The relative behavior of shear velocity, bulk sound speed, and compressional velocity in the mantle: Implications for chemical and thermal structure

2000 ◽  
pp. 63-87 ◽  
Author(s):  
Guy Masters ◽  
Gabi Laske ◽  
Harold Bolton ◽  
Adam Dziewonski
Keyword(s):  
Sound Speed ◽  
Thermal Structure ◽  
Shear Velocity ◽  
Compressional Velocity

Ambiguities in AVO inversion of reflections from a gas‐sand

Geophysics ◽  
1995 ◽  
Vol 60 (1) ◽  
pp. 134-141 ◽  
Author(s):  
Giuseppe Drufuca ◽  
Alfredo Mazzotti
Keyword(s):  
Unique Solution ◽  
Incidence Angle ◽  
Synthetic Data ◽  
Real Data ◽  
Shear Velocity ◽  
Sand Layer ◽  
Borehole Data ◽  
Starting Point ◽  
Wide Range ◽  
Compressional Velocity

We examine the reflections from a thick sand layer embedded in shales deposited in an open marine environment of Miocene age. Borehole data indicate that the sand bed is gas saturated. Making the assumptions of single interface reflections, plane‐wave propagation in elastic and isotropic media, and correct amplitude recovery of the actual seismic data, we try to invert the amplitude variation with offset (AVO) response for the compressional velocity [Formula: see text], shear velocity [Formula: see text], and density [Formula: see text] of the gas‐sand layer, knowing the parameters of the upper layer and the calibration constant. The actual reflections reach incidence angles up to 54 degrees at the farthest offset. Notwithstanding the large range of incidence angles, the outcomes of the inversion are ambiguous for we find many solutions that fit equally well, in a least‐squares sense, the observed AVO response. We present the locus of the solutions as curves in compressional velocity [Formula: see text], shear velocity [Formula: see text], and density [Formula: see text] space. To gain a better understanding of the results, we also perform the same inversion experiment on synthetic AVO data derived from the borehole information. We find that when inverting the AVO response in the same range of incidence angles as in the real data case, the exact solution is found whichever starting point we choose; that is, we have no ambiguity. However, if we limit the incidence angle range, e.g., to 15 degrees, the invention is no longer able to find a unique solution and the set of admissible solutions defines regular curves in [Formula: see text], [Formula: see text], [Formula: see text] space. We infer that residual noise in the recorded data is responsible for the ambiguities of the solutions, and that because of numerical noise, a wide range of incidence angle is required to obtain a unique solution even in noise‐free synthetic data.

Shear velocity logging in slow formations using the Stoneley wave

Geophysics ◽  
1986 ◽  
Vol 51 (1) ◽  
pp. 137-147 ◽  
Author(s):  
Jeffry L. Stevens ◽  
Steven M. Day
Keyword(s):  
Error Estimates ◽  
Shear Velocity ◽  
Low Noise ◽  
Head Wave ◽  
Inversion Method ◽  
Stoneley Wave ◽  
Fluid Viscosity ◽  
High Quality ◽  
Stoneley Waves ◽  
Compressional Velocity

We apply an iterative, linearized inversion method to Stoneley waves recorded on acoustic logs in a borehole. Our objective is to assess inversion of Stoneley wave phase and group velocity as a practical technique for shear velocity logging in slow formations. Indirect techniques for shear logging are of particular importance in this case because there is no shear head wave arrival. Acoustic logs from a long‐spaced sonic tool provided high‐quality, low‐noise data in the 1 to 10 kHz band for this experiment. A shear velocity profile estimated by inversion of a 60 ft (18 ⋅ 3 m) section of full‐wave acoustic data correlates well with the P‐wave log for the section. The inferred shear velocity ranges from 60 to 90 percent of the sound velocity of the fluid. Formal error estimates on the shear velocity are everywhere less than 5 percent. Moreover, application of the same inversion method to synthetic waveforms corroborates these error estimates. Finally, a synthetic acoustic waveform computed from inversion results is an excellent match to the observed waveform. On the basis of these results, we conclude that Stoneley‐wave inversion constitutes a practical, indirect, shear‐logging technique for slow formations. Success of the shear‐logging method depends upon availability of high‐quality, low‐noise waveform data in the 1 to 4 kHz band. Given good prior estimates of compressional velocity and density of the borehole fluid, only rough estimates of borehole radius and formation density and compressional velocity are required. The existing inversion procedure also yields estimates of formation Q inferred from spectral amplitudes of Stoneley waves. This extension of the method is promising, since amplitudes of Stoneley waves in a slow formation are highly sensitive to formation Q. Attenuation caused by formation Q dominates over attenuation caused by fluid viscosity if the viscosity is less than about [Formula: see text]. However, Stoneley‐wave amplitudes are also sensitive to gradients in shear velocity in the direction of propagation. In some cases, correction for the effects of shear‐velocity gradients is required to obtain the formation Q from Stoneley‐wave attenuation.

