Predicting S-wave velocities for unconsolidated sediments at low effective pressure

The elastic properties of sand strongly depend on the grains’ contact stiffness, which can be increased significantly by solid matter and, depending on frequency, viscous fluid acting as contact cement. To calculate seismic velocities in precompacted fluid-cemented sand, we examine how a small amount of viscous fluid at the grain contacts influences their normal and tangential stiffnesses as a function of effective pressure. Using the Hertz-Mindlin approach and considering oscillatory loading in addition to precompaction of a combination of two elastic spheres, we extend the dry-contact elastic theory by a viscoelastic formulation. Here, we describe the radial flow of the fluid cement induced by the oscillations of the grains’ surfaces around the direct contact, a process that leads to a complex normal stiffness and stiffness/frequency dispersion. In the resulting combined model, the low-frequency real part of the complex normal stiffness identical is to the original Hertz-Mindlin expression. The magnitude of the dispersion is governed by the amount of viscous cement; magnitude decreases as effective pressure increases. The frequency of the maximum imaginary part of the normal stiffness is determined mainly by cement viscosity and contact geometry. The tangential contact stiffness virtually is not influenced by the viscous fluid. Comparison of predicted results with data from pulse transmission experiments (500 kHz) on glass beads with two different fluids shows an excellent fit in P-wave velocities [Formula: see text], whereas S-wave velocities [Formula: see text] are systematically overestimated by the model. The experimental results confirm, however, the predicted change with effective pressure in the [Formula: see text] ratio for both examined cases as well as reflect the predicted increase in [Formula: see text] and [Formula: see text], respectively, between the two cases. This implies that our viscoelastic formulation represents a reasonable way to describe the role of viscous cement in sand.

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Improved granular medium model for unconsolidated sands using coordination number, porosity, and pressure relations

Geophysics ◽

10.1190/1.3333539 ◽

2010 ◽

Vol 75 (2) ◽

pp. E91-E99 ◽

Cited By ~ 29

Author(s):

Tanima Dutta ◽

Gary Mavko ◽

Tapan Mukerji

Keyword(s):

Coordination Number ◽

Granular Media ◽

Effective Medium ◽

Unconsolidated Sediments ◽

Data Sets ◽

S Wave ◽

Unconsolidated Sands ◽

Wave Velocities ◽

Effective Medium Model ◽

Medium Model

We have developed a recipe for using closed-form expressions of effective-medium models to predict velocities in unconsolidated sandstones. The commonly used Hertz-Mindlin effective-medium model for granular media often predicts elastic wave velocities that are higher, and [Formula: see text] ratios that are lower, than those observed in laboratory and well log measurements in unconsolidated sediments. We use the extended Walton model, which introduces a parameter [Formula: see text] to represent the fraction of grain contacts that are perfectly adhered. Using the extended Walton model with [Formula: see text] ranging from 0.3 to 1, we obtain new empirical relations between the coordination number (C), porosity, and pressure for P- and S-wave velocities by inverting dynamic measurements on dry, unconsolidated sands. We propose using the extended Walton model [Formula: see text] along with these new C-porosity and C-pressure relations to study the mechanical compaction of unconsolidated sandstones. The model has been tested on two experimental data sets. It provides a reasonable fit to observed P- and S-wave velocities and specifically improves shear-wave predictions.

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A unified effective medium model for gas hydrates in sediments

Geophysics ◽

10.1190/geo2017-0513.1 ◽

2018 ◽

Vol 83 (6) ◽

pp. MR317-MR332 ◽

Cited By ~ 2

Author(s):

Darrell A. Terry ◽

Camelia C. Knapp

Keyword(s):

