The relation between seismic P‐ and S‐wave velocity dispersion in saturated rocks

Seismic velocity dispersionin fluid-saturated rocks appears to be dominated by tow mecahnisms: the large scale mechanism modeled by Biot, and the local flow or squirt mecahnism. The tow mechanisms can be distuinguished by the ratio of P-to S-wave dispersions, or more conbeniently, by the ratio of dynamic bulk to shear compliance dispersions derived from the wave velocities. Our formulation suggests that when local flow denominates, the dispersion of the shear compliance will be approximately 4/15 the dispersion of the compressibility. When the Biot mechanism dominates, the constant of proportionality is much smaller. Our examination of ultrasonic velocities from 40 sandstones and granites shows that most, but not all, of the samples were dominated by local flow dispersion, particularly at effective pressures below 40 MPa.

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The effect of melt on seismic anisotropy of ice polycrystalline aggregates

10.5194/egusphere-egu2020-19134 ◽

2020 ◽

Author(s):

Maria-Gema Llorens ◽

Albert Griera ◽

Paul D. Bons ◽

Enrique Gomez-Rivas ◽

Ilka Weikusta ◽

...

Keyword(s):

Wave Velocity ◽

Large Scale ◽

Seismic Anisotropy ◽

Mechanical Response ◽

Seismic Velocity ◽

Solid Phase ◽

Ice Sheets ◽

P Wave ◽

S Wave ◽

Two Phase

Observations of P-wave (Vp) and S-wave (Vs) velocities in Antarctic and Greenland ice sheets show a strong decrease of 25% of Vs in their deep parts, while Vp remains approximately constant. The drastic Vs decrease corresponds to the basal &#8220;echo free zone&#8221;, where large-scale disturbances and strong preferred ice crystal orientation are found. According to Wittlinger and Farra (2014), the low Vs may be due to the presence of unfrozen liquids resulting from pre-melting at grain joints and/or melting of chemical solutions buried in ice. In this contribution we investigate the evolution of seismic velocity anisotropy during deformation of temperate ice by means of microdynamic numerical simulations. Temperate ice is modelled as a two-phase non-linear viscous aggregate constituted by a solid phase (ice polycrystal) and a liquid phase (melt). The viscoplastic full-field numerical approach (VPFFT-ELLE) (Lebensohn and Rollet, 2020) is used to calculate the mechanical response of the two-phase aggregate, which deforms purely by dislocation glide. Viscoplastic deformation is coupled with dynamic recrystallisation processes, such as grain boundary migration, intracrystalline recovery and polygonisation (Llorens et al., 2017), all driven by the reduction of surface and strain energies. The changes in P- and S-wave velocities are calculated with the AEH-EBSD software (Vel et al., 2016) from single crystal stiffness and microstructural measurements of crystal preferred orientations (CPO) during deformation. Regardless the amount of melt and intensity of recrystallisation, all simulations evolve from a fabric defined by randomly oriented c-axes to a c-axis preferred orientation (CPO) distribution approximately perpendicular to the shear plane.&#160; For purely solid aggregates, the results show that the highest Vp and lowest Vs velocities are rapidly aligned with the CPO (at a shear strain of 1), and then evolve to a strong single maximum with progressive deformation. This alignment has been previously predicted in models, experiments and measured in ice core samples. When melt is present, the maximum and minimum seismic velocities are not aligned with the CPO and both Vp and Vs are considerably lower than in cases without melt. &#160;However, if the bulk modulus of ice is assumed for the melt phase, the presence of melt produces a remarkable decrease in S-wave velocity while Vp is maintained constant. These results suggest that the decrease in S-wave velocity observed at the base of ice sheets could be explained by the presence of overpressured melt, which would be unconnected at triple grain junctions in the ice polycrystal.&#160;References:Wittlinger and Farra. 2014. Polar Science 9, 66-79.Lebensohn and Rollet. 2020. Computational Mat. Sci. 173, 109336.Llorens, et al. 2017. Philosophical Transactions of the Royal Society A, 375, 20150346.Vel, et al. 2016. Computer Methods in Applied Mechanics and Engineering 310, 749-779.&#160;

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Antarctic Upper Mantle Rheology

Geological Society London Memoirs ◽

10.1144/m56-2020-19 ◽

2021 ◽

pp. M56-2020-19

Author(s):

