Seismic Rock Properties Research Articles

Dependence of seismic wave attenuations and velocities in rock on pore fluid properties

Experimental and theoretical results have shown that the influence of pore fluid properties on rock elastic and anelastic properties is mainly explained by two major physical mechanisms. (1) A seismic wave may induce viscous local fluid flow within the pore space, leading to frequency dependent attenuation and velocity dispersion. (2) Polar fluids cause a modulus reduction which may be effective already at small degrees of saturation. Full saturation of the rock pore space prevents local fluid flow but causes a frequency independent stiffening of the rock bulk modulus. This additional effect at higher saturations only influences the velocity of the P-wave. Both mechanisms as well as the additional stiffening effect have to be considered in relating seismic wave properties to the saturant and the degree of saturation. The pore fluid effects on the seismic properties of rocks are discussed in this paper by means of experimental results for ultrasonic waves in Obernkirchener Sandstone saturated with different fluids. Related calculations of the fluid influences on velocity and attenuation demonstrate the effectiveness of the fluid mechanisms in this rock.

Seismic anisotropy and shear-wave splitting in lower-crustal and upper-mantle rocks from the Ivrea Zone—experimental and calculated data

To quantify the relationships between anisotropy, S-wave splitting and tectonics, we determined the seismic properties of lower-crustal and upper-mantle rocks outcropping in the Ivrea Zone (Northern Italy). We obtained P- and S-wave seismic velocities by laboratory direct velocity measurements and/or by calculations based on the modal compositions of the rocks, the lattice preferred orientations (LPOs), and the single crystal stiffness coefficients. Measured P- and S-wave velocities (6.0–7.5 km s −1 and 3.6–4.2 km s −1) are typical of the lower crust. The P-wave anisotropy is in the range 0–10%. Shear-wave birefringence is in the range 0.0–0.6 km s −1, with typical values between 0.0 and 0.2 km s −1. In many cases, the birefringence is clearly related to fabric elements (foliation, lineation). Mafic rocks such as anorthosite or pyroxene-bearing gabbros exhibit low P-wave anisotropies ( < 5%) and low shear-wave birefringences (less than 0.1 km s −1). In contrast, the seismic properties of felsic rocks such as biotite-bearing gneisses and mafic rocks such as amphibolites display high V p anisotropy (10%) and strong birefringence (0.3 km s −1). Biotite and amphibole preferred orientations clearly control seismic anisotropy and particularly shear-wave splitting. In these rocks, maximum splitting is observed in directions parallel to the foliation with the fast split shear wave polarized parallel to the foliation plane. To have an overview of the seismic properties of this lower-crustal section at a broader scale, we calculated from our data the anisotropic seismic properties of several hypothetical samples that are perhaps more representative of the regional anisotropy than each sample individually. For instance, the average lower-crustal sample displays an anisotropy of 5.5% for P waves and a birefringence around 0.14 km s −1 for S waves propagating parallel to the foliation. We observe little splitting for waves propagating at high angle to the foliation.

坑井間トモグラフィによる地下構造探査 その技術と将来性

Crosswell seismic tomography is a new geophysical technique for investigating a subsurface structure. This method is characterized by the geometry that both a seismic source and a receiver are located in boreholes. Energy level of a downhole seismic source is relatively small in order to avoid the damage to well-bores. This restricts the distance between a source well and a receiver well to several hundred meters. However, observed seismic waves contain much higher frequency components than those of surface seismic method. This infers that the crosswell method has a potential to delineate detailed inter-well structure.The analysis of crosswell seismic tomography is to reconstruct inter-well velocity structure from traveltimes and ray paths of observed direct waves. Because ray paths of seismic waves depend on the velocity structure which would be the solution, the analysis is equivalent with solving non-linear equations as an inverse problem. The velocity structure is generally reconstructed by iteratively renewing a velocity model so that the traveltimes calculated from the velocity model converge to observed traveltimes.The crosswell seismic tomography has been applied to resource exploration and environmental studies through the research for investigating the relationship between seismic velocity and physical properties of rocks. The effect of EOR (Enhanced Oil Recovery) processes can be identified by the decrease of seismic velocity. Such a velocity change can be detected by comparing the results of crosswell experiments conducted in several stages of the EOR processesThe crosswell tomography can construct detailed subsurface images in the volcanic area where surface seismic method is not available. This means that the crosswell method will be the best seismic approach to identify fracture zones which forms a geothermal reservoir in vol-canic rocks. In addition, the detection of fracture zones is extremely important to investigate environmental problem of underground waste disposal.Crosswell seismic tomography still has several research problems that should be overcome. Nevertheless, in the future the crosswell seismic method has the possibility to become the best seismic approach to investigate detailed subsurface structure and physical properties of rocks.

