Laboratory Ultrasonic Measurements Research Articles

Ultrasonic measurements on thin samples: numerical modelling

Ultrasonic pulse velocity method is a standard method for measuring elastic properties of rock cores in laboratories. Cylindrical plugs of 40–100 mm length are usually used for such measurements. It was recently shown that thin disc samples (~15 mm in length) were suitable for such measurements in the case of an advanced experimental set-up. Here we present results of numerical simulations to support the outcome of the previous work and to improve the understanding of wave propagation in the samples during laboratory ultrasonic measurements. The finite element method within Abaqus/Explicit (Dassault Systemes, Simulia) is used to simulate wave propagation along the experimental rig and the rock sample caused by transmitted ultrasonic pulse. The computational domain mimics the real geometry. The results of the numerical modelling prove that an S-wave transducer also produces a compressional wave that propagates along the sample and can be recorded by a receiver. Simulations are performed for three configurations used in real laboratory experiments. The numerically simulated waveforms are compared with the signals, recorded during laboratory experiments. Simulated travel times of elastic waves are in a good agreement with experimentally obtained results.

ULTRASONIC ELASTIC CHARACTERISTICS OF SIX KINDS OF METAMORPHIC COALS IN CHINA UNDER ROOM TEMPERATURE AND PRESSURE CONDITIONS

AbstractCoal elasticity is one of the important mechanical characteristics reflecting the material composition and structure of coal. Under the constraint of drilling and well logging, applying seismic exploration to predict elastic characteristics of the coal seams would be of important engineering significance for coal mining and CBM reservoir evaluation and development. In this regard, ultrasonic experiment of coal samples is the basis to realize the seismic inversion of physical properties of coal seam. In this paper, based on 30 pieces of six kinds of metamorphic raw coals collected from six mining areas of Yima, Fukang, Huainan, Pingdingshan, Hebi and Jiaozuo in China, laboratory ultrasonic measurements of these coal samples were respectively conducted in three directions of strike, dip and perpendicular to the beddings of coal seam under room temperature and pressure. The experimental results show: the samples’ velocities of both pressure wave (P‐wave) and shear wave (S‐wave) decrease in turn in the three directions; there are obvious anisotropies of velocities, and the average anisotropy of P‐wave velocity is stronger than S‐wave's. Moreover, larger differences exist among both quality factors and modules of elasticity in the three directions, and the S‐wave quality factors are greater than P‐wave's; with exception of Poisson's ratio, modules of elasticity of coal are less than those common sedimentary rocks. Through the experiment and analysis, it can be demonstrated that Gardener and Castagna formulas are not suitable to represent relations of coal elastic parameters in China's coal fields, and two empirical formulas with higher precision are statistically given.

Multiscale geophysical characterization of an unstable rock mass

Hazard mitigation from rockfalls and landslides is a priority in densely populated areas. A proper characterization of the inner structure of the rock mass is key to the comprehension of the mechanisms enhancing the slope instabilities. To this aim multi-scale geophysical methods can provide a novel and valuable tool for a high-resolution imaging of the internal structure of the rock mass and unique constraints on the physical state of the medium. We present here a cross-hole seismic tomography survey coupled with laboratory ultrasonic velocity measurements and physical properties determination on rock samples to characterize the damaged and potentially unstable granitic cliff of Madonna del Sasso (NW Italy).Results allowed to obtain: i) a lithological interpretation of the velocity field obtained at the site, ii) a systematic correlation of the measured velocities with physical properties (density and porosity) and macroscopic features of the granite (weathering and anisotropy) of the cliff. The multi-scale approach adopted within this study revealed to be crucial for the imaging at depth of the main fractures affecting the cliff (site-scale seismic tests) and for the understanding of the variations in the seismic velocity between altered and intact rock (laboratory-scale tests); similar approaches can be potentially used in further microseismic monitoring studies.