E. R. Ivins ◽

W. van der Wal ◽

D. A. Wiens ◽

A. J. Lloyd ◽

L. Caron

Keyword(s):

Wave Velocity ◽

Upper Mantle ◽

Effective Viscosity ◽

Seismic Velocity ◽

Imaging Techniques ◽

Three Dimensional ◽

Velocity Model ◽

Mantle Flow ◽

S Wave ◽

Velocity Changes

AbstractThe Antarctic mantle and lithosphere are known to have large lateral contrasts in seismic velocity and tectonic history. These contrasts suggest differences in the response time scale of mantle flow across the continent, similar to those documented between the northeastern and southwestern upper mantle of North America. Glacial isostatic adjustment and geodynamical modeling rely on independent estimates of lateral variability in effective viscosity. Recent improvements in imaging techniques and the distribution of seismic stations now allow resolution of both lateral and vertical variability of seismic velocity, making detailed inferences about lateral viscosity variations possible. Geodetic and paleo sea-level investigations of Antarctica provide quantitative ways of independently assessing the three-dimensional mantle viscosity structure. While observational and causal connections between inferred lateral viscosity variability and seismic velocity changes are qualitatively reconciled, significant improvements in the quantitative relations between effective viscosity anomalies and those imaged by P- and S-wave tomography have remained elusive. Here we describe several methods for estimating effective viscosity from S-wave velocity. We then present and compare maps of the viscosity variability beneath Antarctica based on the recent S-wave velocity model ANT-20 using three different approaches.

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The Local Earthquake Tomography of Erzurum (Turkey) Geothermal Area

Earth Sciences Research Journal ◽

10.15446/esrj.v23n3.74921 ◽

2019 ◽

Vol 23 (3) ◽

pp. 209-223 ◽

Cited By ~ 2

Author(s):

Caglar Ozer ◽

Mehmet Ozyazicioglu

Keyword(s):

Wave Velocity ◽

Seismic Velocity ◽

Three Dimensional ◽

P Wave ◽

Geothermal Area ◽

S Wave ◽

Velocity Models ◽

Local Earthquake Tomography ◽

P Wave Velocity ◽

Local Earthquake

Erzurum and its surroundings are one of the seismically active and hydrothermal areas in the Eastern part of Turkey. This study is the first approach to characterize the crust by seismic features by using the local earthquake tomography method. The earthquake source location and the three dimensional seismic velocity structures are solved simultaneously by an iterative tomographic algorithm, LOTOS-12. Data from a combined permanent network comprising comprises of 59 seismometers which was installed by Ataturk University-Earthquake Research Center and Earthquake Department of the Disaster and Emergency Management Authority to monitor the seismic activity in the Eastern Anatolia, In this paper, three-dimensional Vp and Vp/Vs characteristics of Erzurum geothermal area were investigated down to 30 km by using 1685 well-located earthquakes with 29.894 arrival times, consisting of 17.298 P- wave and 12.596 S- wave arrivals. We develop new high-resolution depth-cross sections through Erzurum and its surroundings to provide the subsurface geological structure of seismogenic layers and geothermal areas. We applied various size horizontal and vertical checkerboard resolution tests to determine the quality of our inversion process. The basin models are traceable down to 3 km depth, in terms of P-wave velocity models. The higher P-wave velocity areas in surface layers are related to the metamorphic and magmatic compact materials. We report that the low Vp and high Vp/Vs values are observed in Yedisu, Kaynarpinar, Askale, Cimenozu, Kaplica, Ovacik, Yigitler, E part of Icmeler, Koprukoy, Uzunahmet, Budakli, Soylemez, Koprukoy, Gunduzu, Karayazi, Icmesu, E part of Horasan and Kaynak regions indicated geothermal reservoir.