Calculated and observed seismic behaviour of exposed upper mantle

In order to determine the seismic properties of rocks representing the continental crust/upper mantle transition, a structural and petrographical study of peridotites of the Ivrea-Verbano zone (N. Italy) has been undertaken. The seismic behaviour (Vp, Vs, birefringence, orientation of the fast shear-wave polarisation plane) has been calculated from the modal composition of rock samples, together with the crystallographic preferred orientation and the single crystal elastic constants, using the methods of Mainprice (1990). It is shown that the intrinsic anisotropy of peridotites is mainly due to the lattice preferred orientation (LPO) of olivine and to the competing effects due to the LPO of orthopyroxene. If present, well-oriented hydrous phases such as phlogopite, leads to both P- and S-waves velocities becoming drastically reduced, but the anisotropy and the S-wave splitting (mainly on the foliation plane) is increased. An attempt has been made to apply this petrologic and microstructural approach to understand the seismic properties of ultramafic rocks with the aid of a mobile seismic network set up in the Ivrea-Verbano zone. Five recording units with 3-component geophones were installed on the most representative lithologies of the Ivrea-Verbano zone: kinzigites, gabbros, stronalites, and on metapelites of the adjacent outcrop of the Serie dei Laghi. During five months the seismic network recorded 49 events that were mainly regional earthquakes. The preliminary analysis of the first arrival revealed a high-velocity anomaly near the Finero ultramafic massif. Rough calculations indicated that this anomaly could be due to a shallow-depth, peridotitic body about six kilometres thick. S-wave polarisation analysis was carried out on the horizontal components of the seismograms of four regional earthquakes that showed a first arrival propagating almost vertically at the recording sites. Four earthquakes were chosen in order to reconstruct the orientation of the polarisation plane of the shear phases. The preliminary results of this analysis have shown a qualitatively good agreement between the orientation of the polarisation plane of the fast shear-wave and the strike of the foliation at the station near Finero, located on kinzigites of the Ivrea-Verbano zone, although the relationships between the local structural setting and the polarisation orientations at the other recording sites is not yet so clear.

Seismic-waveform effects of conical points in gradually varying anisotropic media

SUMMARY We describe the effects of anisotropic slowness-surface conical points (acoustic axes) on quasi-shear wavefronts and waveforms in variable elastic media. Conical points have quite complicated geometrical consequences even for a point source in or a wave refracting into a homogeneous anisotropic medium. A hole develops in the fast quasi-shear wavefront and the swallowtail catastrophe plays an important role in the geometry of the slow quasi-shear front, which becomes folded with numerous self-intersections. The two fronts are joined along the rim of the hole. This geometry influences the waveforms, which show Hilbert-transform and diffraction effects. Therefore, standard ray theory is inapplicable even for a uniform medium and the Maslov method is needed to describe waveforms. The introduction of elastic gradients further complicates the geometry of the problem, because rays bend sharply as their slowness approaches that of the axis. An initially smooth, single-valued slow quasi-shear front will evolve in the gradient region into a front which is folded and multivalued and once again the swallowtail is important. However, in contrast to a homogeneous medium, no ‘hole’ develops in the fast quasi-shear front and the slow and fast fronts separate completely. While such geometrical factors are included in the Maslov method, waveforms are also affected by coupling of the fast and slow waves on nearing the axis, where the rays and polarizations rotate most rapidly and their slownesses differ by very little. Numerical examples are presented for a cubic and an orthorhombic material. The differences between these two examples show that the fine structure of ‘continuously varying internal conical refraction’ can vary considerably from material to material, though its basic principles are clearly defined. Waveforms are presented for a point source in a uniform medium and for fast and slow shear waves in a gradient, with and without coupling. Overall, we conclude that the wavefront-folding effects cause the most drastic waveform distortions. Coupling becomes most important when signals merge, as at cuspidal edges or at lower frequencies, since the net waveform could be altered significantly if its components are varied. Conical refraction complicates and yet could be decisive for the identification of seismic anisotropy and rock properties.

