Seismic properties of rock samples from the Pikwitonei granulite belt – God's Lake domain crustal cross section, Manitoba

1996 ◽  
Vol 33 (5) ◽  
pp. 757-768 ◽  
Author(s):  
David M. Fountain ◽  
Matthew H. Salisbury
Keyword(s):  
Cross Section ◽  
Wave Velocity ◽  
Acoustic Impedance ◽  
Lower Crust ◽  
Compressional Wave ◽  
Superior Province ◽  
Mafic Rocks ◽  
Wave Velocities ◽  
Granulite Belt ◽  
Felsic Rocks

Laboratory measurements of compressional and shear wave velocity to confining pressures of 600 MPa for a suite of representative samples collected from the Pikwitonei granulite belt and God's Lake domain, an Archean crustal cross section in the northwestern Superior Province, provide the basis of comparison of these terranes with the seismic characteristics of Archean lower crust. We found that felsic rocks in the Pikwitonei granulite belt and God's Lake domain, which make up the bulk of these terranes, have a similar average compressional wave velocity of 6.5 km/s at 600 MPa, indicating that felsic rocks show little velocity change across the amphibolite–granulite facies transition. Compressional wave velocities for mafic rocks from each terrane are between 7.1 and 7.3 km/s. Apparent Poisson's ratio ranges from 0.24 to 0.26 and 0.26 to 0.28 for felsic and mafic rocks, respectively. These velocity data compare favorably with data for similar lithologies from the Kapuskasing uplift. Using the relative abundances of the constituent lithologies, the weighted average compressional wave velocities of the God's Lake domain and Pikwitonei granulite belt at 600 MPa are 6.56 and 6.63 km/s, respectively. These values, coupled with velocity distribution functions based on the population statistics and relative abundance for each lithology, show that there is no correspondence between the seismic characteristics of the Pikwitonei granulite belt and typical Archean and Proterozoic lower crust. The average properties of the Pikwitonei granulite belt and God's Lake domain, however, correspond well with typical Archean and Proterozoic middle crust. This suggests that either the Pikwitonei granulite belt represents an extreme felsic end member of Archean lower crust or that the deepest levels of the Superior Province crust are not exposed in the Pikwitonei granulite belt. Similar distribution function diagrams for acoustic impedance show that the Pikwitonei granulite belt is characterized by high acoustic impedance contrasts, but the high-impedance component is low in abundance. If the strong reflections observed under the Pikwitonei granulite belt in recent Lithoprobe surveys are not due to other causes, such as favorably oriented bodies of metamorphosed banded iron formation, diabase, or rock units not exposed in this region but present at depth, then they are caused by surprisingly small volumes of mafic metavolcanic rocks.

Compressional‐wave velocities in attenuating media: A laboratory physical model study

Geophysics ◽  
2000 ◽  
Vol 65 (4) ◽  
pp. 1162-1167 ◽  
Author(s):  
Joseph B. Molyneux ◽  
Douglas R. Schmitt
Keyword(s):  
Phase Velocity ◽  
Group Velocity ◽  
Wave Velocity ◽  
Propagation Distance ◽  
Compressional Wave ◽  
Arrival Times ◽  
Wave Velocities ◽  
Amplitude Maximum ◽  
Phase And Group Velocities ◽  
Group Velocities

Elastic‐wave velocities are often determined by picking the time of a certain feature of a propagating pulse, such as the first amplitude maximum. However, attenuation and dispersion conspire to change the shape of a propagating wave, making determination of a physically meaningful velocity problematic. As a consequence, the velocities so determined are not necessarily representative of the material’s intrinsic wave phase and group velocities. These phase and group velocities are found experimentally in a highly attenuating medium consisting of glycerol‐saturated, unconsolidated, random packs of glass beads and quartz sand. Our results show that the quality factor Q varies between 2 and 6 over the useful frequency band in these experiments from ∼200 to 600 kHz. The fundamental velocities are compared to more common and simple velocity estimates. In general, the simpler methods estimate the group velocity at the predominant frequency with a 3% discrepancy but are in poor agreement with the corresponding phase velocity. Wave velocities determined from the time at which the pulse is first detected (signal velocity) differ from the predominant group velocity by up to 12%. At best, the onset wave velocity arguably provides a lower bound for the high‐frequency limit of the phase velocity in a material where wave velocity increases with frequency. Each method of time picking, however, is self‐consistent, as indicated by the high quality of linear regressions of observed arrival times versus propagation distance.