Friction Coefficient ◽

Gas Hydrates ◽

Effective Medium ◽

Unconsolidated Sediments ◽

Sphere Model ◽

S Wave ◽

Wave Velocities ◽

Effective Medium Model ◽

Medium Model ◽

Rough Sphere

A unified effective medium model is developed to incorporate the endpoints of perfectly smooth and infinitely rough sphere components and to allow partitioning between rough and smooth grains. We incorporate the unified model into the framework for gas hydrates in unconsolidated sediments using pore-fluid and rock-matrix configurations for grain placement, while reviewing other developments that have taken place in the past four decades. The unified rock-matrix model is validated with data available from the 2002 Mallik gas hydrates project well 5L-38. Gas-hydrate saturation and neutron-porosity logs from this well are used to generate synthetic P- and S-wave velocity models for several values of the friction coefficient. First, we overlaid crossplots of P- versus S-wave velocities for synthetic and measured velocities, and we compared the match until a good choice was found for the friction coefficient. Second, we plotted the synthetic velocities as separate logs of P- and S-wave velocities for each friction coefficient; the synthetic velocity logs were then overlaid on the measured velocities calculated from the sonic logs. Results of a direct comparison of the synthetic and measured velocity logs provide valuable insights into the validation of the unified effective medium model. Recognizing the significance of the Hertz-Mindlin-type effective medium models for gas hydrates in unconsolidated sediments, we incorporate the previous efforts into a single “unified” model and define a common nomenclature. Although we attempt to assign a single friction coefficient value to each hydrate window, it is not surprising that in a real and heterogeneous environment, the value might vary with depth, as it does here at the larger spatial scales. We determine and quantitatively estimate that gas hydrates in sediments are well-predicted with a friction coefficient closer to a smooth sphere model than a rough sphere model.

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Using P- to S- wave conversions from controlled sources to determine the shear-wave velocity structure along Hikurangi Margin Forearc, New Zealand

10.5194/egusphere-egu2020-11940 ◽

2020 ◽

Author(s):

Pasan Herath ◽

Tim Stern ◽

Martha Savage ◽

Dan Bassett ◽

Stuart Henrys ◽

...

Keyword(s):

New Zealand ◽

Pore Pressure ◽

Shear Wave ◽

P Wave ◽

Unconsolidated Sediments ◽

Eigenvalue Decomposition ◽

Slow Slip ◽

S Wave ◽

Wave Velocities ◽

Basement Rocks

The Hikurangi subduction margin offshore of the east coast of New Zealand displays along-strike variations in subduction-thrust slip behavior. Geodetic observations show that the subduction-thrust of the southern segment of the margin is locked on the 30-100 year scale and the northern segment displays periodic slow-slip on the 1-2 year scale. It is hypothesised that spatial variations in pore-pressure may play a role in this contrasting phenomenon. Higher pore-pressures would result in lower effective stresses, which promote slow-slip of the subduction-thrust. In addition, the presence of a sedimentary wedge with very low shear wave-speeds in the northern Hikurangi margin has been proposed to fit the ultra-long duration of ground motions observed following the 2016 Kaikoura earthquake. Compressional (P-) wave velocities (Vp) of the subsurface provide useful information about the lithological composition. Combined with shear (S-) wave velocities (Vs), the Vp/Vs ratio which is directly related to Poisson&#8217;s ratio can be obtained. This is a diagnostic property of a rock&#8217;s consolidation and porosity. Typical Vp/Vs ratio of consolidated and crystalline rocks range from 1.6 to 1.9 and that of unconsolidated sediments can range from 2.0 to 4.0.We use the controlled sources of R/V Marcus G Langseth recorded by a profile of 49 multi-component ocean bottom seismometers (OBS) along the Hikurangi margin forearc for the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) to derive the Vs structure and estimate the Vp/Vs ratio. The orientations of the horizontal components of each OBS are found by a hodogram analysis and by an eigenvalue-decomposition of the covariance matrix. Using the orientations, the horizontal components of each OBS are rotated into radial and transverse components. P to S converted phases are identified on the radial and transverse components considering their linear moveout, polarisation angle, and ellipticity. We confirm incoming S-waves to OBSs by comparing them with their hydrophone components. We identify both PPS (up-going P-wave after reflection or refraction converts to an S-wave at an interface) and PSS (down-going P-wave from the controlled source converts to an S-wave at an interface) type conversions. The identified conversion interfaces are the sediment-basement interface and the top of the subducting crust. The travel-time delay of a PPS type conversion relative to its P-wave arrival is indicative of Vs above the converting interface. The linear-moveout of PSS type conversions are indicative of Vs along the raypath after the conversion. Preliminary results from the southern Hikurangi margin suggest Vp/Vs ratios of ~1.70 for the basement rocks above the subducting crust and ~1.90 for the sediments overlying the basement rocks. These values indicate that the basement rocks are consolidated and less porous than the overlying sediments.We expect to estimate the Vp/Vs ratios in the northern Hikurangi margin to assess the role played by pore-pressure in the along-strike variation in subduction-thrust slip behavior. We also expect to ascertain the presence and estimate the thickness of the low-velocity sediment wedge in the northern Hikurangi margin.