3-D seismic velocities calculated from lattice-preferred orientation and reflectivity of a lower crustal section: examples of the Val Sesia section (Ivrea zone, northern Italy)

SUMMARY To quantify the seismic properties of lower crustal rocks and to better constrain the origin of the lower crustal seismic reflectivity, we determined the complete 3-D seismic properties of a lower crustal section. Eight representative samples of the main lithologic and structural units outcropping in the Val Sesia (Ivrea zone) were studied in detail. The seismic velocities were calculated using the single crystal stiffness coefficients and the lattice preferred orientation (LPO) of each mineral in all samples. The 21 stiffness coefficients characterizing the elastic behaviour of each rock are determined. Mafic and ultramafic rocks such as pyroxenite and pyroxenebearing gabbros display complex shear wave properties. These rocks are weakly birefringent (maximum 0.1 km s-') and it is difficult to find consistent relationships between the seismic properties and the rock structure. On the other hand, seismic properties of deformed felsic rocks are essentially controlled by mica. They display strong S-wave birefringence (0.3 km s-') and relatively high Vp anisotropy (7.6 per cent). Amphibole also strongly influences the rock birefringence patterns. For both kind of rocks, the foliation is highly birefringent and the fast polarized shear wave is systematically oriented parallel to the foliation. We show that the number of mineral phases in the rock strongly controls the anisotropy. The seismic anisotropy has a complex role in the P-wave reflectivity. Compared to the isotropic case, anisotropy enhances the reflection coefficient for about 60 per cent of the possible lithological interfaces. For 40 per cent of the interfaces, the reflection coefficient is much lower when one considers the medium as anisotropic.

Seismic properties of rocks exposed in the POLAR profile region—constraints on the interpretation of the refraction data

The POLAR profile region comprises the northern part of the Precambrian Baltic Shield. The region is subdivided into a number of tectonic units, which are made up of Archaean gneiss terrains, locally hosting Archaean and/or Palaeoproterozoic supracrustal sequences and belts. We have measured the seismic properties of representative rock samples recovered from surface exposures. The experimental data include the measurement of elastic wave velocities (VP, VS) and densities at confining pressures up to 600 MPa and temperatures up to 600°C (at 600 MPa), the determination of the pressure and temperature derivatives of VP and VS, and the determination of velocity anisotropy. The petrophysical data are compared with the chemical and mineral composition of the rocks. Using a regional geotherm, velocity-depth profiles have been calculated for the different lithologies, and the calculated in-situ velocity data have been used to interpret lithologically the seismic models for VP, VS, and evaluated from the seismic refraction data. There is experimental evidence, that the upper crust (< 10 km depth) is dominated by quartzofeldspatic gneisses. The high-velocity body below the surface comprising the Lapland Granulite Belt and parts of the Kittila Greenstone Belt can be correlated with amphibolite and granulite lithologies. The velocity structure of the seismically homogeneous middle crust (10-30 km) is best explained by a uniform quartz-dioritic composition. The boundary between the middle and the lower crust (at about 30 km) which is marked by a pronounced increase of the P-wave velocity, is due to a change in the overall chemical composition from acidic to more mafic and to increasing metamorphic grade, i.e. the transition from amphibolite facies to more mafic granulite facies lithologies.