A rock-physics investigation of unconsolidated saline permafrost: P-wave properties from laboratory ultrasonic measurements

Saline permafrost is sensitive to thermal disturbances and is prone to subsidence, which renders it a major source of geohazard in Arctic coastal environments. Seismic methods could be used to map and monitor saline permafrost at scales of geotechnical interests because of the ice-content dependencies of seismic properties. We have developed a comprehensive study of the ultrasonic P-wave properties (i.e., velocity and attenuation) of synthetic saline permafrost samples for a range of salinities and temperatures, and measurements conducted on a fine-grained permafrost core obtained from Barrow, Alaska. The resulting data consist of P-wave properties presented as functions of temperature and salinity. Notable observations include the following: P-wave velocities showed marked reductions in the presence of dissolved salts and complex variations resulting from the water-to-ice phase transitions; strong P-wave attenuation was present in the temperature intervals in which the samples were partially frozen. When presented as functions of ice saturation, the data sets lead us to two key findings: (1) neither a purely cementing nor a purely pore-filling model of the pore-scale distributions of ice could adequately fit the observed velocity data and (2) although the velocities increase monotonically with increasing ice saturations, P-wave attenuation reaches a maximum at intermediate ice saturations—contrary to the ordinary expectation of decreasing attenuation with increasing velocities. The observed ice-content dependencies of P-wave properties, along with the implications on the probable pore-scale distributions of ice, provide a valuable basis for rock-physics modeling, which in turn could facilitate seismic characterizations of saline permafrost.

Multichannel analysis of surface-waves and integration of downhole acoustic televiewer imaging, ultrasonic Vs and Vp, and vertical seismic profiling in an NEHRP-standard classification, South of Concordia, Kansas, USA

Seismic site characteristics, as pertaining to earthquake hazard reduction, are a function of the subsurface elastic moduli and the geologic structures. This study explores how multiscale (surface, downhole, and laboratory) datasets can be utilized to improve “constrained” average Vs30 (shear-wave velocity to a 30-meter depth). We integrate borehole, surface and laboratory measurements for a seismic site classification based on the standards of the National Earthquake Hazard Reduction Program (NEHRP). The seismic shear-wave velocity (Vs30) was derived from a geophysical inversion workflow that utilized multichannel analysis of surface-waves (MASW) and downhole acoustic televiewer imaging (DATI). P-wave and S-wave velocities, based on laboratory measurements of arrival times of ultrasonic-frequency signals, supported the workflow by enabling us to calculate Poisson's ratio, which was incorporated in building an initial model for the geophysical inversion of MASW. Extraction of core samples from two boreholes provided lithology and thickness calibration of the amplitudes of the acoustic televiewer imaging for each layer. The MASW inversion, for calculating Vs sections, was constrained with both ultrasonic laboratory measurements (from first arrivals of Vs and Vp waveforms at simulated in situ overburden stress conditions) and the downhole acoustic televiewer (DATV) amplitude logs. The Vs30 calculations enabled categorizing the studied site as NEHRP-class “C” — very dense soil and soft rock. Unlike shallow fractured carbonates in the studied area, S-wave and P-wave velocities at ultrasonic frequency for the deeper intact shale core-samples from two boreholes were in better agreement with the corresponding velocities from both a zero-offset vertical seismic profiling (VSP) and inversion of Rayleigh-wave velocity dispersion curves.

Poroelastic finite-difference modeling for ultrasonic waves in digital porous cores

Scattering attenuation in short wavelengths has long been interesting to geophysicists. Ultrasonic coda waves, observed as the tail portion of ultrasonic wavetrains in laboratory ultrasonic measurements, are important for such studies where ultrasonic waves interact with small-scale random heterogeneities on a scale of micrometers, but often ignored as noises because of the contamination of boundary reflections from the side ends of a sample core. Numerical simulations with accurate absorbing boundary can provide insight into the effect of boundary reflections on coda waves in laboratory experiments. The simulation of wave propagation in digital and heterogeneous porous cores really challenges numerical techniques by digital image of poroelastic properties, numerical dispersion at high frequency and strong heterogeneity, and accurate absorbing boundary schemes at grazing incidence. To overcome these difficulties, we present a staggered-grid high-order finite-difference (FD) method of Biot’s poroelastic equations, with an arbitrary even-order (2L) accuracy to simulate ultrasonic wave propagation in digital porous cores with strong heterogeneity. An unsplit convolutional perfectly matched layer (CPML) absorbing boundary, which improves conventional PML methods at grazing incidence with less memory and better computational efficiency, is employed in the simulation to investigate the influence of boundary reflections on ultrasonic coda waves. Numerical experiments with saturated poroelastic media demonstrate that the 2L FD scheme with the CPML for ultrasonic wave propagation significantly improves stability conditions at strong heterogeneity and absorbing performance at grazing incidence. The boundary reflections from the artificial boundary surrounding the digital core decay fast with the increase of CPML thicknesses, almost disappearing at the CPML thickness of 15 grids. Comparisons of the resulting ultrasonic coda Qsc values between the numerical and experimental ultrasonic S waveforms for a cylindrical rock sample demonstrate that the boundary reflection may contribute around one-third of the ultrasonic coda attenuation observed in laboratory experiments.