Sonic logging of compressional‐wave velocities in a very slow formation

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1627-1633 ◽  
Author(s):  
Bart W. Tichelaar ◽  
Klaas W. van Luik
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Compressional Wave ◽  
P Wave ◽  
Dipole Source ◽  
Frequency Spectra ◽  
Unconsolidated Sands ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Sonic Logging

Borehole sonic waveforms are commonly acquired to produce logs of subsurface compressional and shear wave velocities. To this purpose, modern borehole sonic tools are usually equipped with various types of acoustic sources, i.e., monopole and dipole sources. While the dipole source has been specifically developed for measuring shear wave velocities, we found that the dipole source has an advantage over the monopole source when determining compressional wave velocities in a very slow formation consisting of unconsolidated sands with a porosity of about 35% and a shear wave velocity of about 465 m/s. In this formation, the recorded compressional refracted waves suffer from interference with another wavefield component identified as a leaky P‐wave, which hampers the determination of compressional wave velocities in the sands. For the dipole source, separation of the compressional refracted wave from the recorded waveforms is accomplished through bandpass filtering since the wavefield components appear as two distinctly separate contributions to the frequency spectrum: a compressional refracted wave centered at a frequency of 6.5 kHz and a leaky P‐wave centered at 1.3 kHz. For the monopole source, the frequency spectra of the various waveform components have considerable overlap. It is therefore not obvious what passband to choose to separate the compressional refracted wave from the monopole waveforms. The compressional wave velocity obtained for the sands from the dipole compressional refracted wave is about 2150 m/s. Phase velocities obtained for the dispersive leaky P‐wave excited by the dipole source range from 1800 m/s at 1.0 kHz to 1630 m/s at 1.6 kHz. It appears that the dipole source has an advantage over the monopole source for the data recorded in this very slow formation when separating the compressional refracted wave from the recorded waveforms to determine formation compressional wave velocities.

VELOCITY OF COMPRESSIONAL WAVES IN POROUS MEDIA AT PERMAFROST TEMPERATURES

Geophysics ◽  
1968 ◽  
Vol 33 (4) ◽  
pp. 584-595 ◽  
Author(s):  
A. Timur
Keyword(s):  
Porous Media ◽  
Wave Velocity ◽  
Compressional Wave ◽  
Freezing Process ◽  
Rock Matrix ◽  
Compressional Wave Velocity ◽  
Compressional Waves ◽  
Average Equation ◽  
Wave Velocities ◽  
Time Average

Measurements of velocity of compressional waves in consolidated porous media, conducted within a temperature range of 26 °C to −36 °C, indicate that: (1) compressional wave velocity in water‐saturated rocks increases with decreasing temperature whereas it is nearly independent of temperature in dry rocks; (2) the shapes of the velocity versus temperature curves are functions of lithology, pore structure, and the nature of the interstitial fluids. As a saturated rock sample is cooled below 0 °C, the liquid in pore spaces with smaller surface‐to‐volume ratios (larger pores) begins to freeze and the liquid salinity controls the freezing process. As the temperature is decreased further, a point is reached where the surface‐to‐volume ratio in the remaining pore spaces is large enough to affect the freezing process, which is completed at the cryohydric temperature of the salts‐water system. In the ice‐liquid‐rock matrix system, present during freezing, a three‐phase, time‐average equation may be used to estimate the compressional wave velocities. Below the cryohydric temperature, elastic wave propagation takes place in a solid‐solid system consisting of ice and rock matrix. In this frozen state, the compressional wave velocity remains constant, has its maximum value, and may be estimated through use of the two‐phase time average equation. Limited field data for compressional wave velocities in permafrost indicate that pore spaces in permafrost contain not only liquid and ice, but also gas. Therefore, before attempting to make velocity estimates through the time‐average equations, the natures and percentages of pore saturants should be investigated.