Effect of oil under in-situ conditions on the seismic properties of rocks : Clark, V A GeophysicsV57, N7, July 1992, P894–901

Effect of oil under in-situ conditions on the seismic properties of rocks : Clark, V A GeophysicsV57, N7, July 1992, P894–901

Seismic properties of pore fluids

Pore fluids strongly influence the seismic properties of rocks. The densities, bulk moduli, velocities, and viscosities of common pore fluids are usually oversimplified in geophysics. We use a combination of thermodynamic relationships, empirical trends, and new and published data to examine the effects of pressure, temperature, and composition on these important seismic properties of hydrocarbon gases and oils and of brines. Estimates of in‐situ conditions and pore fluid composition yield more accurate values of these fluid properties than are typically assumed. Simplified expressions are developed to facilitate the use of realistic fluid properties in rock models. Pore fluids have properties that vary substantially, but systematically, with composition, pressure, and temperature. Gas and oil density and modulus, as well as oil viscosity, increase with molecular weight and pressure, and decrease with temperature. Gas viscosity has a similar behavior, except at higher temperatures and lower pressures, where the viscosity will increase slightly with increasing temperature. Large amounts of gas can go into solution in lighter oils and substantially lower the modulus and viscosity. Brine modulus, density, and viscosities increase with increasing salt content and pressure. Brine is peculiar because the modulus reaches a maximum at a temperature from 40 to 80°C. Far less gas can be absorbed by brines than by light oils. As a result, gas in solution in oils can drive their modulus so far below that of brines that seismic reflection bright spots may develop from the interface between oil saturated and brine saturated rocks.

The effect of oil under in‐situ conditions on the seismic properties of rocks

Direct hydrocarbon indicators (DHIs) on seismic sections are commonly thought to be diagnostic only of gas. However, oil sands can also generate DHIs such as bright spots and flat events since oils under in‐situ conditions can contain large amounts of solution gas. This dissolved gas substantially decreases the velocity of sound and the density of the oils as compared to measurements of these properties at surface conditions. Hydrocarbon indicators caused by oil sands are investigated by first measuring the elastic properties of an oil as a function of gas‐oil ratio, next, calculating the elastic properties of additional oil compositions under in‐situ conditions using standard pressure‐volume‐temperature (PVT) measurements, and then calculating the compressional velocity in oil‐saturated rocks for several typical oils using Gaasmann’s equation. The potential for seismic anomalies caused by oil‐saturated rocks is higher than thought because the properties of oil under reservoir conditions can differ significantly from those of surface oils. Specifically: 1) The properties of oil depend on its composition: the higher the API gravity and the gas‐to‐oil ratio (GOR), the lower the density and velocity of sound (adiabatic bulk modulus) and the lower the velocity of a rock saturated with the oil. 2) Calculations of oil‐sand velocities using the in situ properties of oils show that areas having light oils and/or poorly consolidated rocks are the most likely areas in which to encounter oil DHIs. Since overpressured areas can have both poorly consolidated rocks and high GOR oils, they are especially prone to large oil responses.

Laboratory measurements of the seismic properties of sedimentary rocks

Abstract A laboratory technique for the measurement of compressional and shear wave velocities and attenuations (quality factors) of sedimentary rocks at pressures up to 70 MPa and at frequencies up to 1 MHz was published by Winkler and Plona in 1982. In the method the rock sample is enclosed between buffer rods. The arrival times and amplitudes of the signals reflected from its top and base are compared. A similar technique has been used for many years in the non-destructive testing of metals and plastics. This paper describes the use of a single-frequency pulsed sine wave for the velocity and attenuation measurements, in addition to the transient pulse used in the Schlumberger method. Measurements were made on samples with accurately known compressional and shear wave properties and on replicated rock samples. The results demonstrate that the determinations of velocities are accurate to ± 0.3%, and that the determinations of attenuation are accurate to ±0.1 dB/cm at 0.85 MHz.

