Seismic velocities of unconsolidated sands: Part 2 — Influence of sorting- and compaction-induced porosity variation

Geophysics ◽  
2007 ◽  
Vol 72 (1) ◽  
pp. E15-E25 ◽  
Author(s):  
Michael A. Zimmer ◽  
Manika Prasad ◽  
Gary Mavko ◽  
Amos Nur
Keyword(s):  
Compressional Wave ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Seismic Velocities ◽  
Porosity Variation ◽  
Unconsolidated Sands ◽  
Wave Velocities ◽  
Grain Size Distributions ◽  
Gassmann Fluid Substitution ◽  
Water Saturated

Unaccounted-for porosity variation in unconsolidated sediments can cloud the interpretation of the sediment’s seismic velocities for factors such as fluid content and pressure. However, an understanding of the effects of porosity variation on the velocities can permit the remote characterization of porosity with seismic methods. We present the results of a series of measurements designed to isolate the effects of sorting- and compaction-induced porosity variation on the seismic velocities and their pressure dependences in clean, unconsolidated sands. We prepared a set of texturally similar sand and glass-bead samples with controlled grain-size distributions to cover an initial porosity range from 0.26 to 0.44. We measured the compressional- and shear-wave velocities and porosity of dry samples over a series of hydrostatic pressure cycles from [Formula: see text]. Over this rangeof porosities, the velocities of the dry samples at a given pressure vary by [Formula: see text]. However, the water-saturated compressional-wave velocities, modeled with Gassmann fluid substitution, demonstrate a consistent increase with decreasing porosity. In both the dry and water-saturated cases, the porosity trend at a given pressure is approximately described by the isostress (harmonic) average between the moduli of the highest-porosity sample at that pressure and the moduli of quartz, the predominant mineral component of the samples. Empirical power-law fit coefficients describing the pressure dependences of the dry bulk, shear, and constrained (P-wave) moduli from each sample also demonstrate no significant, systematic relationship with the porosity. The porosity dependence of the water-saturated bulk and constrained moduli is primarily contained in the empirical coefficient representing the modulus at zero pressure.

Seismic velocities of unconsolidated sands: Part 1 — Pressure trends from 0.1 to 20 MPa

Geophysics ◽  
2007 ◽  
Vol 72 (1) ◽  
pp. E1-E13 ◽  
Author(s):  
Michael A. Zimmer ◽  
Manika Prasad ◽  
Gary Mavko ◽  
Amos Nur
Keyword(s):  
Shear Wave ◽  
Pressure Dependence ◽  
Glass Bead ◽  
Theoretical Models ◽  
Compressional Wave ◽  
P Wave ◽  
Seismic Velocities ◽  
Unconsolidated Sands ◽  
Wave Velocities ◽  
Shear Wave Velocities

Knowledge of the pressure dependences of seismic velocities in unconsolidated sands is necessary for the remote prediction of effective pressures and for the projection of velocities to unsampled locations within shallow sand layers. We have measured the compressional- and shear-wave velocities and bulk, shear, and P-wave moduli at pressures from [Formula: see text] in a series of unconsolidated granular samples including dry and water-saturated natural sands and dry synthetic sand and glass-bead samples. The shear-wave velocities in these samples demonstrate an average pressure dependence approximately proportional to the fourth root of the effective pressure [Formula: see text], as commonly observed at lower pressures. For the compressional-wave velocities, theexponent in the pressure dependence of individual dry samples is consistently less than the exponent for the shear-wave velocity of the same sample, averaging 0.23 for the dry sands and 0.20 for the glass-bead samples. These pressure dependences are generally consistent over the entire pressure range measured. A comparison of the empirical results to theoretical predictions based on Hertz-Mindlin effective-medium models demonstrates that the theoretical models vastly overpredict the shear moduli of the dry granular frame unless the contacts are assumed to have no tangential stiffness. The models also predict a lower pressure exponent for the moduli and velocities [Formula: see text] than is generally observed in the data. We attribute this discrepancy in part to the inability of the models to account for decreases in the amount of slip or grain rotation occurring at grain-to-grain contacts with increasing pressure.

