scholarly journals Experimental study of acoustic anisotropy and birefringence in dry and saturated Fontainebleau sandstone

Geophysics ◽  
1990 ◽  
Vol 55 (11) ◽  
pp. 1455-1465 ◽  
Author(s):  
M. Zamora ◽  
J. P. Poirier
Keyword(s):  
Crack Density ◽  
Uniaxial Stress ◽  
P Waves ◽  
Thermally Induced ◽  
S Waves ◽  
Velocity Anisotropy ◽  
Fontainebleau Sandstone ◽  
Plane Normal ◽  
Acoustic Anisotropy ◽  
Scanning Electron

The velocities of ultrasonic P, SH, and SV waves have been measured in two perpendicular directions, in samples of Fontainebleau sandstone as received or thermally cracked, dry, or saturated, under uniaxial stress. We have investigated the effect of cracking, saturation, and uniaxial stress on the velocity of P and S waves in two orthogonal directions (anisotropy) and the velocity of S waves with two orthogonal polarizations in each direction of propagation (birefringence). The effect of cracking, saturation, and uniaxial stress on Poisson’s ratio has also been investigated. The velocity anisotropy is larger for S waves than for P waves and practically disappears in saturated samples. Birefringence is attenuated in saturated samples. Inversion of the results using Crampin’s model gives values of the crack densities in three directions, in qualitative agreement with the state of cracking observed by scanning electron microscopy. In particular, the crack density is found to be near zero in sandstones with rounded pores only. After thermally induced cracking the crack density is found to be ≈20 percent; uniaxial stress closes the cracks in the plane normal to the stress. Also, in naturally cracked samples the crack density is found to be quite high. Uniaxial stress causes the density of cracks to decrease, mostly in the plane normal to the stress.

Elastic velocity anisotropy in the presence of an anisotropic initial stress

1972 ◽  
Vol 62 (5) ◽  
pp. 1183-1193 ◽  
Author(s):  
F. A. Dahlen
Keyword(s):  
Initial Stress ◽  
Elastic Anisotropy ◽  
Body Wave ◽  
Homogeneous Medium ◽  
Body Waves ◽  
P Waves ◽  
S Waves ◽  
Velocity Anisotropy ◽  
First Order ◽  
Wave Velocities

Abstract The effect of a homogeneous anisotropic initial stress on the propagation of infinitesimal amplitude elastic body waves in a perfectly elastic, homogeneous medium is investigated. If the medium is inherently isotropic in the reference configuration and if the magnitude τ0 of the deviatoric part of the initial static stress is small compared to the rigidity μ of the medium, then the apparent body-wave velocities of P waves are unaffected by the initial stress to first order in τ0/μ. The apparent body-wave velocities of S waves are rendered anisotropic to first order, and this effect is described explicitly. It is concluded that the direct effect of an anisotropic initial stress cannot contribute appreciably to the observed velocity anisotropy of horizontally propagating P waves in the oceanic upper mantle. Those observations require an inherent elastic anisotropy of the oceanic uppermantle material.

Numerical simulation of azimuthal acoustic logging in a borehole penetrating a rock formation boundary

Geophysics ◽  
2016 ◽  
Vol 81 (3) ◽  
pp. D283-D291 ◽  
Author(s):  
Peng Liu ◽  
Wenxiao Qiao ◽  
Xiaohua Che ◽  
Xiaodong Ju ◽  
Junqiang Lu ◽  
...  
Keyword(s):  
Domain Model ◽  
P Wave ◽  
S Wave ◽  
Angle Variation ◽  
P Waves ◽  
Acoustic Logging ◽  
S Waves ◽  
Rock Formation ◽  
Logging Tool ◽  
Difference Time