Monte Carlo Simulation of Stress-Associated Scattering Attenuation from Laboratory Ultrasonic Measurements

Abstract Seismic coda waves scattered by small‐scale heterogeneities contain information on stress changes of the medium, because of changes in the physical state of materials. Based on the ultrasonic measurements under different stresses for a cylindrical sandstone sample, we investigate the influence of stress changes on ultrasonic S ‐coda attenuation and aim to characterize its stress/frequency‐dependent pattern. Considering the complexity of ultrasonic coda waveforms measured from finite‐size rock samples in laboratory experiments, the Monte Carlo simulation is employed to synthesize ultrasonic envelopes, which act by incorporating the effect of multiple scatterings and boundary reflections on coda waves. The optimal simulation parameters, estimated by minimizing the residual between the observed and synthesized envelopes, indicate that the rock sample under study presents moderate heterogeneities. The relationship between attenuation and stress is similar for direct and coda S waves and remains stable in the range of high‐effective stresses around 30–60 MPa, with less stress sensitivity. Enhanced attenuation for both types of waves occurs at lower‐effective stresses, but with coda attenuation much faster and stronger, presenting a quite different nonlinear behavior with respect to stress. Coda attenuation increases drastically at extremely low‐effective stresses below 15 MPa because of the increase in rock compliance, showing much greater sensitivity to high‐pore pressure than intrinsic attenuation. This study improves our understanding of the mechanism of ultrasonic coda attenuation and its scaling dependence on stress and frequency.

A reference Earth model for the heat‐producing elements and associated geoneutrino flux

The recent geoneutrino experimental results from KamLAND (Kamioka Liquid Scintillator Antineutrino Detector) and Borexino detectors reveal the usefulness of analyzing the Earth's geoneutrino flux, as it provides a constraint on the strength of the radiogenic heat power, and this, in turn, provides a test of compositional models of the bulk silicate Earth (BSE). This flux is dependent on the amount and distribution of heat‐producing elements (HPEs: U, Th, and K) in the Earth's interior. We have developed a geophysically based, three‐dimensional global reference model for the abundances and distributions of HPEs in the BSE. The structure and composition of the outermost portion of the Earth, the crust and underlying lithospheric mantle, are detailed in the reference model; this portion of the Earth has the greatest influence on the geoneutrino fluxes. The reference model combines three existing geophysical models of the global crust and yields an average crustal thickness of 34.4 ± 4.1 km in the continents and 8.0 ± 2.7 km in the oceans, and a total mass (in 1022 kg) of oceanic, continental, and bulk crust is 0.67 ± 0.23, 2.06 ± 0.25, and 2.73 ± 0.48, respectively. In situ seismic velocity provided by CRUST 2.0 allows us to estimate the average composition of the deep continental crust by using new and updated compositional databases for amphibolite and granulite facies rocks in combination with laboratory ultrasonic velocities measurements. An updated xenolithic peridotite database is used to represent the average composition of continental lithospheric mantle. Monte Carlo simulation is used to predict the geoneutrino flux at 16 selected locations and to track the asymmetrical uncertainties of radiogenic heat power due to the log‐normal distributions of HPE concentrations in crustal rocks.