Complete seismic properties obtained from microcrack fabrics and textures in an amphibolite from the Ivrea zone, Western Alps, Italy

Laboratory seismic measurements of compressional wave velocities ( V p ) and shear-wave splitting ( ΔV s ) have been carried out on a clinopyroxene amphibolite from the Ivrea Zone (Western Alps, Italy) at pressures up to 600 MPa. From the texture of the major minerals (hornblende, plagioclase and pyroxene) and the respective elastic constants, the elastic properties (Voigt average) and seismic anisotropies were calculated. At high confining pressure where the effect of microcracks is eliminated, V p and shear-wave velocities ( V s , V ss ) are in good agreement with the calculated velocities reflecting texture-induced anisotropies. Microcracks are most effective at low confining pressure, but are largely closed at about 200–300 MPa. Microscopic studies revealed that the crack fabric is characterized by oriented transgranular and cleavage cracks. The bulk anisotropy at low pressure conditions was computed by superposing the texture-induced anisotropy with the crack-induced anisotropy. These calculated values are consistent with the measured values, indicating that anisotropy of V p and shear-wave splitting is affected by both texture and oriented microcracks. Based on detailed fabric analyses the presented method allows to describe the complete elastic and seismic properties of crystalline rocks, providing an ideal data basis for three-dimensional seismic modelling to investigate the influence of anisotropic media on the reflectivity.

Seismic properties of rocks with distributions of small cracks : Peacock, S; Hudson, JA Geophys JV102, N2, Aug 1990, P471–484

Seismic properties of rocks with distributions of small cracks : Peacock, S; Hudson, JA Geophys JV102, N2, Aug 1990, P471–484

Seismic rock properties for rock mass descriptions and fluid flow monitoring : Nur, A Proc International Symposium on Rock at Great Depth, Pau, 28–31 August 1989V3, P1081–1102. Publ Rotterdam: A A Balkema, 1989

Seismic rock properties for rock mass descriptions and fluid flow monitoring : Nur, A Proc International Symposium on Rock at Great Depth, Pau, 28–31 August 1989V3, P1081–1102. Publ Rotterdam: A A Balkema, 1989

Seismic Velocities In Carbonate Rocks

Abstract Seismic velocities were measured in the laboratory vs pressure, temperature, and saturation on over 80 cores from various carbonate reservoirs in Alberta, using an ultrasonic pulse transmission method. Porosity, permeability, electrical properties, and capillary pressure were also measured in order to characterize the pore systems. All the measured data and other information are compiled into a database. The results show that both permeabilities and seismic velocities are related to the porosities of the cores, although the data are scattered. Liquid saturation (e.g. oil and water) increases the compressional velocity but has only nominal effect on the shear velocity. The magnitude of the increase in compressional velocities with liquid saturation is dependent upon the pore geometry of the rocks, reservoir pressure, as well as the distribution and properties of the saturating fluids. The velocity data are also fit to the time-average equation and the Gassmann equation. The results are usually unsatisfactory, which mean that the time-average equation used in sonic log interpretation is inadequate. For log interpretation, it might be desirable to establish a porosity-velocity relationship specific to carbonate reservoirs, instead of using the time-average equation. Furthermore, the Gassmann equation does not adequately explain the laboratory results. In most of the cores measured, the compressional velocities increase by more than 5% with oil saturation, with a subsequent decrease by over 5% when flooded with a hydrocarbon solvent. This suggests that seismic monitoring of injected gases and hydrocarbon solvent in carbonate reservoirs should be possible. Introduction Seismic properties or rocks are very important to petroleum exploration and production. In seismic exploration, velocity and density contrast between two rock layers causes reflection of seismic waves at the interface. The reflected seismic waves bring out information on the structure of the earth's crust, so that the seismic interpreter can look for potential hydrocarbon traps from the seismic profile. In the process of seismic exploration, a thorough understanding of seismic velocities in rocks plays a vital role in the success of seismic methods. In recent years, seismic methods have found other applications with success in petroleum production assessment, reservoir characterization, and enhanced oil recovery (EOR) and production monitoring. In seismic monitoring applications, the velocity contrast caused by various EOR and production processes is the physical basis of the method. Laboratory results have shown that large velocity decreases are found in heavy oil sands as temperature increases(1–5), in oil-saturated rocks when flooded with carbon dioxide (CO2(6,7) and in oil-saturated rocks when flooded with gas and hydrocarbon solvent(8,9). Furthermore, laboratory result also show large differences between the compressional velocities in gas and liquid (e.g. water or oil) saturated rocks(10). These results open the door for using seismic methods to monitor EOR and production processes. In the literature, most of the seismic velocity data were measured in sandstones and sand. Little has been published on seismic properties of carbonate rocks. Rafavich er al.(11) measured 93 Mission Canyon formation carbonate cores from four wells in the Williston basin field, North Dakota, which appears to be the only systematic study of seismic velocities in carbonates.

