017

In order to investigate the characteristics of the anisotropic stratum, a multi-azimuth seismic r... more In order to investigate the characteristics of the anisotropic stratum, a multi-azimuth seismic refraction technique is proposed in this study since the travel time anomaly of the refraction wave induced by this anisotropic stratum will be large for a far offset receiver. To simplify the problem, a two-layer (isotropy-horizontal transverse isotropy) model is considered. A new travel time equation of the refracted P-wave propagation in this two-layer model is derived, which is the function of the phase and group velocities of the horizontal transverse isotropic stratum. In addition, the measured refraction wave velocity in the physical model experiment is the group velocity. The isotropic intercept time equation of a refraction wave can be directly used to estimate the thickness of the top (isotropic) layer of the two-layer model, because the contrast between the phase and group velocities of the horizontal transverse isotropic medium is seldom greater than 10% in the earth. If the contrast between the phase and group velocities of an anisotropic medium is small, the approximated travel time equation of a refraction wave is obtained. This equation is only dependent on the group velocity of the horizontal transverse isotropic stratum. The elastic constants A 11 , A 13 and A 33 and the Thomsen anisotropic parameter ε of the horizontal transverse isotropic stratum can be estimated using this multi-azimuth seismic refraction technique. Furthermore, under a condition of weak anisotropy, the Thomsen anisotropic parameter δ of the horizontal transverse isotropic stratum can be estimated by this technique as well.

Exploration Geophysics, 2016

This study uses ultrasonic physical modelling to test the accuracies of numerical calculations of... more This study uses ultrasonic physical modelling to test the accuracies of numerical calculations of traveltimes and conversion-point (CP) positions for P-SV wave propagation in a horizontal transversely isotropic (TI) medium. Study results show that the traveltimes and CP positions for P-SV wave propagation on the isotropic plane of a TI medium computed using Fermat’s minimum-time principle are the same as those of using the isotropic non-hyperbolic moveout equation and the isotropic CP equation. However, for P-SV wave propagation on the symmetry-axis plane of a TI medium, the arrival times and CP positions of SV-waves are difficult to determine by any ray methods when the propagation directions of SV-waves are within the cuspoidal SV-wave group velocities zone. But the first arrival times and the propagation of the dominant energy of P-SV waves can still be analysed by ray methods. Based on the calculation of Fermat’s minimum-time principle, if the source-receiver offset is greater t...

$Research paper thumbnail of Azimuthal variation of converted-wave amplitude in a reservoir with vertically aligned fractures − a physical model study$

Geophysical Prospecting, 2016

The existence of fractures not only provides space for oil and gas to reside in but also creates ... more The existence of fractures not only provides space for oil and gas to reside in but also creates pathways for their migration. Accurate description of a fractured reservoir is thus an important subject of exploration for geophysicists and petroleum engineers. In reflection seismology, a reservoir of parallel vertical fractures is often considered a transversely isotropic medium with its symmetry axis horizontally oriented and its physical properties varying in azimuth on the horizontal symmetry-axis plane. In the history of fractured reservoir exploration, azimuthal variation in the P-wave amplitude, velocity, and fractional difference of the split S-waves have been popular seismic attributes used to delineate characteristics and extract information from the reservoir. Instead of analysing the reflection signatures of P-wave and S-wave, the objective of this study is to demonstrate the azimuthal variation of the converted wave (C-wave) amplitude in a fractured reservoir. To facilitate our objective, both common offset and end-on shooting reflection experiments were conducted in different azimuths on the horizontal symmetry-axis plane of a horizontal transverse isotropic model. In the acquired profile, reflections of P-wave, PS 1-wave (C 1-), and a mixture of PS 2-(C 2-) and S 1-waves were observed and identified. Thereafter, the laboratory observations were Hilbert transformed to compute the reflectivity strength of the relative events. Results show that the reflectivity strengths of both P-and C 1-waves are consistently weakened from the direction of the layering strike to the layering normal. However, the azimuthal variation of the C 1-wave amplitude is more significant than that of the P-wave and can be considered another effective seismic attribute for orienting the fracture strike of a reservoir that consists of vertically aligned fractures.

Exploration Geophysics, 2017

In active tectonic regions, the primary formations are often tilted and subjected to the processe... more In active tectonic regions, the primary formations are often tilted and subjected to the processes of folding and/or faulting. Dipping formations may be categorised as tilted transverse isotropy (TTI). While carrying out hydrocarbon exploration in areas of orogenic structures, mispositioning and defocusing effects in apparent reflections are often caused by the tilted transverse isotropy of the overburden. In this study, scaled physical modelling was carried out to demonstrate the behaviours of seismic wave propagation and imaging problems incurred by transverse isotropic (TI) overburdens that possess different orientations of the symmetry axis. To facilitate our objectives, zero-offset reflections were acquired from four stratum-fault models to image the same structures that were overlain by a TI (phenolite) slab. The symmetry axis of the TI slab was vertical, tilted or horizontal. In response to the symmetry axis orientations, spatial shifts and asymmetrical diffraction patterns i...

Physica A: Statistical Mechanics and its Applications, 2008

We propose a new relationship between the slip and waiting time of real earthquake series. We cal... more We propose a new relationship between the slip and waiting time of real earthquake series. We calculated the Hurst exponents for both time series of slip and waiting time of earthquake sequence in Taiwan CWB (Central Weather Bureau) catalogue. Our findings suggest a good correlation with a correlation coefficient of about 0.8 between the two exponents. Such a good correlation is highly similar to the ones expected from time-or slip-predictable earthquake recurrence models and suggests that the recurrence of real seismicity could be reduced to the time-or slip-predictability in certain sense. This paper, thus, initiates a new direction reconsidering earthquake recurrence.