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Velocity dispersion between ultrasonic and seismic frequencies in brine‐saturated reservoir sandstones

Geophysics ◽

10.1190/1.1451700 ◽

2002 ◽

Vol 67 (1) ◽

pp. 254-258 ◽

Cited By ~ 27

Author(s):

Michael S. King ◽

J. Robert Marsden

Keyword(s):

High Porosity ◽

S Wave ◽

Local Flow ◽

Effective Stresses ◽

Wave Velocities ◽

Flow Phenomena ◽

Seismic Frequencies ◽

Reservoir Sandstones ◽

Poroelastic Theory ◽

Low Porosity

Ultrasonic P‐ and S‐wave velocities have been measured on 44 specimens from core samples of relatively clean sandstones recovered from hydrocarbon reservoirs. Ten specimens have porosities less than 10%, and 34 have porosities in the range 20–30%. Velocities were measured with the specimens in both their dry and brine‐saturated states, under hydrostatic effective stresses to 60 MPa for the low‐porosity specimens and to 40 MPa for those of the high‐porosity set. Gassmann's poroelastic theory is found to account for changes in velocity for the low‐porosity set at 60 MPa effective stress when the dry specimens are fully saturated with brine. The velocities used for “dry” specimens in Gassmann's theory must, however, be those measured after the dry specimens have first adsorbed a small quantity of moisture. For saturated specimens at effective stresses of 40 MPa or less, local flow phenomena due to the presence of open microcracks are assumed to be responsible for the measured velocities being appreciably higher than those predicted theoretically.

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Comparison of P‐ and S‐wave velocities and Q’S from VSP and sonic log data

Geophysics ◽

10.1190/1.1443541 ◽

1994 ◽

Vol 59 (10) ◽

pp. 1512-1529 ◽

Cited By ~ 22

Author(s):

Gopa S. De ◽

Donald F. Winterstein ◽

Mark A. Meadows

Keyword(s):

Velocity Dispersion ◽

P Wave ◽

S Wave ◽

S Waves ◽

Wave Velocities ◽

Sonic Log ◽

Transit Times ◽

Wave Slope ◽

Northern Alberta ◽

Q Values

We compared P‐ and S‐wave velocities and quality factors (Q’S) from vertical seismic profiling (VSP) and sonic log measurements in five wells, three from the southwest San Joaquin Basin of California, one from near Laredo, Texas, and one from northern Alberta. Our purpose was to investigate the bias between sonic log and VSP velocities and to examine to what degree this bias might be a consequence of dispersion. VSPs and sonic logs were recorded in the same well in every case. Subsurface formations were predominantly clastic. The bias found was that VSP transit times were greater than sonic log times, consistent with normal dispersion. For the San Joaquin wells, differences in S‐wave transit times averaged 1–2 percent, while differences in P‐wave transit times averaged 6–7 percent. For the Alberta well, the situation was reversed, with differences in S‐wave transit times being about 6 percent, while those for P‐waves were 2.5 percent. For the Texas well, the differences averaged about 4 percent for both P‐ and S‐waves. Drift‐curve slopes for S‐waves tended to be low where the P‐wave slopes were high and vice versa. S‐wave drift‐curve slopes in the shallow California wells were 5–10 μs/ft (16–33 μs/m) and the P‐wave slopes were 15–30 μs/ft (49–98 μs/m). The S‐wave slope in sandstones in the northern Alberta well was up to 50 μs/ft (164 μs/m), while the P‐wave slope was about 5 μs/ft (16 μs/m). In the northern Alberta well the slopes for both P‐ and S‐waves flattened in the carbonate. In the Texas well, both P‐ and S‐wave drifts were comparable. We calculated (Q’s) from a velocity dispersion formula and from spectral ratios. When the two Q’s agreed, we concluded that velocity dispersion resulted solely from absorption. These Q estimation methods were reliable only for Q values smaller than 20. We found that, even with data of generally outstanding quality, Q values determined by standard methods can have large uncertainties, and negative Q’s may be common.

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Ambient lower mantle structure and composition inferred from seismic tomography, convection models, and geochemistry.

10.5194/egusphere-egu2020-11806 ◽

2020 ◽

Author(s):

Grace E. Shephard ◽

John Hernlund ◽

Christine Houser ◽

Reidar Trønnes ◽

Fabio Crameri

Keyword(s):