EFFECT OF POROSITY, GRAIN CONTACTS, AND CEMENT ON COMPRESSIONAL WAVE VELOCITY THROUGH SYNTHETIC SANDSTONES

Geophysics ◽  
1961 ◽  
Vol 26 (1) ◽  
pp. 77-84 ◽  
Author(s):  
Andris Viksne ◽  
Joseph W. Berg ◽  
Kenneth L. Cook
Keyword(s):  
Wave Velocity ◽  
Atmospheric Pressure ◽  
Cement Content ◽  
Pore Space ◽  
Compressional Wave ◽  
Velocity Measurements ◽  
Compressional Wave Velocity ◽  
Ottawa Sand ◽  
Wave Velocities ◽  
The Relationship

Compressional wave velocities through 36 synthetic sandstone cores were measured and related to several of their physical properties, namely, porosity, manufacturing pressure, grain contacts, and amount of cement. The cores were composed of Ottawa sand grains averaging 0.12 mm in diameter and commercial Grefco cement; the manufacturing pressure was varied from 4,000 to 10,000 psi; the cement content by volume was varied from 10 to 100 percent; the effective porosities ranged between 2.1 and 30.4 percent; and the compressional wave velocities ranged between 9,170 and 17,420 ft.sec. All velocity measurements were taken at room temperature and atmospheric pressure using cores that contained only air in the pore space. The results are presented in graphic form, showing the relationship between the compressional wave velocity and manufacturing pressure, porosity, and cement content. For Grefco cement contents between 10.0 and 17.5 percent, the compressional wave velocity is controlled by the manufacturing pressure and the porosity. A change in manufacturing pressure of 1,000 psi changed the compressional wave velocity by one percent for cores having porosities of about 23 percent and by about 3 percent for cores having porosities of about 28 percent. A decrease in porosity of one percent increased the velocity by an average of 1.4 percent for effective porosities between 23 and 26 percent. The velocity is also dependent, to a great extent, on the number of grain contacts which is intimately associated with the manufacturing pressure, and the cement content which is intimately associated with the porosity. For cement contents greater than 17.5 percent by volume, the sand grains float in the cement, and the analogy between the synthetic sandstone cores and natural sandstones is questionable.

Shear‐wave logging to enhance seismic modeling

Geophysics ◽  
1991 ◽  
Vol 56 (12) ◽  
pp. 2129-2138 ◽  
Author(s):  
M. A. Payne
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Field Data ◽  
Compressional Wave ◽  
Oil Sand ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Amplitude Versus Offset ◽  
Amplitude Versus

In an effort to understand better the amplitude variation with offset for reflections from an oil sand and the sensitivity of the AVO response to shear‐wave velocity variations, I studied synthetic and field gathers collected from an onshore field in the Gulf of Mexico basin. A wave‐equation‐based modeling program generated the synthetic seismic gathers using both measured and estimated shear‐wave velocities. The measured shear‐wave velocities came from a quadrupole sonic tool. The estimated shear‐wave velocities were obtained by applying published empirical and theoretical equations which relate shear‐wave velocities to measured compressional‐wave velocities. I carefully processed the recorded seismic data with a controlled‐amplitude processing stream. Comparison of the synthetic gathers with the processed field data leads to the conclusion that the model containing the measured shear‐wave velocities matches the field data much better than the model containing the estimated shear‐wave velocities. Therefore, existing equations which relate shear‐wave velocities to compressional‐wave velocities yield estimates which are not sufficiently accurate for making quantitative comparisons of synthetic and field gathers. Even small errors in the shear‐wave velocities can have a large impact on the output. Such errors can lead to an incomplete and perhaps inaccurate understanding of the amplitude‐versus‐offset response. This situation can be remedied by collecting shear‐wave data for use in amplitude‐versus‐offset modeling, and for building databases to generate better shear‐wave velocity estimator equations.

scholarly journals Vestiges of underplating and assembly in the central North China Craton based on S-wave velocities