Sequential Measurement of Compressional and Shear Velocities of Rock Samples Under Triaxial Pressure

1969 ◽  
Vol 9 (04) ◽  
pp. 378-394 ◽  
Author(s):  
K.P. Desai ◽  
D.P. Helander
Keyword(s):  
Acoustic Wave ◽  
Resonant Frequency ◽  
Wave Velocity ◽  
Resonance Method ◽  
Shear Velocity ◽  
Berea Sandstone ◽  
Rock Samples ◽  
Wave Velocities ◽  
Compressional Velocity ◽  
Wave Properties

Abstract A laboratory measuring system was designed that can precisely and sequential measure both compressional and shear velocities of rock samples under identical conditions of stress distribution and stress history. This is required if accurate and realistic dynamic elastic properties of rocks are to be determined. The hysteresis effect on velocity pressure characteristics of rock was determined to pressure characteristics of rock was determined to illustrate this point. Lead titanate zirconate transducers were used for measuring compressional wave velocity, and AC-cut quartz transducers were used for measuring shear wave velocity. The system was tested using samples of standard material such as aluminum, steel, brass and lucite. Measurements obtained were accurate within 1 percent. percent. Compressional and shear velocities were measured sequentially on 10 samples of Berea sandstone and two samples of Bartlesville sandstone. It was found that 1. Both compressional and shear velocities increased with an increase in applied external pressure. pressure. 2. Compressional velocity depends upon both external (Pe) and internal (Pi) pressure. 3. Shear velocity depends only upon the differential pressure (Pne-Pe-Pi). 4. The nature of the fluid saturant had little effect on compressional velocity. 5. Shear velocity decreased with an increase in the density of the saturant. 6. The Berea sandstone indicated very little anisotropy. 7. The Bartlesville sandstone showed definite anisotropy. Introduction The various properties of an acoustic wave trainvelocity, amplitude, frequency, etc. may be modified, sometimes quite severely by the media through which the wave has traveled. This suggests the use of wave properties to determine, at least in part, the nature of the material through which the part, the nature of the material through which the wave has passed. To accomplish this successfully requires a reliable technique to for obtaining accurate values of all acoustic wave properties. One purpose of this paper is to describe a recently developed system that can precisely and sequentially record acoustic compressional and shear energies as functions both of time and of frequency. One example of the utility of this system is the accurate measurement of compressional and shear velocities through rock samples subjected to triaxial, i.e., simultaneous but independent vertical, circumferential and pore pressure. Since acoustic velocity and elasticity are closely interrelated, such a system would help to determine realistically the elastic properties of rock samples in the laboratory. METHODS FOR THE INDEPENDENT MEASUREMENT OF COMPRESSIONAL AND SHEAR WAVE VELOCITIES Currently there are two suitable nondestructive laboratory techniques for measuring wave velocity through a rock sample under pressure. One is by the resonance method and the other is by the pulse technique. In the resonance method a sample, in the form of a thin wire, rod, or plate, is make to vibrate in the longitudinal, torsional or flexural mode. Resonant frequency is determined by recording the amplitude of vibration as a function of applied frequency; the amplitude is maximum at resonant frequency. For isotropic materials the relationships between resonance frequencies, elastic moduli and acoustic wave velocities are well known. SPEJ p. 378

Acoustic wave propagation in a fluid‐filled borehole surrounded by a formation with stress‐relief‐induced anisotropy

Geophysics ◽  
1993 ◽  
Vol 58 (9) ◽  
pp. 1257-1269 ◽  
Author(s):  
Lasse Renlie ◽  
Arne M. Raaen
Keyword(s):  
Wave Propagation ◽  
Acoustic Wave ◽  
Shear Wave ◽  
Transverse Isotropy ◽  
Shear Velocity ◽  
Stress Relief ◽  
Stoneley Wave ◽  
Acoustic Wave Propagation ◽  
Damaged Zone ◽  
Compressional Velocity