Damage and seismic velocity structure of pulverized rocks near the San Andreas Fault

AbstractA combination of seismic refraction tomography, laboratory ultrasonic velocity measurements, and microstructural observations was used to study the shallow velocity structure of a strand of the San Andreas fault (SAF) just south of Littlerock, California. The examined site has a strongly asymmetric damage structure with respect to the SAF core. The conglomerates to the southwest show little to no damage, whereas a ~100 m wide damage zone exists to the northeast with a ~50 m wide zone of pulverized granite adjacent to the fault core. Seismic P‐wave velocities of the damaged and pulverized granite were investigated over a range of scales. In situ seismic velocity imaging was performed on three overlapping profiles normal to the SAF with lengths of 350 m, 50 m, and 25 m. In the laboratory, ultrasonic velocities were measured on centimeter‐ to decimeter‐sized samples taken along the in situ profiles. The samples were also investigated microstructurally. Micro‐scale fracture damage intensifies with increasing proximity to the fault core, allowing a subdivision of the damage zone into several sections. Laboratory‐derived velocities in each section display varying degrees of anisotropy, and combined with microfracture analysis suggest an evolving damage fabric. Pulverized rocks close to the fault exhibit a preferred fault‐parallel orientation of microfractures, resulting in the lowest P‐wave velocity orientated in fault‐perpendicular direction. Closest to the fault, pulverized rocks exhibit a gouge‐like fabric that is transitional to the fault core. Comparison of absolute velocities shows a scaling effect from field to laboratory for the intact rocks. A similar scaling effect is absent for the pulverized rocks, suggesting that they are dominated by micro‐scale damage. Fault‐parallel damage fabrics are consistent with existing models for pulverized‐rock generation that predict strong dynamic reductions in fault‐normal stress. Our observations provide important constraints for theoretical models and imaging fault damage properties at depth using remote methods.

A new velocity-pressure-compaction model for uncemented sediments

SUMMARY Knowledge of the pressure dependence of rock properties is useful for a wide range of earth science problems, especially related to pore pressure changes caused by fluid injection or withdrawal, as often occurs in groundwater, hydrocarbon and CO2 sequestration reservoirs. A long-standing problem is that theoretical models of velocity-pressure response often do not match laboratory measurements, and alternately, empirical regressions fit to such data do not extrapolate accurately to wider pressure ranges since they have little or no physical basis. Accurate determination of the dry rock frame properties at low effective pressure is a key aspect of the problem, particularly when ultrasonic laboratory measurements are not available in this pressure range. We present a new model to describe the pressure sensitivity of the bulk and shear moduli for uncemented sedimentary rocks. Our model incorporates effects of sedimentary compaction and critical porosity, including a relationship to account for porosityanddensitychangewithpressure.Themodelistestedonlaboratorymeasurementsfor various rock samples and fits well over a wide range of pressures. The new velocity‐pressure model should be useful for improved prediction and interpretation of pressure-dependent rock properties and seismic data.

減衰特性解明の超音波室内実験における散乱現象の影響について

減衰特性解明を目的とした超音波室内実験においては，測定媒体の内部構造をX線CTや核磁気共鳴画像技術を使用することによりミクロスケール不均質性を評価し，散乱減衰を適切に評価できる技術的素地が拓かれつつある。その一方で，地震波減衰解析のための減衰モデルや解析手法がその仮定とする前提を十分検証することなく適用される例が少なくない。本論説では，まず減衰解析で一般的に用いられる，指数関数で表現される減衰モデルを詳述し，その仮定を明らかにする。さらに減衰を表現するQ値 (1サイクルの波が有するエネルギーとその1サイクルの過程で失われるエネルギーとの比)は，現象論的な量であることを指摘する。次に，減衰解析手法で最も頻繁に適用されるspectral ratio法を例にして，手法が前提している要件について詳述する。また，減衰解析の際に必要な波形切り出しによる任意性の問題なども指摘する。最後に，減衰解析における散乱減衰の影響について，いくつかの室内実験例を基に，散乱減衰の重要性とその扱い方の難しさについて指摘し，特に散乱効果が最も大きいミー散乱過程においては減衰解析の視点から散乱減衰の理論やモデルの再考の必要性があることを述べる。