SEG Technical Program Expanded Abstracts 1999, 1999

We are now working on the propagation of qS-wave in an orthorhombic anisotropic medium (OAM) thro... more We are now working on the propagation of qS-wave in an orthorhombic anisotropic medium (OAM) through laboratory work. The aim of this work is trying to get a close view of the degenerate of quasi-shear (qS-) wave in the OAM. To perform the objective of this work, a pulsetransmission experimental setup was designed to measure the direct arrivals of the elastic energy. The transmission experiments were done from one principal (symmetry) axis to another. Our laboratory data clearly demonstrate the degeneration of the qS-wave propagation in the OAM. Compare to transversely isotropic media (TIM), the splitting phenomenon of the qS-wave, which was not shown along the axis of an infinite-fold of symmetry planes in a TIM, was observed in all three principal axes of the OAM.

SEG Technical Program Expanded Abstracts 2000, 2000

Journal of Testing and Evaluation, 2014

A spherically focused transducer (SFT) can concentrate the ultrasounds to a focal zone that yield... more A spherically focused transducer (SFT) can concentrate the ultrasounds to a focal zone that yields a better lateral resolution at a certain axial range. Unfortunately, this result is always accompanied by a loss of resolution outside the focal zone. The synthetic aperture focusing technique (SAFT) is a type of digital signal processing technique that is commonly based on the point-source wavefront backpropagation (WFBP) theory to improve the axial resolution of the image. However, the ultrasound is refracted when it propagates through the couplant–specimen interface to detect the flaws in specimens; thus, the capability of a virtual point-source SAFT in refocusing ultrasonic energy is deteriorated caused by the phase aberration by an SFT. Therefore, in this study, the finite source element (FSE) refraction corrected WFBP SAFT imaging method will be introduced to reduce the phase aberration effect when refocusing a defocused ultrasonic image. Study results show that the axial resolution of the image is largely improved by this technique, but improvement is not yet attained in the lateral resolution. In addition, the signal-to-noise ratio of the image is enhanced. Therefore, for cases with a strong phase aberration, the proposed method is recommended for obtaining better image resolution.

SEG Technical Program Expanded Abstracts 1996, 1996

SEG Technical Program Expanded Abstracts 1997, 1997

International Journal of Hydrogen Energy, 2014

The main purpose of this study is the characteristic and nature of current generation with a pure... more The main purpose of this study is the characteristic and nature of current generation with a pure culture of single cell in a sediment microbial fuel cell. A sediment microbial fuel cell with an air-cathode system was studied for a prolonged period of time. The current maintained a steady increase throughout the entire time period and reached to its peak of 1.82 mA with power density of 29,024.65 mW/cm 2 at day 35. Water parameters such as salinity and pH were observed throughout the entire time period for better understanding. Operation of water parameter had been done after stabilization of current output for every measurement. The electron transfer pathway was assessed by cyclic voltammetry study. A low current density was observed due to profound internal resistance (141 U), and the reason for which was ohmic losses. A linear relationship was observed between current density and power density. Phylogenetic analysis was performed with 16S rRNA to identify the studied organism.

$Research paper thumbnail of A physical model study of converted wave amplitude variation in a reservoir of systematically aligned vertical fractures$

International journal of molecular sciences, 2012

Microbial fuel cells (MFCs) represent a novel technology for wastewater treatment with electricit... more Microbial fuel cells (MFCs) represent a novel technology for wastewater treatment with electricity production. Electricity generation with simultaneous nitrate reduction in a single-chamber MFC without air cathode was studied, using glucose (1 mM) as the carbon source and nitrate (1 mM) as the final electron acceptor employed by Bacillus subtilis under anaerobic conditions. Increasing current as a function of decreased nitrate concentration and an increase in biomass were observed with a maximum current of 0.4 mA obtained at an external resistance (R(ext)) of 1 KΩ without a platinum catalyst of air cathode. A decreased current with complete nitrate reduction, with further recovery of the current immediately after nitrate addition, indicated the dependence of B. subtilis on nitrate as an electron acceptor to efficiently produce electricity. A power density of 0.0019 mW/cm(2) was achieved at an R(ext) of 220 Ω. Cyclic voltammograms (CV) showed direct electron transfer with the involve...

International Journal of Molecular Sciences, 2011

In this study the-green chemistry‖ use of the biosurfactant surfactin for the synthesis of calciu... more In this study the-green chemistry‖ use of the biosurfactant surfactin for the synthesis of calcium phosphate using the reverse microemulsion technique was demonstrated. Calcium phosphates are bioactive materials that are a major constituent of human teeth and bone tissue. A reverse microemulsion technique with surfactin was used to produce nanocrystalline brushite particles. Structural diversity (analyzed by SEM and TEM) resulted from different water to surfactin ratios (W/S; 250, 500, 1000 and 40,000). The particle sizes were found to be in the 16-200 nm range. Morphological variety was observed in the as-synthesized microemulsions, which consisted of nanospheres (~16 nm in diameter) and needle-like (8-14 nm in diameter and 80-100 nm in length) noncalcinated particles. However, the calcinated products included nanospheres (50-200 nm in diameter), oval (~300 nm in diameter) and nanorod (200-400 nm in length) particles. FTIR and XRD

Journal of Testing and Evaluation, 2013

ABSTRACT