The stress relief associated with the drilling of a borehole may lead to an anisotropic formation in the vicinity of the borehole, where the properties in the radial direction differ from those in the axial and tangential directions. Thus, axial and radial compressional acoustic velocities are different, and similarly, the velocity of an axial shear‐wave depends on whether the polarization is radial or tangential. A model was developed to describe acoustic wave propagation in a borehole surrounded by a formation with stress‐relief‐induced radial transverse isotropy (RTI). Acoustic full waveforms due to a monopole source are computed using the real‐axis integration method, and dispersion relations are found by tracing poles in the [Formula: see text] plane. An analytic expression for the low‐frequency Stoneley wave is developed. The numerical results confirm the expectations that the compressional refraction is mainly given by the axial compressional velocity, while the shear refraction arrival is due to the shear wave with radial polarization. As a result, acoustic logging in an RTI formation, will indicate a higher [Formula: see text] ratio than that existing in the virgin formation. It also follows that the shear velocity may be a better indicator of a mechanically damaged zone near the borehole than the compressional velocity. The Stoneley‐wave velocity was found to decrease with the increasing degree of RTI.

AVO in transversely Isotropic media—An overview

Geophysics ◽  
1994 ◽  
Vol 59 (5) ◽  
pp. 775-781 ◽  
Author(s):  
J. P. Blangy
Keyword(s):  
Transversely Isotropic ◽  
Shear Velocity ◽  
Velocity Shear ◽  
Type I ◽  
Amplitude Variation With Offset ◽  
Type Iii ◽  
Transversely Isotropic Media ◽  
Isotropic Media ◽  
Compressional Velocity ◽  
Ti Media

The amplitude variation with offset (AVO) responses of elastic transversely isotropic media are sensitive to contrasts in both of Thomsen’s anisotropic parameters δ and ε. The equation describing P-P reflections indicates that the smaller the contrasts in isotropic properties (compressional velocity, shear velocity, and density) and the larger the contrasts in δ and ε across an interface of reflection, the greater the effects of anisotropy on the AVO signature. Contrasts in δ are most important under small‐to‐medium angles of incidence as previously described by Banik (1987), while contrasts in ε can have a strong influence on amplitudes for the larger angles of incidence commonly encountered in exploration (20 degrees and beyond). Consequently, using Rutherford and Williams’ AVO classification of isotropic gas sands, type I gas sands overlain by a transversely isotropic (TI) shale exhibit a larger decrease in AVO than if the shale had been isotropic, and type III gas sands overlain by a transversely isotropic (TI) shale exhibit a larger increase in AVO than if the shale had been isotropic. Furthermore, it is possible for a “type III” isotropic water sand to exhibit an “unexpected) increase in AVO if the overlying shale is sufficiently anisotropic. More quantitative AVO interpretations in TI media require considerations of viscoelastic TI in addition to elastic TI and lead to complicated integrated earth models. However, when elastic and viscoelastic TI have the same axis of symmetry in a shale overlying an isotropic sand, both elastic and viscoelastic TI affect the overall AVO response in the same direction by constructively increasing/decreasing the isotropic component of the AVO response. Continued efforts in this area will lead to more realistic reservoir models and hopefully answer some of the unexplained pitfalls in AVO interpretation.

scholarly journals Pressure and fluid saturation prediction in a multicomponent reservoir using combined seismic and electromagnetic imaging

Geophysics ◽  
2003 ◽  
Vol 68 (5) ◽  
pp. 1580-1591 ◽  
Author(s):  
G. Michael Hoversten ◽  
Roland Gritto ◽  
John Washbourne ◽  
Tom Daley
Keyword(s):  
Electrical Conductivity ◽  
Water Saturation ◽  
Saturation Pressure ◽  
Shear Velocity ◽  
Velocity Shear ◽  
Rock Properties ◽  
Velocity Change ◽  
Archie’S Law ◽  
Fluid Saturation ◽  
Compressional Velocity

This paper presents a method for combining seismic and electromagnetic (EM) measurements to predict changes in water saturation, pressure, and CO2 gas/oil ratio in a reservoir undergoing CO2 flood. Crosswell seismic and EM data sets taken before and during CO2 flooding of an oil reservoir are inverted to produce crosswell images of the change in compressional velocity, shear velocity, and electrical conductivity during a CO2 injection pilot study. A rock‐properties model is developed using measured log porosity, fluid saturations, pressure, temperature, bulk density, sonic velocity, and electrical conductivity. The parameters of the rock‐properties model are found by an L1‐norm simplex minimization of predicted and observed differences in compressional velocity and density. A separate minimization, using Archie's law, provides parameters for modeling the relations between water saturation, porosity, and electrical conductivity. The rock‐properties model is used to generate relationships between changes in geophysical parameters and changes in reservoir parameters. Electrical conductivity changes are directly mapped to changes in water saturation; estimated changes in water saturation are used along with the observed changes in shear‐wave velocity to predict changes in reservoir pressure. The estimation of the spatial extent and amount of CO2 relies on first removing the effects of the water saturation and pressure changes from the observed compressional velocity changes, producing a residual compressional velocity change. This velocity change is then interpreted in terms of increases in the CO2/oil ratio. Resulting images of the CO2/oil ratio show CO2‐rich zones that are well correlated to the location of injection perforations, with the size of these zones also correlating to the amount of injected CO2. The images produced by this process are better correlated to the location and amount of injected CO2 than are any of the individual images of change in geophysical parameters.