Experimental verification of the fracture density and shear‐wave splitting relationship using synthetic silica cemented sandstones with a controlled fracture geometry

ABSTRACTWe present laboratory ultrasonic measurements of shear‐wave splitting from two synthetic silica cemented sandstones. The manufacturing process, which enabled silica cementation of quartz sand grains, was found to produce realistic sandstones of average porosity 29.7 ± 0.5% and average permeability 29.4 ± 11.3 mD. One sample was made with a regular distribution of aligned, penny‐shaped voids to simulate meso‐scale fractures in reservoir rocks, while the other was left blank. Ultrasonic shear waves were measured with a propagation direction of 90° to the coincident bedding plane and fracture normal. In the water saturated blank sample, shear‐wave splitting, the percentage velocity difference between the fast and slow shear waves, of <0.5% was measured due to the bedding planes (or layering) introduced during sample preparation. In the fractured sample, shear‐wave splitting (corrected for layering anisotropy) of 2.72 ± 0.58% for water, 2.80 ± 0.58% for air and 3.21 ± 0.58% for glycerin saturation at a net pressure of 40 MPa was measured. Analysis of X‐ray CT scan images was used to determine a fracture density of 0.0298 ± 0.077 in the fractured sample. This supports theoretical predictions that shear‐wave splitting (SWS) can be used as a good estimate for fracture density in porous rocks (i.e., SWS = 100εf, where εf is fracture density) regardless of pore fluid type, for wave propagation at 90° to the fracture normal.

Theory and laboratory experiments of elastic wave scattering by dry planar fractures

Remote sensing of fractures with elastic waves is important in fields ranging from seismology to nondestructive testing. In many geophysical applications, fractures control the flow of fluids such as water, hydrocarbons or magma. While previous analytic descriptions of scattering mostly deal with very large or very small fractures (compared to the dominant wavelength), we present an analytic solution for the scattering of elastic waves from a fracture of arbitrary size. Based on the linear slip model for a dry fracture, we derive the scattering amplitude in the frequency domain under the Born approximation for all combinations of incident and scattered wave modes. Our analytic results match laser-based ultrasonic laboratory measurements of a single fracture in clear plastic, allowing us to quantify the compliance of a fracture. Copyright © 2011 by the American Geophysical Union.

Dynamic elastic properties of coal

Laboratory ultrasonic velocity measurements of different types of coal demonstrate that their dynamic elastic properties depend on coal rank and applied effective pressure. In spite of the growing interest in coal beds as targets for methane production, the high abundance in sedimentary sequences and the strong influence that they have on seismic response, little data are available on the acoustic properties of coal. Velocities were measured in core plugs parallel and perpendicular to lamination surfaces as a function of confining pressure up to [Formula: see text] in loading and unloading cycles. P- and S-wave velocities and dry bulk and dry shear moduli increase as coal rank increases. Thus, bituminous coal and cannel show lower velocities and moduli than higher ranked coals such as semianthracite and anthracite. The [Formula: see text] relationship for dry samples is linear and covers a relatively wide range of effective pressures and coal ranks. However, there is a pressure dependence on the elastic properties of coal for confining pressures below [Formula: see text]. This pressure sensitivity is related to the presence of microcracks. Finally, the data show that coal has an intrinsic anisotropy at confining pressures above [Formula: see text], the closing pressure for most of the microcracks. This intrinsic anisotropy at high pressures might be due to fine lamination and preferred orientation of the macerals.

Laboratory measurements of the physical properties of Triassic Evaporites from Central Italy and correlation with geophysical data