We have developed a new 3D acoustic logging tool (3DAC). To examine the azimuthal resolution of 3DAC, we have evaluated a 3D finite-difference time-domain model to simulate a case in which the borehole penetrated a rock formation boundary when the tool worked at the azimuthal-transmitting-azimuthal-receiving mode. The results indicated that there were two types of P-waves with different slowness in waveforms: the P-wave of the harder rock (P1) and the P-wave of the softer rock (P2). The P1-wave can be observed in each azimuthal receiver, but the P2-wave appears only in the azimuthal receivers toward the softer rock. When these two types of rock are both fast formations, two types of S-waves also exist, and they have better azimuthal sensitivity compared with P-waves. The S-wave of the harder rock (S1) appears only in receivers toward the harder rock, and the S-wave of the softer rock (S2) appears only in receivers toward the softer rock. A model was simulated in which the boundary between shale and sand penetrated the borehole but not the borehole axis. The P-wave of shale and the S-wave of sand are azimuthally sensitive to the azimuth angle variation of two formations. In addition, waveforms obtained from 3DAC working at the monopole-transmitting-azimuthal-receiving mode indicate that the corresponding P-waves and S-waves are azimuthally sensitive, too. Finally, we have developed a field example of 3DAC to support our simulation results: The azimuthal variation of the P-wave slowness was observed and can thus be used to reflect the azimuthal heterogeneity of formations.

An analytical solution to separate P‐waves and S‐waves in VSP wavefields

Geophysics ◽  
1995 ◽  
Vol 60 (4) ◽  
pp. 955-967 ◽  
Author(s):  
Hiroshi Amano
Keyword(s):  
Analytical Solution ◽  
Formal Solution ◽  
Seismic Profile ◽  
Synthetic Seismograms ◽  
Third Order ◽  
P Waves ◽  
Practical Applications ◽  
S Waves ◽  
The Third ◽  
Z Domain

An analytical solution to separate P‐waves and S‐waves in vertical seismic profile (VSP) wavefields is derived using combinations of certain terms of the formal solution for forward VSP modeling. Some practical applications of this method to synthetic seismograms and field data are investigated and evaluated. Little wave distortion is recognized, and the weak wavefield masked by dominant wavetrains can be extracted with this method. The decomposed wavefield is expressed in the frequency‐depth (f-z) domain as a linear combination of up to the third‐order differential of traces, which is approximated by trace differences in the practical separation process. In general, five traces with single‐component data are required in this process, but the same process is implemented with only three traces in the acoustic case. Two‐trace extrapolation is applied to each edge of the data gather to enhance the accuracy of trace difference. Since the formulas are developed in the f-z domain, the influence of anelasticity can be taken into account, and the calculation is carried out fast enough with the benefit of the fast Fourier transform (FFT).

Performance of composite sandwich structures under thermal cycling

2019 ◽  
Vol 54 (2) ◽  
pp. 271-283
Author(s):  
Sandesh Rathnavarma Hegde ◽  
Mehdi Hojjati
Keyword(s):  
Crack Formation ◽  
Mechanical Performance ◽  
Crack Density ◽  
Free Edge ◽  
Element Analysis ◽  
Thermally Induced ◽  
Composite Sandwich ◽  
Composite Sandwich Structures ◽  
Number Of Cycles ◽  
Microscopic Inspection

Effect of thermally induced microcracks on mechanical performance of a space grade laminated sandwich panel is investigated. A simple non-contact setup using liquid nitrogen is developed to subject the material to low temperature of −170℃ with cooling rate of 24℃/min. Then the samples are exposed to the elevated temperature of 150℃ inside oven. Microcracks formation and propagation are monitored through microscopic observation of cross-section during the cycling. Flatwise tensile test is performed after a number of cycles. A correlation is made between number of cycles and flatwise mechanical strength after quantifying the microcracks. It is observed that the crack formation gets saturated at about 40 cycles, avoiding the need to conduct more thermal cycles. Microcrack formation both at the free edge and middle of laminate was observed. The crack density at the middle was comparatively less than the ones found on the free edges. Results for non-contact cooling are compared with samples from direct nitrogen contact cooling. Microscopic inspection and flatwise test show differences between contact and non-contact cooled samples. Flatwise tensile strength for non-contact cooled samples shows 15% reduction, while the contact cooled samples have about 30% decrease in bond strength. A 3D finite element analysis is conducted to qualitatively identify the location of stress concentration which can be possible sites of crack formation. Good agreement is observed between the model and experimental results.

scholarly journals Explicit Q expressions for inhomogeneous P- and SV-waves in isotropic viscoelastic media

2019 ◽  
Vol 17 (2) ◽  
pp. 300-312 ◽  
Author(s):  
Xu Liu ◽  
Stewart Greenhalgh ◽  
Bing Zhou ◽  
Huijian Li
Keyword(s):  
Energy Density ◽  
Dissipation Factor ◽  
Energy Balance Equation ◽  
Dissipated Energy ◽  
P Waves ◽  
S Waves ◽  
Viscoelastic Media ◽  
Inhomogeneous Waves ◽  
Inhomogeneity Parameter ◽  
P And Sv Waves