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.

Seismic velocities and electrical resistivity of recent volcanics and their dependence on porosity, temperature, and water saturation

Geophysics ◽  
1981 ◽  
Vol 46 (10) ◽  
pp. 1415-1422 ◽  
Author(s):  
A. W. Ibrahim ◽  
George V. Keller
Keyword(s):  
Water Saturation ◽  
Strong Dependence ◽  
Electric Resistivity ◽  
P Wave ◽  
Seismic Velocities ◽  
Fine Grained ◽  
Wave Velocities ◽  
Electrical Resistivities ◽  
P Wave Velocities ◽  
Water Saturated

Variation of P‐wave velocities and electrical resistivities of several suites of water‐saturated recent volcanics was investigated. Both P‐velocities and resistivities exhibited strong dependence on porosity. Resistivity was also dependent upon degree of water saturation and temperature. P‐wave velocities, while showing a strong dependence on porosity, appear to be independent of water saturation and temperature. Volcanics, in general, exhibit higher resistivities compared to other igneous rocks and sediments. Electric resistivity of fine‐grained basalts is anomalously low, probably due to higher content of disseminated iron. Pyroclastics and volcanic breccia, on the other hand, exhibit higher resistivities in relation to fine‐grained basalts.

Seismic Velocities and Attenuation from Borehole Measurements near the Parkfield Prediction Zone, Central California

1989 ◽  
Vol 5 (3) ◽  
pp. 513-537 ◽  
Author(s):  
James F. Gibbs ◽  
Edward F. Roth
Keyword(s):  
P Wave ◽  
Depth Interval ◽  
Seismic Velocities ◽  
S Wave ◽  
Wave Source ◽  
Central California ◽  
Unconsolidated Sands ◽  
Wave Velocities ◽  
Average Shear ◽  
Parkfield Earthquake

Shear (S)- and compressional (P)- wave velocities were measured to a depth of 195 m in a borehole near the San Andreas fault where a recurrence of a moderate Parkfield earthquake is predicted. S-wave velocities determined from orthogonal directions of the S-wave source show velocity differences of approximately 20 percent. An average shear-wave Q of 4 was determined in relatively unconsolidated sands and gravels of the Paso Robles Formation in the depth interval 57.5-102.5 m.

Seismic mapping and modeling of near‐surface sediments in polar areas

Geophysics ◽  
2003 ◽  
Vol 68 (2) ◽  
pp. 566-573 ◽  
Author(s):  
Tor Arne Johansen ◽  
Per Digranes ◽  
Mark van Schaack ◽  
Ida Lønne
Keyword(s):  
Water Saturation ◽  
Geological Mapping ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Polar Regions ◽  
Seismic Velocities ◽  
S Wave ◽  
Delta Front ◽  
Near Surface ◽  
Water Saturated

A knowledge of permafrost conditions is important for planning the foundation of buildings and engineering activities at high latitudes and for geological mapping of sediment thicknesses and architecture. The freezing of sediments is known to greatly affect their seismic velocities. In polar regions the actual velocities of the upper sediments may therefore potentially reveal water saturation and extent of freezing. We apply various strategies for modeling seismic velocities and reflectivity properties of unconsolidated granular materials as a function of water saturation and freezing conditions. The modeling results are used to interpret a set of high‐resolution seismic data collected from a glaciomarine delta at Spitsbergen, the Norwegian Arctic, where the upper subsurface sediments are assumed to be in transition from unfrozen to frozen along a transect landward from the delta front. To our knowledge, this is the first attempt to study pore‐fluid freezing from such data. Our study indicates that the P‐ and S‐wave velocities may increase as much as 80–90% when fully, or almost fully, water‐saturated unconsolidated sediments freeze. Since a small amount of frozen water in the voids of a porous rock can lead to large velocity increases, the freezing of sediments reduces seismic resolution; thus, the optimum resolution is obtained at locations where the sediments appear unfrozen. The reflectivity from boundaries separating sediments of slightly different porosity may depend more strongly on the actual saturation rather than changes in granular characteristics. For fully water‐saturated sediments, the P‐wave reflectivity decreases sharply with freezing, while the reflectivity becomes less affected as the water saturation is lowered. Thus, a combination of velocity and reflectivity information may reveal saturation and freezing conditions.