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A simple method of predicting S-wave velocity

Geophysics ◽

10.1190/1.2357833 ◽

2006 ◽

Vol 71 (6) ◽

pp. F161-F164 ◽

Cited By ~ 37

Author(s):

Myung W. Lee

Keyword(s):

Wave Velocity ◽

P Wave ◽

Unconsolidated Sediments ◽

S Wave ◽

Amplitude Analysis ◽

Simple Method ◽

Low Frequencies ◽

Wave Velocities ◽

Aspect Ratios ◽

P Wave Velocity

Prediction of shear-wave velocity plays an important role in seismic modeling, amplitude analysis with offset, and other exploration applications. This paper presents a method for predicting S-wave velocity from the P-wave velocity on the basis of the moduli of dry rock. Elastic velocities of water-saturated sediments at low frequencies can be predicted from the moduli of dry rock by using Gassmann’s equation; hence, if the moduli of dry rock can be estimated from P-wave velocities, then S-wave velocities easily can be predicted from the moduli. Dry rock bulk modulus can be related to the shear modulus through a compaction constant. The numerical results indicate that the predicted S-wave velocities for consolidated and unconsolidated sediments agree well with measured velocities if differential pressure is greater than approximately [Formula: see text]. An advantage of this method is that there are no adjustable parameters to be chosen, such as the pore-aspect ratios required in some other methods. The predicted S-wave velocity depends only on the measured P-wave velocity and porosity.

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

Geophysics ◽

10.1190/1.1443537 ◽

1994 ◽

Vol 59 (1) ◽

pp. 87-92 ◽

Cited By ~ 35

Author(s):

Gary Mavko ◽

Diane Jizba

Keyword(s):

Wave Velocity ◽

Large Scale ◽

Velocity Dispersion ◽

Seismic Velocity ◽

Ultrasonic Velocities ◽

S Wave ◽

Local Flow ◽

Wave Velocities ◽

Shear Compliance

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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Nonlinear behavior of soil sediments identified by using borehole records observed at the Ashigra Valley, Japan

Bulletin of the Seismological Society of America ◽

10.1785/bssa0850061821 ◽

1995 ◽

Vol 85 (6) ◽

pp. 1821-1834

Author(s):

Toshimi Satoh ◽

Toshiaki Sato ◽

Hiroshi Kawase

Keyword(s):

Shear Strain ◽

Main Part ◽

Nonlinear Behavior ◽

Strong Motion ◽

S Wave ◽

Ground Shaking ◽

Wave Velocities ◽

The One ◽

Soil Sediments ◽

Strong Ground

Abstract We evaluate the nonlinear behavior of soil sediments during strong ground shaking based on the identification of their S-wave velocities and damping factors for both the weak and strong motions observed on the surface and in a borehole at Kuno in the Ashigara Valley, Japan. First we calculate spectral ratios between the surface station KS2 and the borehole station KD2 at 97.6 m below the surface for the main part of weak and strong motions. The predominant period for the strong motion is apparently longer than those for the weak motions. This fact suggests the nonlinearity of soil during the strong ground shaking. To quantify the nonlinear behavior of soil sediments, we identify their S-wave velocities and damping factors by minimizing the residual between the observed spectral ratio and the theoretical amplification factor calculated from the one-dimensional wave propagation theory. The S-wave velocity and the damping factor h (≈(2Q)−1) of the surface alluvial layer identified from the main part of the strong motion are about 10% smaller and 50% greater, respectively, than those identified from weak motions. The relationships between the effective shear strain (=65% of the maximum shear strain) calculated from the one-dimensional wave propagation theory and the shear modulus reduction ratios or the damping factors estimated by the identification method agree well with the laboratory test results. We also confirm that the soil model identified from a weak motion overestimates the observed strong motion at KS2, while that identified from the strong motion reproduces the observed. Thus, we conclude that the main part of the strong motion, whose maximum acceleration at KS2 is 220 cm/sec2 and whose duration is 3 sec, has the potential of making the surface soil nonlinear at an effective shear strain on the order of 0.1%. The S-wave velocity in the surface alluvial layer identified from the part just after the main part of the strong motion is close to that identified from weak motions. This result suggests that the shear modulus recovers quickly as the shear strain level decreases.

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Jacobian matrix for the inversion of P- and S-wave velocities and its accurate computation method

Science China Earth Sciences ◽

10.1007/s11430-010-4124-7 ◽

2010 ◽

Vol 54 (5) ◽

pp. 647-654 ◽

Cited By ~ 6

Author(s):

FuPing Liu ◽

XianJun Meng ◽

YuMei Wang ◽

GuoQiang Shen ◽

ChangChun Yang

Keyword(s):

Jacobian Matrix ◽

Computation Method ◽

S Wave ◽

Wave Velocities ◽

Accurate Computation

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