VARIATION WITH DEPTH IN SHALLOW AND DEEP WATER MARINE SEDIMENTS OF POROSITY, DENSITY AND THE VELOCITIES OF COMPRESSIONAL AND SHEAR WAVES

Geophysics ◽  
1957 ◽  
Vol 22 (3) ◽  
pp. 523-552 ◽  
Author(s):  
John E. Nafe ◽  
Charles L. Drake
Keyword(s):  
Marine Sediments ◽  
Deep Water ◽  
Shear Velocity ◽  
Original Data ◽  
Limited Range ◽  
Simple Relation ◽  
Deep Basin ◽  
Depth Relationship ◽  
Compressional Velocity ◽  
Range Of Variation

In a study of the dependence of the velocity of compressional waves in marine sediments upon the thickness of overburden, the velocity‐depth relationship in shelf sediments is shown to be distinctly different from that in deep basin sediments. The difference between the two cases may be illustrated by comparing the straight lines that best represent the data. These are [Formula: see text] shallow water, [Formula: see text] deep water where V is in km/sec and Z is in kilometers. Shallow and deep water are defined arbitrarily to be under 100 fathoms and over 1,500 fathoms respectively. The observed variation of average compressional velocity in the shallow and deep water sediments, taken together with the known limited range of variation of velocity for a given porosity, yields limits in turn upon the porosity‐depth dependence in the two environments. It is shown that at the same depth of overburden porosity is much greater in deep water sediments than in shallow. A physical argument is presented to show that there is implicit in the observed narrow range of variation of velocity with porosity a simple relation between porosity and rigidity. Thus quantitative estimates of shear velocity may be made from compressional velocity alone. In this way the original data are used to place rather narrow limits on the depth variation of shear velocity, porosity, and density. A number of comparisons with observation are employed to test the conclusions at each stage of the discussion.

scholarly journals Formation of large low shear velocity provinces through the decomposition of oxidized mantle

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Wenzhong Wang ◽  
Jiachao Liu ◽  
Feng Zhu ◽  
Mingming Li ◽  
Susannah M. Dorfman ◽  
...  
Keyword(s):  
High Pressure ◽  
Lower Mantle ◽  
Single Phase ◽  
Ferric Iron ◽  
Thermal Structure ◽  
Shear Velocity ◽  
Mantle Mineral ◽  
Deep Earth ◽  
Low Shear ◽  
Lowermost Mantle

AbstractLarge Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO3–10 mol% Fe2O3 system, rather than forming a homogeneous single phase. The Fe3+-rich bridgmanite has substantially lower velocities and a higher VP/VS ratio than MgSiO3 bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe3+-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe3+-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.

scholarly journals Log-based prediction of pore pressure and its relationship to compressional velocity, shear velocity and Poisson’s ratio in gas reservoirs; Case study from selected gas wells in Iran

2021 ◽  
Vol 266 ◽  
pp. 01015
Author(s):  
E. R. Nikou ◽  
H. Aghaei ◽  
M. Ghaedi ◽  
H. Jafarpour
Keyword(s):  
Fluid Pressure ◽  
Shear Velocity ◽  
Oil Reservoirs ◽  
Velocity Shear ◽  
Fluid Type ◽  
Gas Wells ◽  
Pore Fluid Pressure ◽  
Gas Reservoirs ◽  
Compressional Velocity

A precise identification of pore fluid pressure (PP) is of great significance, specifically, in terms of drilling safety and reservoir management. Despite numerous work have been carried out for prediction of PP in oil reservoirs, but there still exists a tangible lack of such work in gas hosting rocks. The present study aims to discuss and evaluate the application of a number of existing methods for prediction of PP in two selected giant carbonate gas reservoirs in south Iran. For this purpose, PP was first estimated based on the available conventional log data and later compared with the PP suggested by Reservoir Formation Test (RFT) and other bore data. At the end, it has been revealed that while PP prediction is highly dependent on the type of litho logy in carbonates, the effect of fluid type is negligible. Moreover, the velocity correlations work more efficiently for the pure limestone/dolomite reservoirs compared with the mixed ones.

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