Lower Mantle ◽

Subduction Zones ◽

Large Scale ◽

Seismic Velocity ◽

Seismic Signal ◽

High Viscosity ◽

S Wave ◽

Velocity Models ◽

Reference Velocity ◽

Seismic Anomalies

The lower mantle can be grouped into high, low, and average (i.e., ambient) seismic velocity domains at each depth, based on the amplitude and polarity of wavespeed perturbations (% &#948;lnVs, % &#948;lnVp). Many studies focus on elucidating the thermo-chemical and structural origins of fast and slow domains, in particular. Subducted slabs are associated with fast seismic anomalies throughout the mantle, and reconstructed palaeo-positions of Cenozoic to Mesozoic subduction zones agrees with seismically imaged deep slabs. Conversely, slow wavespeed domains account for the two antipodal LLSVPs in the lowermost mantle, which are potentially long-lived features, as well as rising hot mantle above the LLSVPs and discrete mantle plumes. However, low-amplitude wavespeeds (close to the reference velocity models) are often overlooked By comparing multiple P- and S-wave tomographic models individually, and through &#8220;vote maps&#8221;, we reveal the depth-dependent characteristics and the geometry of ambient structures, and compare them to numerical convection models. The ambient velocity domains may contain early refractory and bridgmantic mantle with elevated Si/(Mg+Fe) and Mg/Fe ratios (BEAMS; bridgmanite-enriched mantle structures). They could have formed by early basal magma ocean (BMO) fractionation during a period of core-BMO exchange of SiO2 (from core to BMO) and FeO (from BMO to core), or represent cumulates of BMO crystallization with bridgmanite as the liquidus phase. The high viscosity of bridgmanitic material may promote its convective aggregation and stabilise the large-scale, degree-2 convection pattern. Despite its high viscosity, bridgmanitic material, representing a primitive and refractory reservoir for primordial-like He and Ne components, might be entrained in vigorous, deep-rooted plumes. The restriction of a weak seismic signal, ascribed to iron spin-pairing in ferropericlase, to the fast and slow domains, supports the notion that the ambient lower mantle domains are bridgmanitic.

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Wavefield simulation and data-acquisition-scheme analysis for LWD acoustic tools in very slow formations

Geophysics ◽

10.1190/1.3552929 ◽

2011 ◽

Vol 76 (3) ◽

pp. E59-E68 ◽

Cited By ~ 23

Author(s):

Hua Wang ◽

Guo Tao

Keyword(s):

Wave Velocity ◽

Cutoff Frequency ◽

P Wave ◽

Flexural Wave ◽

S Wave ◽

Low Frequencies ◽

Wave Velocities ◽

Dipole System ◽

P Wave Velocity ◽

Wavefield Simulation

Propagating wavefields from monopole, dipole, and quadrupole acoustic logging-while-drilling (LWD) tools in very slow formations have been studied using the discrete wavenumber integration method. These studies examine the responses of monopole and dipole systems at different source frequencies in a very slow surrounding formation, and the responses of a quadrupole system operating at a low source frequency in a slow formation with different S-wave velocities. Analyses are conducted of coherence-velocity/slowness relationships (semblance spectra) in the time domain and of the dispersion characteristics of these waveform signals from acoustic LWD array receivers. These analyses demonstrate that, if the acoustic LWD tool is centralized properly and is operating at low frequencies (below 3 kHz), a monopole system can measure P-wave velocity by means of a “leaky” P-wave for very slow formations. Also, for very slow formations a dipole system can measure the P-wave velocity via a leaky P-wave and can measure the S-wave velocity from a formation flexural wave. With a quadrupole system, however, the lower frequency limit (cutoff frequency) of the drill-collar interference wave would decrease to 5 kHz and might no longer be neglected if the surrounding formation becomes a very slow formation, with S-wave velocities at approximately 500 m/s.

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Acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection

Geophysics ◽

10.1190/1.1468600 ◽

2002 ◽

Vol 67 (2) ◽

pp. 405-412 ◽

Cited By ~ 55

Author(s):

Manika Prasad

Keyword(s):