The Triassic Evaporites (TE) of the Umbria–Marche Apennines, a sedimentary succession made up of a sequence of alternating sulphates (anhydrites and gypsum) and dolostones, represent a key lithology in terms of sealing properties and earthquake triggering. Here we: (1) report laboratory measurements of density, porosity, Vp, Vs, seismic anisotropy and permeability at effective confining pressures from 0 to 100 MPa, conducted on samples of TE collected from both outcrops and boreholes; and (2) attempt to upscale the laboratory results to larger scale geophysical investigation, such as Vp sonic logs, seismic tomography and in situ measurements of pore-fluid pressure. The average laboratory P-wave velocity is 6.0 km/s for dolostones, 4.6 km/s for gypsum–dolostones and 5.8 km/s for anhydrites, at ambient pressure. As effective confining pressure is increased up to 100 MPa, the average P-wave velocity increases to 7.0 km/s for dolostones, 5.3 km/s for gypsum and 6.4 km/s for anhydrites. Vp/Vs ratios appear to be independent of confining pressure, with average values of ∼ 1.8 to ∼ 2.2 for sulphates and ∼ 1.9 to ∼ 2.2 for dolostones, respectively, for dry and saturated conditions. All samples are characterized by very low permeability (10 − 18 m 2 to 10 − 21 m 2), with the higher values for gypsum–dolostones and fractured dolostones samples. The Vp profiles obtained from ultrasonic laboratory measurements match well the in situ Vp profiles measured using sonic logs. In the laboratory, the Vp/Vs ratio increases when pressurized pore fluids are present, in agreement with 4D seismic tomography that relates the increase in Vp/Vs ratio to the migration of fluids during the 1997 Umbria–Marche seismic sequence ( M max ∼ 6.0) that nucleates within the TE. Our low-permeability values are consistent with deep borehole measurements of high pore-fluid pressure trapped within the Triassic Evaporites.

An investigation of the use of chaotic signals in active sonar

Many natural effects are now known to be chaotic rather than random, i.e., are generated by simple non-linear systems. The paper reports the outcome of a theoretical and experimental study to assess whether signals based on chaotic time series offer advantages in active sonar. It is noted that for conventional detection, the detailed structure of the signal is unimportant. Any advantage will come from novel effects due to interactions with the target or the environment. An overview is given of apparently novel behavior in echoes from a resonant target which were first noted in simulations and then observed in laboratory ultrasonic measurements. An explanation in terms of linear systems theory is outlined. The limitations which inhibit the use of advanced non-linear techniques for detection will be discussed and illustrated with reference to the results of analyzing data measured in an outdoor acoustic test facility.

Effects of pore fluid pressure on the seismic response of a fractured carbonate reservoir

An effective medium model for the stress-dependent seismic properties of fractured reservoirs is developed here on the basis of a combination of a general theory of viscoelastic waves in rock-like composites with recently published formulae for deformation of communicating and interacting cavities (interconnected pores/cracks and fractures at finite concentration) under drained loading. The inclusion-based model operates with spheroidal cavities at two different length scales; namely, the microscopic scale of the pores and (grain-boundary) cracks, and the mesoscopic scale of the fractures (controlling the flow of fluid). The different cavity types can in principle have any orientation and aspect ratio, but the microscopic pores/cracks and mesoscopic fractures were here assumed to be randomly and vertically oriented, respectively. By using three different aspect ratios for the relatively round pores (representing the stiff part of the pore space) and a distribution of aspect ratios for the relatively flat cracks (representing the compliant part of the pore space), we obtained a good fit between theoretical predictions and ultrasonic laboratory measurements on an unfractured rock sample under dry conditions. By using a single aspect ratio for the mesoscopic fractures, we arrived at a higher-order microstructural model of fractured porous media which represents a generalization of the first-order model developed by Chapman et al. (2002,2003). The effect of cavity size was here modelled under the assumption that the characteristic time for wave-induced (squirt) flow at the scale of a particular cavity (pore/crack vs. fracture) is proportional with the relevant scale-size. In the modelling, we investigate the effect of a decreasing pore pressure with constant confining pressure (fixed depth), and hence, increasing effective pressure. The analysis shows that the attenuation-peak due to the mesoscopic fractures in the reservoir will move downward in frequency as the effective pressure increases. In the range of seismic frequencies, our modelling indicates that the P-wave velocities may change by more than 20% perpendicular to the fractures and close to 10% parallel to the fractures. In comparison, the vertical S-wave velocities change by only about 5% for both polarization directions (perpendicular and parallel to the fractures) when the effective pressure increases from 0 to 15 MPa. This change is mainly due to the overall change in porosity with pressure. The weak pressure dependence is a consequence of the fact that the S waves will only sense if the fractures are open or not, and since all the fractures have the same aspect ratio, they will close at the same effective pressure (which is outside the analysed interval).