Seismic properties of rocks from an exposure of extended continental crust—new laboratory measurements from the Ivrea Zone

Compressional wave velocities for new samples from the Ivrea-Verbano Zone (IVZ) and Strona-Ceneri Zone (SCZ) have been measured to 600 MPa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains. These data, coupled with previous measurements, show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of SCZ and IVZ schists and gneisses. IVZ paragneisses form the basis of a linear velocity-density relationship for metagraywackes of various metamorphic grade. This relationship falls close to that expected for mafic rocks and overlaps velocity-density data for IVZ mafic gneisses. One-dimensional near-vertical reflection models, based on these data and recent structural reconstructions of the terrain, show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite-granulite facies paragneiss, mafic gneiss and ultramafic rocks. Transparent upper crustal levels in the model result from the overall similarity of velocities for schists, gneisses and granites from the structurally and lithologically complex SCZ. A mean lower crustal velocity of 7.1 km/s is calculated for the part of the IVZ composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex. The velocity-density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from P-wave velocity data in the absence of other constraints.

Seismic properties of rocks with distributions of small cracks

SUMMARY Hudson's theory of multiple scattering for the dynamic properties of cracked rock is applied to media containing cracks of varying orientation, radius and aspect ratio. Angular variation of velocity and attenuation due to parallel and nearly parallel cracks and cracks with normals at a fixed angle to a given axis are presented. Anisotropy gradually decreases as the distribution of orientations is widened from exactly parallel to random. Increasing the aspect ratio of thin saturated cracks generally causes the elastic properties to approach those due to dry (gas-filled) cracks. For fluid-filled cracks the line singularity where the birefringent shear waves have equal velocity moves closer to the symmetry axis as either the distribution of orientations is widened or the aspect ratio is increased. Calculated attenuations due to Rayleigh scattering and viscous dissipation in crack-filling fluids are many orders of magnitude less than reported attenuations in the Earth. Viscous dissipation varies linearly with frequency and aspect ratio of thin fluid-filled cracks. A power-law distribution of crack radii, intended to represent that found in the Earth's crust, has no effect on the angular variations of seismic velocities or viscous attenuation but scattering attenuation is sensitive to the power-law exponent. Lognormal distributions of aspect ratio, also corresponding to observations in real rock, affect velocity as well as attenuation. Velocities derived from the theory are compared with those measured in laboratory specimens with known geometries of cracks.

Density of the ocean crust

The density structure and average density of the oceanic crust have implications for various geological and geophysical problems, including interpretations of gravity data, the variation of lithospheric bouyancy in relation to age, and role and fate of the crust in subduction. But no systematic evaluation of oceanic crustal density has been made and estimates range from 2.85 to 3.0 Mg m−3 (refs 1–3). We have made an evaluation based on seismic refraction data in combination with drilling results, laboratory studies of seismic properties of oceanic and ophiolitic rocks, and ophiolite lithostratigraphy. Our preferred value for the mean density of the oceanic crust is 2.89±0.04 Mg m−3