Abstract We derive explicit expressions for the dissipation factors of inhomogeneous P and SV-waves in isotropic viscoelastic media. The Q−1 values are given as concise and simple functions of material parameters and the wave inhomogeneity parameter using two different definitions. Unlike homogenous waves, inhomogeneous waves may have significant differences in the values of dissipation factors because of different definitions. For example, under one of the three dissipation factor definitions that Q−1 is equal to the time-averaged dissipated-energy density divided by twice the time-averaged strain-energy density, it is found and proved that the dissipation factor of SV-waves is totally independent of the inhomogeneity parameter. For materials in which P-waves are normally more dissipative than S-waves (e.g. a porous reservoir), the dissipation factors of P-waves tend to decrease with increasing degree of inhomogeneity. Based on Buchan's classic real value energy balance equation, a parallel investigation is conducted for each step similar to that based on the Carcione equations, including derivation of explicit formulas (with inhomogeneity angle representing the degree of inhomogeneity of a plane wave), and dissipation curves calculations. We also obtain an inhomogeneity independent formula of $Q_{\, SV}^{ - 1}$, and exactly the same phase velocity and attenuation dispersion results for the example material.

Shallow near-surface effects

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. T221-T231 ◽  
Author(s):  
Christine E. Krohn ◽  
Thomas J. Murray
Keyword(s):  
Time Delay ◽  
Velocity Gradient ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
P Waves ◽  
Large Computer ◽  
Near Surface ◽  
S Waves ◽  
High Attenuation ◽  
Pulse Broadening

The top 6 m of the near surface has a surprisingly large effect on the behavior of P- and S-waves. For unconsolidated sediments, the P-wave velocity gradient and attenuation can be quite large. Computer modeling should include these properties to accurately reproduce seismic effects of the near surface. We have used reverse VSP data and computer simulations to demonstrate the following effects for upgoing P-waves. Near the surface, we have observed a large time delay, indicating low velocity ([Formula: see text]), and considerable pulse broadening, indicating high attenuation ([Formula: see text]). Consequently, shallowly buried geophones have greater high-frequency bandwidth compared with surface geophones. In addition, there is a large velocity gradient in the shallow near surface (factor of 10 in 5 m), resulting in the rotation of P-waves to the vertical with progressively smaller amplitudes recorded on horizontal phones. Finally, we have found little indication of a reflection or ghost from the surface, although downgoing reflections have been observed from interfaces within the near surface. In comparison, the following have been observed for upgoing S-waves: There is a small increase in the time delay or pulse broadening near the surface, indicating a smaller velocity gradient and less change in attenuation. In addition, the surface reflection coefficient is nearly one with a prominent surface ghost.

Australian national ocean bottom seismograph fleet advances conventional exploration

2017 ◽  
Vol 57 (2) ◽  
pp. 738
Author(s):  
Alexey Goncharov ◽  
Michal Malinowski ◽  
Dejan Sekulic ◽  
Ashby Cooper ◽  
Peter Chia ◽  
...  
Keyword(s):  
Regional Scale ◽  
Seismic Survey ◽  
Ocean Bottom ◽  
P Waves ◽  
Crust And Upper Mantle ◽  
Seismic Surveys ◽  
S Waves ◽  
Industry Standard ◽  
Ocean Bottom Seismographs ◽  
Obs Data

A fleet of new Australian ocean bottom seismographs (OBSs) have broadband frequency range, and similar instruments are available at only five or six institutions globally. These OBSs are multi-purpose devices able to record passive-source seismic data (earthquakes, ambient noise) as well as active-source (airgun generated) data and, at the same time, to monitor seismic survey noise and whale calls for environmentally responsible exploration. OBS data collected during commercial seismic surveys in Australian waters prove that it is possible to image the velocity distribution of the whole crust and upper mantle from analysis of both reflected and refracted phases generated by an industry-standard broadband airgun array. This means that valuable information on a regional scale can be obtained as a by-product of commercial seismic surveys. Three-component recording capability of OBSs allows analysis of S-waves in addition to the P-waves that are conventionally used in marine reflection surveys.

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