Wave Velocity ◽

P Wave ◽

Acoustic Measurements ◽

S Wave ◽

Deepwater Drilling ◽

Unconsolidated Sands ◽

Wave Velocities ◽

P Wave Velocity ◽

Shallow Water Flows ◽

Low Pressures

Shallow water flows and over‐pressured zones are a major hazard in deepwater drilling projects. Their detection prior to drilling would save millions of dollars in lost drilling costs. I have investigated the sensitivity of seismic methods for this purpose. Using P‐wave information alone can be ambiguous, because a drop in P‐wave velocity (Vp) can be caused both by overpressure and by presence of gas. The ratio of P‐wave velocity to S‐wave velocity (Vp/Vs), which increases with overpressure and decreases with gas saturation, can help differentiate between the two cases. Since P‐wave velocity in a suspension is slightly below that of the suspending fluid and Vs=0, Vp/Vs and Poisson's ratio must increase exponentially as a load‐bearing sediment approaches a state of suspension. On the other hand, presence of gas will also decrease Vp but Vs will remain unaffected and Vp/Vs will decrease. Analyses of ultrasonic P‐ and S‐wave velocities in sands show that the Vp/Vs ratio, especially at low effective pressures, decreases rapidly with pressure. At very low pressures, Vp/Vs values can be as large as 100 and higher. Above pressures greater than 2 MPa, it plateaus and does not change much with pressure. There is significant change in signal amplitudes and frequency of shear waves below 1 MPa. The current ultrasonic data shows that Vp/Vs values can be invaluable indicators of low differential pressures.

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Effects of fluid-shear resistance and squirt flow on velocity dispersion in rocks

Geophysics ◽

10.1190/geo2014-0304.1 ◽

2015 ◽

Vol 80 (2) ◽

pp. D99-D110 ◽

Cited By ~ 11

Author(s):

Nishank Saxena ◽

Gary Mavko

Keyword(s):

Velocity Dispersion ◽

Seismic Velocity ◽

Laboratory Data ◽

Elastic Solid ◽

High Viscosity ◽

New Method ◽

Biot Theory ◽

Local Flow ◽

Shear Relaxation ◽

High Frequency Effects

Laboratory measurements of rocks saturated with high-viscosity fluids (such as heavy-oil, bitumen, magma, kerogen, etc.) often exhibit considerable seismic velocity dispersion, which is usually underestimated by the Biot theory. Over the years, grain-scale dispersion mechanisms such as squirt (local-flow) and shear relaxation (nonzero shear stress in the pore fluid) have been more successful in explaining the measured dispersion. We developed a new method to quantify the combined high-frequency effects of squirt and shear dispersion on the effective moduli of rocks saturated with viscous fluids. Viscous fluid at high frequencies was idealized as an elastic solid of finite shear modulus, hydraulically locked in stiff and soft pores. This method entailed performing solid substitution in stiff pores of a dry rock frame, which itself was unrelaxed due to solid-filled soft pores. The unrelaxed frame stiffness solutions required information on the pressure dependency of the rock stiffness and porosity. This method did not have any adjustable parameters, and all required inputs can be directly measured. With various laboratory and numerical examples, we noted that accounting for combined effects of squirt and shear relaxation was necessary to explain laboratory-measured velocities of rocks saturated with fluids of high viscosity. Predictions of the new method were in good agreement with the laboratory data.

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Measurement of the shear slowness of slow formations from monopole logging-while-drilling sonic logs

Geophysics ◽

10.1190/geo2019-0236.1 ◽

2019 ◽

Vol 85 (1) ◽

pp. D45-D52

Author(s):

Yuanda Su ◽

Xinding Fang ◽

Xiaoming Tang

Keyword(s):

Numerical Modeling ◽

Wave Velocity ◽

Fluid Velocity ◽

S Wave ◽

Data Set ◽

Acoustic Logging ◽

Wave Velocities ◽

Logging While Drilling ◽

Sonic Logs ◽

Refracted Waves

Acoustic logging-while-drilling (LWD) is used to measure formation velocity/slowness during drilling. In a fast formation, in which the S-wave velocity is higher than the borehole-fluid velocity, monopole logging can be used to obtain P- and S-wave velocities by measuring the corresponding refracted waves. In a slow formation, in which the S-wave velocity is less than the borehole-fluid velocity, because the fully refracted S-wave is missing, quadrupole logging has been developed and used for S-wave slowness measurement. A recent study based on numerical modeling implies that monopole LWD can generate a detectable transmitted S-wave in a slow formation. This nondispersive transmitted S-wave propagates at the formation S-wave velocity and thus can be used for measuring the S-wave slowness of a slow formation. We evaluate a field example to demonstrate the applicability of monopole LWD in determining the S-wave slowness of slow formations. We compare the S-wave slowness extracted from a monopole LWD data set acquired in a slow formation and the result derived from the quadrupole data recorded in the same logging run. The results indicated that the S-wave slowness can be reliably determined from monopole LWD sonic data in fairly slow formations. However, we found that the monopole approach is not applicable to very slow formations because the transmitted S-wave becomes too weak to detect when the formation S-wave slowness is much higher than the borehole-fluid slowness.

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