scholarly journals Subsonic near-surface P-velocity and low S-velocity observations using propagator inversion

Geophysics ◽  
2005 ◽  
Vol 70 (4) ◽  
pp. R15-R23 ◽  
Author(s):  
Robbert van Vossen ◽  
Andrew Curtis ◽  
Jeannot Trampert
Keyword(s):  
Shear Velocity ◽  
Confining Pressure ◽  
Compressional Wave ◽  
P Wave ◽  
Soil Conditions ◽  
Detailed Knowledge ◽  
S Wave ◽  
Near Surface ◽  
Unconsolidated Sands ◽  
Wave Velocities

Detailed knowledge of near-surface P- and S-wave velocities is important for processing and interpreting multicomponent land seismic data because (1) the entire wavefield passes through and is influenced by the near-surface soil conditions, (2) both source repeatability and receiver coupling also depend on these conditions, and (3) near-surface P- and S-wave velocities are required for wavefield decomposition and demultiple methods. However, it is often difficult to measure these velocities with conventional techniques because sensitivity to shallow-wave velocities is low and because of the presence of sharp velocity contrasts or gradients close to the earth's free surface. We demonstrate that these near-surface P- and S-wave velocities can be obtained using a propagator inversion. This approach requires data recorded by at least one multicomponent geophone at the surface and an additional multicomponent geophone at depth. The propagator between them then contains all information on the medium parameters governing wave propagation between the geophones at the surface and at depth. Hence, inverting the propagator gives local estimates for these parameters. This technique has been applied to data acquired in Zeist, the Netherlands. The near-surface sediments at this site are unconsolidated sands with a thin vegetation soil on top, and the sediments considered are located above the groundwater table. A buried geophone was positioned 1.05 m beneath receivers on the surface. Propagator inversion yielded low near-surface velocities, namely, 270 ± 15 m/s for the compressional-wave velocity, which is well below the sound velocity in air, and 150 ± 9 m/s for the shear velocity. Existing methods designed for imaging deeper structures cannot resolve these shallow material properties. Furthermore, velocities usually increase rapidly with depth close to the earth's surface because of increasing confining pressure. We suspect that for this reason, subsonic near-surface P-wave velocities are not commonly observed.

Constraining helicopter electromagnetic models of the Okavango Delta with seismic-refraction and seismic-reflection data

Geophysics ◽  
2014 ◽  
Vol 79 (3) ◽  
pp. B123-B134 ◽  
Author(s):  
Fabienne Reiser ◽  
Joel E. Podgorski ◽  
Cedric Schmelzbach ◽  
Heinrich Horstmeyer ◽  
Alan G. Green ◽  
...  
Keyword(s):  
Seismic Reflection ◽  
Lower Layer ◽  
Seismic Refraction ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Saturated Sand ◽  
Resistive Layer ◽  
Wave Velocities ◽  
Reflection Data ◽  
Water Saturated