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Haoyu Tian ◽  
Chuansong He
Keyword(s):  
Group Velocity ◽  
Wave Velocity ◽  
Crustal Structure ◽  
North China Craton ◽  
Lower Crust ◽  
North China ◽  
Velocity Structure ◽  
S Wave ◽  
Middle Crust ◽  
Wave Velocities

AbstractThe destruction of the North China Craton (NCC) is a controversial topic among researchers. In particular, the crustal structure associated with the craton’s destruction remains unclear, even though a large number of seismic studies have been carried out in this area. To investigate the crustal structure and its dynamic implications, we perform noise tomography in the central part of the NCC. In this study, continuous vertical-component waveforms spanning one year from 112 broadband seismic stations are used to obtain the group velocity dispersion curves of Rayleigh waves at different periods, and surface wave tomography is employed to extract the Rayleigh wave group velocity distributions at 9–40 s. Finally, the S-wave velocity structure at depths of 0–60 km is determined by the inversion of pure-path dispersion data. The results show obvious differences in the crustal structure among the Western Block (WB), the Trans-North China Orogen (TNCO) and the Eastern Block (EB). The lower crust of the northern part of the EB exhibits a high-velocity S-wave anomaly, which may be related to magmatic underplating in the lower crust induced by an upwelling mantle plume. The S-wave velocity of the WB is lower than that of the TNCO in the upper and middle crust and is lower than that of both the TNCO and the EB in the lower crust. The crust of the TNCO shows higher S-wave velocities than the WB and EB in the upper and middle crust, and its overall S-wave velocity structure is clearly different from those of the WB and EB, implying that the crustal structure of the TNCO may contain vestiges of the Paleoproterozoic collision between the WB and EB and their subsequent assembly. This study marks the first time these findings are identified for the NCC.

Relationships between compressional‐wave and shear‐wave velocities in clastic silicate rocks

Geophysics ◽  
1985 ◽  
Vol 50 (4) ◽  
pp. 571-581 ◽  
Author(s):  
J. P. Castagna ◽  
M. L. Batzle ◽  
R. L. Eastwood
Keyword(s):  
Bulk Modulus ◽  
Shear Wave ◽  
Wave Velocity ◽  
Velocity Ratio ◽  
Shear Velocity ◽  
Compressional Wave ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Water Saturated ◽  
Silicate Rocks

New velocity data in addition to literature data derived from sonic log, seismic, and laboratory measurements are analyzed for clastic silicate rocks. These data demonstrate simple systematic relationships between compressional and shear wave velocities. For water‐saturated clastic silicate rocks, shear wave velocity is approximately linearly related to compressional wave velocity and the compressional‐to‐shear velocity ratio decreases with increasing compressional velocity. Laboratory data for dry sandstones indicate a nearly constant compressional‐to‐shear velocity ratio with rigidity approximately equal to bulk modulus. Ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water‐saturated and dry trends. For dry rigidity equal to dry bulk modulus, Gassmann’s equations predict velocities in close agreement with data from the water‐saturated rock.

THE ELASTICITY OF THE LARGE ARTERIES IN HYPERTENSION

1952 ◽  
Vol 30 (2) ◽  
pp. 125-129
Author(s):  
J. P. Adamson ◽  
J. Doupe
Keyword(s):  
Blood Pressure ◽  
Pulse Wave Velocity ◽  
Wave Velocity ◽  
Pulse Wave ◽  
Diastolic Pressure ◽  
Arterial Elasticity ◽  
Large Arteries ◽  
Wave Velocities

Intra-arterial pressures and pulse wave velocities were measured in 18 subjects whose auscultatory diastolic pressures ranged from 45 to 120 mm. Hg. Various methods were used to lower the blood pressure in the hypertensive and to raise it in nonhypertensive subjects so that pulse wave velocities might be compared in all subjects at a common diastolic pressure. The pulse wave velocities were calculated for a diastolic pressure of 80 mm. Hg. No significant differences were found between hypertensive and nonhypertensive subjects. It was concluded that a defect of arterial elasticity as gauged by pulse wave velocity is not a factor in the pathogenesis of hypertension.

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