Laboratory measurement of P-wave velocity in crustal and upper mantle xenoliths from Ichino-megata, NE Japan: ultrabasic hydrous lower crust beneath the NE Honshu arc

Ultrasonic laboratory measurements of P-wave velocity (Vp) were carried out up to 1.0 GPa in a temperature range of 25–400 °C for crustal and mantle xenoliths of Ichino-megata, northeast Japan. The rocks used in the present study cover a nearly entire range of lithological variation of the Ichino-megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast (NE) Honshu arc. The Vp values measured at 25 °C and 1.0 GPa are 6.7–7.2 km/s for the hornblende gabbros (38.6–46.9 wt.% SiO 2), 7.2 km/s for the hornblende-pyroxene gabbro (43.8 wt.% SiO 2), 6.9–7.3 km/s for the amphibolites (36.1–44.3 wt.% SiO 2), 8.0–8.1 km/s for the spinel lherzolites (46.2–47.2 wt.% SiO 2) and 6.30 km/s for the biotite granite (72.1 wt.% SiO 2). Combining the present data with the Vp profile of the NE Honshu arc [Iwasaki, T., Kato, W., Moriya, T., Hasemi, A., Umino, N., Okada, T., Miyashita, K., Mizogami, T., Takeda, T., Sekine, S., Matsushima, T., Tashiro, K., Miyamachi, H. 2001. Extensional structure in northern Honshu Arc as inferred from seismic refraction/wide-angle reflection profiling. Geophys. Res. Lett. 28 (12), 2329–2332], we infer that the 15 km thick lower crust of the NE Honshu arc is composed of amphibolite and/or hornblende (±pyroxene) gabbro with ultrabasic composition. The present study suggests that the Vp range of the lower crustal layer (6.6–7.0 km/s) in the NE Honshu arc, which is significantly lower than that obtained from various seismic measurements (e.g. the northern Izu-Bonin-Mariana arc: 7.1–7.3 km/s), is due to the thick hydrous lower crustal layer where hornblende, plagioclase and magnetite are dominant.

Sonic velocity and grain contact properties in reservoir sandstones

The purpose of this study was to examine the effect of grain contacts and quartz cementation with regard to acoustic wave propagation in sandstones. Grain contacts have been considered essential when relating acoustic velocities to physical rock properties, and a parameter numerically representing the contact area between individual grains (contact length) has been measured. The method used involves digital petrographic microscopy pictures analysed by image analysis software. Other parameters, such as grain size, number of grain-to-grain contacts, cracks, clay content and porosity, have also been closely examined. The results showed that the contact area between sand grains may be useful for explaining velocity variations during the initial stages of quartz cementation and grain framework stiffening. Continued increase in grain-contact area by chemical compaction will have less influence on the sonic velocity compared to variations in porosity. The Garn Fm. from the Norne Field (2.6–2.7 km burial depth), which is in the early phase of initial grain framework stiffening by quartz cementation, and the Garn Fm. from the Kristin Field (4.6–4.7 km burial depth), which is thoroughly quartz cemented, were chosen as sample materials. Log-derived velocities, in addition to some laboratory ultrasonic velocity measurements, were used in this study.

Elastic wave propagation along a set of parallel fractures

Previous studies on elastic wave propagation in fractured media have demonstrated that a single planar fracture supports fracture interface waves and that two plane parallel fractures support fracture channel waves. Here, the results are presented for plane wave propagation through an infinite number of plane parallel fractures with equal fracture spacing and fracture stiffnesses. Analysis of the dispersion equations for this fractured system demonstrates that these waves exhibit symmetric and antisymmetric particle motions, degenerate to classical Rayleigh‐Lamb plate waves when the fractures are completely open, and possess dispersive velocities that are functions of both the fracture stiffness and spacing. Time‐frequency analysis performed on a series of laboratory ultrasonic transmission measurements on a fractured rock analog shows good agreement with the theoretical predictions.