Electrical resistivity models derived from exceptionally high-quality helicopter transient electromagnetic data recorded across the Okavango Delta in Botswana, one of the world’s great inland deltas or megafans, include three principal layers: (1) an upper heterogeneous layer of dry and water-saturated sand, (2) an intermediate electrically conductive layer that likely comprises saline-water-saturated sand and clay, and (3) a lower fan-shaped electrically resistive layer of freshwater-saturated sand/gravel and/or crystalline basement. If part of the lower layer comprises a freshwater aquifer, it would be evidence for a recently proposed Paleo Okavango Megafan and a major new source of freshwater. In an attempt to constrain the interpretation of the lower layer, we acquired two high-resolution seismic refraction and reflection data sets at each of two investigation sites: one near the center of the delta and one along its western edge. The interface between unconsolidated sediments and basement near the center of the delta is well defined by an [Formula: see text] to [Formula: see text] increase in P-wave velocities, a change in seismic reflection facies, and a strong continuous reflection. This interface is about 45 m deeper than the top of the lower resistive layer, thus providing support for the Paleo Okavango Megafan hypothesis. Subhorizontal seismic reflectors are additional evidence for a sedimentary origin of the upper part of the lower resistive layer. In contrast to the observations at the delta’s center, the interface between unconsolidated sediments and basement along its western edge, which is also defined by a [Formula: see text] to [Formula: see text] increase in P-wave velocities and a continuous reflection, coincides with the top of the resistive layer.

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

Geophysics ◽  
2002 ◽  
Vol 67 (2) ◽  
pp. 405-412 ◽  
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.

Reduced seismic velocities in the source zone of New Madrid earthquakes

1988 ◽  
Vol 78 (4) ◽  
pp. 1491-1509
Author(s):  
Haydar J. Al-Shukri ◽  
Brian J. Mitchell
Keyword(s):  
Compressional Wave ◽  
External Pressure ◽  
P Wave ◽  
Source Zone ◽  
Fault System ◽  
Earthquake Activity ◽  
Depth Range ◽  
Seismic Velocities ◽  
Crustal Rocks ◽  
New Madrid

Abstract A three-dimensional inversion of P-wave travel-time residuals from local earthquakes reveals a remarkable pattern of low seismic velocities in crustal rocks immediately adjacent to the active portions of the New Madrid fault system. Seismic velocities are lowest in regions of greatest concentration of earthquake activity near two intersections of linear trends in seismicity. The maximum reduction in compressional wave velocity is at least 7 per cent in the upper 5 km of the crust and at least 4 per cent in the depth range of 5 to 14 km. The reductions are consistent with a velocity decrease which would be expected if crustal rocks in the source zone contain fluid-filled cracks in which pore pressure is a substantial fraction of external pressure. The presence or absence of such fluids may explain why some portions of the faults in and surrounding the upper Mississippi Embayment are active while others are not.

Reflection and transmission of waves at a fluid/porous‐medium interface

Geophysics ◽  
2002 ◽  
Vol 67 (1) ◽  
pp. 282-291 ◽  
Author(s):  
Arthur I. M. Denneman ◽  
Guy G. Drijkoningen ◽  
David M. J. Smeulders ◽  
Kees Wapenaar
Keyword(s):  
Porous Medium ◽  
P Wave ◽  
Reflection And Transmission Coefficients ◽  
Sand Layer ◽  
Reflection And Transmission ◽  
Transmission Coefficients ◽  
Wave Velocities ◽  
Medium Interface ◽  
Water Saturated ◽  
Silt Layer

We study the wave properties at a fluid/porous‐medium interface by using newly derived closed‐form expressions for the reflection and transmission coefficients. We illustrate the usefulness of these relatively simple expressions by applying them to a water/porous‐medium interface (with open‐pore or sealed‐pore boundary conditions), where the porous medium consists of (1) a water‐saturated clay/silt layer, (2) a water‐saturated sand layer, (3) an air‐filled clay/silt layer, or (4) an air‐filled sand layer. We observe in the frequency range 5 Hz–20 kHz that the fast P‐wave and S‐wave velocities in the four porous materials are indistinguishable from the corresponding frequency‐independent ones calculated using Gassmann relations. Consequently, for these frequencies we would expect the reflection and transmission coefficients for the four water/porous‐medium interfaces to be similar to the ones for corresponding interfaces between water and effective elastic media (described by Gassmann wave velocities). This expectation is not fulfilled in the case of an interface between water and an air‐filled porous layer with open pores. A close examination of the expressions for the reflection and transmission coefficients shows that this unexpected result is because of the large density difference between water and air.

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